Chemistry Topic Quiz

Why the correct answer is correct: Pure water has a fixed, exact boiling point of 100 °C. If a liquid boils at any other temperature (like 103 °C), it contains impurities. Therefore, the boiling point is the only piece of evidence here that proves the water is not pure.

Why the other answers are wrong:

• B & C: Cobalt(II) chloride paper turning pink only proves that water is present. It does not tell you if the water is pure or mixed with something else.
• D: The fact that the paper turned pink proves the liquid does contain water.

Helpful Tips:

• Purity = Physical Constants: To check if a substance is pure, always look at its boiling point or melting point.
• Presence = Chemical Tests: Chemical tests (like cobalt chloride paper) only confirm what a substance is, not how pure it is.
• Common Pitfall: Don’t confuse "water" with "pure water"! Many liquids contain water but are actually mixtures (like salt water or vinegar).

Why Answer A is Correct

• Boiling Point: Pure substances have a specific, fixed boiling point. Measuring this is the standard way to check for purity. If water contains impurities, its boiling point will increase.
• The Formula: "Iron(II)" tells you the iron ion has a 2+ charge ($Fe^{2+}$). The phosphate ion has a 3– charge ($PO_4^{3-}$). To make the compound neutral, you need three $Fe^{2+}$ ions (+6) to balance two $PO_4^{3-}$ ions (–6), giving $Fe_3(PO_4)_2$.

Why the Others are Wrong Other options are incorrect if they suggest the test "identifies elements" (boiling points only identify substances/purity) or use the wrong formula, such as $Fe_2(PO_4)_3$, which is Iron(III) phosphate.

Tips & Pitfalls

• The Roman Numeral Trap: The (II) tells you the charge of the metal, not the number of atoms in the formula.
• Brackets: Always put brackets around polyatomic ions (like $PO_4$) if there is more than one of them.

Why the Correct Answer (A) is Right:

• Purpose: Anhydrous copper(II) sulfate is a white powder that turns blue when it touches water. This makes it a perfect test to confirm the presence of water.
• Formula: Iron(II) means the iron ion has a $2+$ charge ($Fe^{2+}$). The phosphate ion has a $3-$ charge ($PO_4^{3-}$). To balance these charges to zero, you need three iron ions ($3 \times +2 = +6$) and two phosphate ions ($2 \times -3 = -6$), resulting in $Fe_3(PO_4)_2$.

Why the Others are Wrong:

• Purity: Chemical tests like copper(II) sulfate only show if water is there; they cannot prove water is pure. Only physical tests (like a boiling point of exactly $100^\circ\text{C}$) prove purity.
• Wrong Formulas: Formulas like $FePO_4$ represent Iron(III) phosphate, not Iron(II).

💡 Top Tip: Always use the Roman numeral in the name to find the metal's charge. "Anhydrous" simply means "dry" or "without water"!

EXPLANATION:

The name iron(II) sulfate gives you the answer! The Roman numeral (II) tells you the iron ion has an oxidation state of +2. Because $FeSO_4$ is a neutral compound (it has no overall charge), the sum of all oxidation states in the formula must be zero.

Why other choices are wrong:

• Any choice suggesting iron is +3 is incorrect because the Roman numeral specifically identifies it as iron(II).
• Any choice claiming the sum of oxidation states is anything other than zero is wrong; all stable chemical compounds are electrically neutral.

Helpful Tips:

• The Shortcut: For transition metals (like iron or copper), the Roman numeral is your "cheat code" for the oxidation state.
• The Zero Rule: If you don't see a plus or minus sign on the formula, the total sum must be 0.
• Common Pitfall: Don’t confuse the small "4" in $SO_4$ with the charge; that just counts the oxygen atoms!

EXPLANATION:

Why A is correct: This relies on the Rule of Neutrality. In a stable compound like iron(II) sulfate ($FeSO_4$), the total charge must equal zero. Because the sulfate ion ($SO_4$) is a polyatomic ion with a fixed charge of $2-$, the iron ion must have a charge of $2+$ to balance it out ($+2 + -2 = 0$).

Why the others are incorrect:

• Color: While iron(II) often looks green, color is a qualitative observation and can change depending on the environment; it isn't a definitive calculation.
• Mass/Moles: These tell you the quantity of the substance, not the specific charge of the ions.
• Atomic Number: This identifies the element (iron has 26 protons), but it doesn't change when iron loses electrons to form an ion.

Helpful Tip: Memorize common polyatomic ions like sulfate ($SO_4^{2-}$) and nitrate ($NO_3^-$). They are the "keys" to unlocking the oxidation state of the metal they are bonded to!

EXPLANATION:

Why Answer A is Correct: The question states that iron(II) sulfate is produced. In chemistry, the Roman numeral (II) in the name tells you the charge and oxidation state of the metal ion. Therefore, iron has an oxidation state of +2 in the salt ($FeSO_4$).

Why other answers are incorrect:

• Formula Errors: Any option suggesting the formula is $Fe_2(SO_4)_3$ is wrong; that is iron(III) sulfate. The correct formula here is $FeSO_4$.
• Gas Identification: If an option suggests the gas is $CO_2$ or $O_2$, it is incorrect. Reactive metals + dilute acid always produce hydrogen gas ($H_2$).
• Redox Confusion: Iron starts as a neutral atom (0) and becomes $Fe^{2+}$. Since it loses electrons, it is oxidized, not reduced.

Helpful Tip: Always use the Roman numeral in the product's name as a "cheat sheet" for the oxidation state. Also, remember the "Squeaky Pop" test—it only identifies hydrogen!

EXPLANATION:

Why the correct answer is A: Pure substances have a specific, fixed boiling point. Pure water boils at exactly $100^\circ\text{C}$. When impurities (like salt) are added to water, the boiling point increases. Because this sample boils at $102^\circ\text{C}$, the temperature deviation proves the liquid is a mixture, not a pure substance.

Why other answers are incorrect:

• The Cobalt(II) chloride test: Turning from blue to pink only proves that water is present. It is a chemical test for the identity of water, but it cannot determine purity. A mixture of salt and water would still turn the paper pink.
• Boiling at $102^\circ\text{C}$: This cannot be used to claim the liquid is pure, as $102^\circ\text{C}$ does not match the known boiling point of pure water.

Helpful Tip: Remember: Chemical tests (like cobalt chloride) identify what a substance is, while Physical properties (exact melting and boiling points) determine how pure it is.

Why A is correct: Pure substances have fixed, specific physical properties. Pure water has a "sharp" boiling point of exactly 100 °C (at standard pressure). If any impurities are dissolved in the liquid, the boiling point will increase or occur over a range of temperatures.

Why the others are wrong:

• Chemical tests (like anhydrous copper sulfate turning blue) only prove that water is present. They cannot tell you if the water is pure or mixed with other substances.
• Visual or pH tests are unreliable because many different colorless liquids can have a neutral pH of 7 without being pure water.

Helpful Tip: Always distinguish between a test for presence (Is water there?) and purity (Is it only water?). To prove purity, you must always check a physical constant like the boiling or melting point.

Why Choice A is Correct:

• Pure Water: Pure substances have a fixed, sharp boiling point. Pure water boils at exactly 100 °C. If water contains impurities, it will boil over a range of temperatures rather than a single point.
• The Formula: Iron(II) indicates an $Fe^{2+}$ ion. The phosphate ion is $PO_4^{3-}$. To balance the charges so the compound is neutral, you need three iron ions ($3 \times +2 = +6$) and two phosphate ions ($2 \times -3 = -6$). This results in $Fe_3(PO_4)_2$.

Why other choices are wrong: Options stating water boils over a "range" of temperatures are describing mixtures, not pure substances. Formulas like $FePO_4$ or $Fe_2(PO_4)_3$ are incorrect because the ionic charges are not properly balanced.

Helpful Tip: Use the "Swap and Drop" method for formulas! Take the charge of the iron (2) and the phosphate (3), swap them, and write them as subscripts.

EXPLANATION:

Why the correct answer is A: Pure substances have a fixed, specific boiling point. If a water sample boils exactly at its standard boiling point (100°C at sea level), it confirms the sample is pure. Impurities usually raise the boiling point.

For the formula, Iron(II) indicates an $\text{Fe}^{2+}$ ion. The phosphate ion is $\text{PO}_4^{3-}$. To make the compound neutral, you must balance the charges: three $\text{Fe}^{2+}$ ions $(+6)$ and two $\text{PO}_4^{3-}$ ions $(-6)$ equal zero. This gives us $\text{Fe}_3(\text{PO}_4)_2$.

Why the others are wrong:

• Purity: Measuring boiling point doesn't change or create purity; it only verifies it.
• Formula: Formulas like $\text{FePO}_4$ or $\text{Fe}_2\text{PO}_4$ are incorrect because the positive and negative charges do not balance out to zero.

Helpful Tip: The Roman numeral (II) tells you the charge of the iron, not the number of atoms in the formula! Always use the "criss-cross" method to balance ionic charges.

Why A is correct: In chemistry, all neutral compounds must have a total charge of zero. In iron(II) sulfate ($FeSO_4$), the iron ion has an oxidation state of $+2$, and the sulfate group ($SO_4$) has a total oxidation state of $-2$. When you add these together (along with the individual sulfur and oxygen states), the sum is exactly zero. This confirms the compound is chemically balanced.

Why the others are wrong:

• Incorrect Formulas: Answers suggesting formulas like $Fe_2SO_4$ are wrong because the charges would not balance to zero.
• Wrong Oxidation States: Statements claiming the iron is $Fe^{3+}$ contradict the "iron(II)" part of the name.
• Physical Properties: Purity checks using boiling points are often distractors; $FeSO_4$ is a solid that decomposes when heated rather than boiling like a simple liquid.

Helpful Tip: The Roman numeral (II) specifically tells you the charge of the metal is $+2$. Always use this to check if a formula is balanced!

EXPLANATION:

Why the correct answer is correct: Pure substances have fixed physical properties, such as specific melting and boiling points. Pure water has a unique boiling point of exactly 100 °C (at standard pressure). If a liquid boils at this precise temperature, it confirms the substance is pure.

Why other options are incorrect: Common distractors usually involve chemical tests, such as anhydrous copper(II) sulfate turning blue or cobalt chloride paper turning pink. These tests only prove that water is present; they do not prove it is pure. For example, salty seawater would still pass these chemical tests, but it is not pure water.

Helpful Tip:

• To test for purity: Look for "sharp" or "exact" physical constants (boiling/melting points).
• To test for presence: Look for chemical color changes.
• Common Pitfall: Don’t confuse "water" with "pure water"! Any mixture containing water will react chemically, but only pure water boils at 100 °C.

Why A is correct: The chemical test (cobalt chloride turning pink) proves the liquid contains water. However, pure substances have fixed, sharp boiling points. Pure water boils at exactly 100 °C. Because this liquid boils at 104 °C, it must contain impurities (like salt or sugar) that have raised the boiling point.

Why the others are wrong:

• The liquid is not "pure": Even though it contains water, the higher boiling point (104 °C) proves it is a mixture.
• Chemical tests vs. Purity: Cobalt(II) chloride only confirms the presence of water. It cannot tell you if the water is pure; only a physical test (boiling/melting point) can do that.

Helpful Tips:

• Presence vs. Purity: Use cobalt chloride or copper(II) sulfate to see if water is there. Use boiling point to see if it is pure.
• Impurity Effect: Impurities always increase the boiling point and decrease the melting point.

Why the correct answer is correct: In chemistry, pure substances have a "sharp" or fixed boiling point. This means they turn into gas at one specific temperature. If the liquid is pure water, the thermometer will stay at exactly 100 °C until every drop has boiled away.

Why the other options are wrong:

• Chemical tests: Using cobalt(II) chloride paper or copper(II) sulfate only proves that water is present. It doesn’t tell you if the water is pure or mixed with something else.
• Physical appearance: Just because a liquid is colorless or odorless doesn’t mean it’s pure. Many clear liquids (like vinegar or salt water) are mixtures.

Helpful Tips:

• Pure = Fixed: Always look for "fixed temperature" or "sharp melting/boiling point" when identifying pure substances.
• Impure = Range: Impure substances (mixtures) melt or boil over a range of temperatures (e.g., 101 °C to 105 °C).

EXPLANATION:

Hydrated iron(II) sulfate ($FeSO_4 \cdot xH_2O$) contains water trapped within its crystals. Heating it releases this water as steam, which then condenses into a colorless liquid.

• Why A is correct: Pure water has a fixed, sharp boiling point of exactly 100 °C. The name "iron(II) sulfate" uses the Roman numeral (II) to show the iron is in the +2 oxidation state. Removing the water to make the salt "anhydrous" does not change the charge of the iron ion.
• Why others are wrong:
  • Boiling point: A value like 102 °C would indicate the water is impure.
  • Oxidation state: Options showing +3 are incorrect because the starting material is specifically an iron(II) salt.

Student Tip: The Roman numeral in a compound’s name is a "cheat code"—it tells you the oxidation state immediately! Also, remember that only pure substances have a single, fixed boiling point.

Why the Correct Answer is A: The cobalt(II) chloride paper turning pink only proves that water is present. It does not tell you if the water is pure. To check for purity, we look at physical properties. Pure water has a fixed boiling point of exactly 100 °C. Because this liquid boils at 105 °C, the higher boiling point confirms that the water contains impurities.

Why the others are wrong:

• Chemical tests (like the paper test) cannot prove purity; they only identify the substance.
• If a statement suggests the paper shows the water is pure, it is incorrect because the paper reacts the same way to both tap water and distilled water.
• A boiling point of 105 °C always indicates an impure mixture.

Helpful Tip: Think of it this way: Chemical tests identify what the substance is, but Physical tests (boiling/melting points) show how pure it is. Pure substances have one specific, sharp boiling point!

Why A is correct:

1. The Water Test: Anhydrous cobalt(II) chloride paper is a specific test for water. When it turns from blue to pink, it confirms that water is present in the sample.
2. The Purity Test: Pure substances have fixed, exact boiling points. Pure water boils at exactly 100 °C. Because this liquid boils at 103 °C, it must be a mixture (impure). Impurities typically raise the boiling point of a liquid.

Why other options are wrong:

• It isn't pure water because the boiling point is not exactly 100 °C.
• It isn't a different pure liquid because only water turns cobalt(II) chloride paper pink.

Top Tip: Always use two steps for these questions:

• Chemical tests (like color changes) tell you what the substance is.
• Physical tests (like boiling/melting points) tell you how pure it is.

Why the correct answer is correct: Pure water has a fixed boiling point of 100°C. Because this sample boils at 102°C, it must be impure (impurities like salt raise the boiling point of a liquid).

For the formula, "Iron(II)" tells you the iron ion has a +2 charge ($Fe^{2+}$). The phosphate ion always has a -3 charge ($PO_4^{3-}$). To make the compound neutral, you need three $Fe^{2+}$ (+6 total) and two $PO_4^{3-}$ (-6 total), giving you $Fe_3(PO_4)_2$.

Why the others are wrong: Any option stating the water is "pure" is incorrect because pure substances boil at exact temperatures. Formulas like $FePO_4$ are incorrect because the charges are not balanced for Iron(II).

Tips & Pitfalls:

• Boiling Point: Remember: Impurities raise boiling points but lower melting points.
• The "Criss-Cross" Rule: Swap the numerical charges of the ions to find the subscripts for the formula!

Why A is Correct:

1. Chemical Formula: Iron(II) indicates an iron ion with a +2 charge ($\text{Fe}^{2+}$). The phosphate ion has a -3 charge ($\text{PO}_4^{3-}$). To create a neutral compound, the charges must balance to zero. You need three $\text{Fe}^{2+}$ ions ($+6$ total) and two $\text{PO}_4^{3-}$ ions ($-6$ total), resulting in $\text{Fe}_3(\text{PO}_4)_2$.
2. Boiling Point: Pure substances have a fixed, specific boiling point. Testing if water boils at exactly $100^\circ\text{C}$ (at sea level) is a standard way to check for purity.

Why Others are Wrong: Other options may list $\text{Fe}_2(\text{PO}_4)_3$, which is Iron(III) phosphate, or claim the boiling point determines volume or density. Boiling point is a physical property used specifically to identify substances or their purity.

Top Tip: The Roman numeral (II) tells you the charge of the metal ion, not the number of atoms in the formula!

EXPLANATION:

Why the correct answer (A) is correct: The change of anhydrous cobalt(II) chloride from blue to pink is a specific chemical test for the presence of water. This reaction proves that the liquid contains water molecules.

Why the other options are incorrect:

• It is NOT "pure water": Pure water has a fixed boiling point of exactly 100 °C. Because this liquid boils at 102 °C, it is an impure mixture (water containing a dissolved substance).
• It is NOT "not water": Even though the boiling point is higher than 100 °C, the pink color change confirms water is definitely present.

Helpful Tip & Common Pitfall: Students often confuse testing for water with testing for purity.

• Chemical tests (like cobalt chloride or copper(II) sulfate) only tell you if water is present.
• Physical tests (like boiling point or freezing point) tell you if the water is pure.

Why it’s correct: The Roman numeral (II) indicates that the iron ion has a 2+ charge ($\text{Fe}^{2+}$). Since chloride ions have a 1- charge ($\text{Cl}^-$), you need two chlorides to balance one iron ion, resulting in $\text{FeCl}_2$. Regarding purity, pure substances have a sharp, constant boiling point. Pure water boils at exactly 100°C; if it stays at this temperature while boiling, it is pure.

Why other options are wrong:

• $\text{FeCl}_3$ is the formula for Iron(III) chloride, not Iron(II).
• If the water were impure, it would boil over a range of temperatures (e.g., 102°C–105°C) rather than at one constant point.

Tips & Pitfalls:

• Roman Numerals: These tell you the charge of the metal, not the number of atoms.
• Purity Rule: Impurities always raise the boiling point and lower the melting point of a substance.

This question tests two key concepts: chemical tests for water and physical tests for purity.

Why A is correct:

1. Presence of Water: Cobalt(II) chloride paper turns from blue to pink specifically when it reacts with water. This confirms the liquid contains water.
2. Purity: Pure water has a fixed boiling point of exactly 100 °C. Because this sample boils at 103 °C, it must contain dissolved impurities (like salt), which raise the boiling point.

Why other answers are incorrect:

• It cannot be pure water because the boiling point is not 100 °C.
• It cannot be water-free because the cobalt chloride changed color.

Top Tip: Always distinguish between a chemical test (color change) which identifies a substance, and a physical test (boiling/melting point) which identifies if that substance is pure! Pure substances have sharp, exact boiling points.

Why the correct answer is correct: This question tests your knowledge of chemical tests and purity.

1. The Water Test: Anhydrous cobalt(II) chloride turning from blue to pink is the standard chemical test that confirms the presence of water.
2. Boiling Point: Pure water has a fixed boiling point of exactly 100 °C. Because this liquid boils at 103 °C, it must be impure. Adding a solute (like salt) to water increases its boiling point.

Why other answers are incorrect:

• B: It cannot be pure water because pure water must boil at 100 °C.
• C & D: These are incorrect because the cobalt(II) chloride test proves that water is definitely present.

💡 Tip: Always look at the boiling point to determine purity! Pure substances have a sharp, specific boiling point. If the temperature is higher or lower than the standard value, the substance is impure.

Correct Answer: A

Why it’s correct: In chemistry, we check the melting point to confirm a sample is pure. Pure substances melt at a single, sharp temperature, whereas impurities cause them to melt over a range of lower temperatures.

To find the formula for iron(II) phosphate, you must balance the charges:

• Iron(II) has a $+2$ charge ($Fe^{2+}$).
• Phosphate has a $-3$ charge ($PO_4^{3-}$). To make the total charge zero, you need three $Fe^{2+}$ ions ($+6$) and two $PO_4^{3-}$ ions ($-6$), resulting in $Fe_3(PO_4)_2$.

Why other options are wrong:

• Melting points do not measure "reactivity" or "density."
• Formulas like $FePO_4$ would be for iron(III) phosphate, where the charges are $+3$ and $-3$.

Top Tip: The Roman numeral (II) tells you the charge of the metal ion ($+2$). Always use brackets when you have more than one polyatomic ion (like phosphate) in a formula!

Why Answer A is Correct The cobalt(II) chloride test is a specific chemical test for the presence of water. Anhydrous cobalt(II) chloride is naturally blue, but when it touches water molecules, it reacts and turns pink. This happens even if the water is mixed with other substances (impure).

Why the other answers are incorrect Physical properties, like boiling point, are used to check purity. Pure water boils at exactly 100 °C. Because this sample boils at 105 °C, it is impure, meaning the boiling point alone cannot confirm that the liquid is water—it only tells us the liquid isn't pure. Similarly, being "colorless" or "clear" isn't enough proof, as many other liquids look like water.

Top Tip

• Chemical tests (like cobalt chloride) identify what a substance is.
• Physical tests (like boiling point) identify how pure a substance is.

Why it’s correct: Pure substances have a specific, fixed boiling point. At standard pressure, pure water always boils at exactly 100 °C. Because this liquid boils at 104 °C, it contains impurities (like salt or sugar) that have raised the boiling point. Therefore, it cannot be pure water.

Why the others are wrong:

• Any claim that the liquid is pure water is incorrect because 104 °C does not match the known boiling point of water.
• Claims that the boiling point doesn't matter are wrong because boiling point is a reliable way to test if a substance is pure or a mixture.

Helpful Tip: Think of impurities as "anchors" that hold liquid molecules back, making it harder for them to turn into gas. This is why impurities increase the boiling point and decrease the melting point!

Why Answer A is Correct The name Iron(II) sulfate tells us the iron ion has a 2+ charge ($\text{Fe}^{2+}$). Since the sulfate ion always has a 2− charge ($\text{SO}_4^{2-}$), they balance perfectly in a 1:1 ratio, giving the formula $\text{FeSO}_4$. Regarding purity, pure substances have specific, fixed boiling and melting points. If a substance is pure, it will boil at one exact temperature rather than across a range.

Why the Others are Incorrect

• Formulas like $\text{Fe}_2\text{SO}_4$ or $\text{Fe}(\text{SO}_4)_2$ are wrong because the positive and negative charges do not balance out to zero.
• Any statement suggesting that a "range of temperatures" indicates purity is incorrect; a boiling range actually identifies an impure mixture.

Tips & Pitfalls

• The Roman Numeral Trick: The (II) refers to the charge of the iron, not the number of iron atoms in the formula.
• Pure vs. Impure: Remember: Fixed = Pure, Range = Impure.

Why A is correct: Pure substances have fixed, "sharp" boiling and melting points. At standard pressure, pure water boils at exactly $100^\circ\text{C}$ and freezes at $0^\circ\text{C}$. If any impurities (like salt) are present, the boiling point will change.

Why others are wrong: Chemical tests—like turning anhydrous copper sulfate blue or cobalt chloride paper pink—only prove that water is present. They cannot prove it is pure; even salty water would pass these tests. Similarly, being "colorless" or having a "pH of 7" applies to many other liquids and doesn't guarantee purity.

Top Tip: To check for purity, look for a specific boiling or melting point. To check for identity (is it water?), use a chemical test.

Common Pitfall: Don’t assume "clear and odorless" means pure. Many colorless liquids, like ethanol or dilute acids, look exactly like water but are not!

Pure substances have fixed, specific melting and boiling points. Because pure water always boils at exactly 100 °C (at standard pressure), this "sharp" boiling point confirms its purity. If any impurities (like salt) were present, the boiling point would increase.

Why other answers are wrong:

• Chemical tests (such as turning anhydrous copper(II) sulfate blue or cobalt chloride paper pink) only confirm that water is present. They do not prove purity; even sea water would pass these tests.
• pH tests only show that a liquid is neutral. Many impure mixtures can have a pH of 7.

Helpful Tip: To remember this, think: Chemical tests identify what a substance is; Physical tests (boiling/melting points) confirm how pure it is. Pure substances boil at one specific temperature, while mixtures boil over a range of temperatures.

To confirm water is pure, we check its physical properties. Pure substances have a sharp, fixed boiling point. For water at standard pressure, this is exactly 100 °C. If the water is impure, it will boil at a higher temperature or over a range.

For the chemical formula, Iron(II) tells us the iron ion has a +2 charge ($Fe^{2+}$). The phosphate ion is a polyatomic ion with a -3 charge ($PO_4^{3-}$). To make the compound neutral (zero charge), we use the "swap and drop" method:

• Three $Fe^{2+}$ ions ($+6$ total) balance two $PO_4^{3-}$ ions ($-6$ total).
• This gives us $Fe_3(PO_4)_2$.

Why others are wrong:

• Options suggesting different boiling points describe impure water.
• Formulas like $FePO_4$ use Iron(III), not Iron(II).

Top Tip: The Roman numeral (II) represents the charge of the metal, not the number of atoms in the formula!

Why A is correct: The standard chemical test for water uses anhydrous copper(II) sulfate. This powder is naturally white, but it turns blue when it reacts with water to become hydrated.

To determine the formula for iron(II) phosphate, you must balance the ionic charges:

• Iron(II) has a $2+$ charge ($\text{Fe}^{2+}$).
• Phosphate has a $3-$ charge ($\text{PO}_4^{3-}$). To make the compound neutral, you need three iron ions ($+6$ total) and two phosphate ions ($-6$ total). This gives the formula $\text{Fe}_3(\text{PO}_4)_2$.

Why others are wrong: Incorrect options often suggest that copper sulfate turns pink (that is the test using cobalt chloride!) or use the wrong charge for Iron (such as $\text{FePO}_4$, which is iron(III) phosphate).

Top Tip: The Roman numeral (II) tells you the charge of the metal ion is $2+$. Always use brackets when your formula requires more than one polyatomic ion, like $(\text{PO}_4)_2$.

Why A is Correct: All chemical compounds are electrically neutral. The Roman numeral (II) indicates that the iron ion has a +2 charge ($Fe^{2+}$). The phosphate ion is a polyatomic ion with a -3 charge ($PO_4^{3-}$). To make the total charge zero, you need a ratio that balances:

• 3 Iron ions: $3 \times (+2) = +6$
• 2 Phosphate ions: $2 \times (-3) = -6$ Combining these gives the neutral formula $Fe_3(PO_4)_2$.

Why other answers are incorrect: Other options likely suggest incorrect ratios (like $FePO_4$) or state that the formula changes based on purity. In chemistry, a compound’s formula is a fixed ratio determined by charge balance, regardless of how pure the sample is.

Common Pitfall: The Roman numeral (II) tells you the charge of the iron, NOT the number of iron atoms! Use the "criss-cross method" (swap the charge numbers to become the other ion's subscript) to find the formula quickly.

Why Answer A is Correct: In chemistry, the Roman numeral (II) tells us that the iron ion has a 2+ charge ($Fe^{2+}$). The phosphate ion always has a 3– charge ($PO_4^{3-}$). To create a neutral compound, the total positive and negative charges must balance out. By using three $Fe^{2+}$ ions (total +6) and two $PO_4^{3-}$ ions (total –6), the charges cancel out perfectly, giving us the formula $Fe_3(PO_4)_2$.

Why the others are wrong: Other options typically suggest incorrect formulas (like $FePO_4$, which is Iron(III) phosphate) or misidentify physical vs. chemical properties. A chemical formula represents a chemical property because it describes the substance's composition and how its atoms bond.

Student Tip: Always remember that the Roman numeral represents the charge of the metal, not the number of atoms! Use the "swap and drop" method with the charges to find the correct subscripts quickly.

Why it’s correct: Pure substances have a fixed, specific boiling point. For pure water, this is exactly 100 °C (at standard pressure). If water boils at a higher temperature, such as 103 °C, it means impurities (like salt or minerals) are dissolved in it, which "trap" the water molecules and require more energy to turn them into gas.

Why other answers are wrong:

• Any claim that the water is "pure" is incorrect because pure substances do not have a range of boiling points; they are exact.
• Saying 103 °C is "close enough" is a mistake; even a small deviation proves the substance is a mixture.

Tips & Pitfalls:

• The Rule: Impurities increase the boiling point but decrease the melting point.
• Pitfall: Don’t assume boiling means a liquid is pure. All liquids boil—the temperature is the only way to tell if it's pure!

Why Answer A is correct: The Roman numeral (II) tells you the Iron ion has a 2+ charge ($Fe^{2+}$). The sulfate ion always has a 2- charge ($SO_4^{2-}$). Because these charges are equal and opposite, they balance perfectly in a 1:1 ratio, giving the formula FeSO₄.

In chemistry, the melting point is a standard test for purity. A pure solid melts at one specific, "sharp" temperature. If the substance is impure, it will melt at a lower temperature and over a wider range.

Why other options are wrong:

• Formulas like $Fe_2SO_4$ or $Fe(SO_4)_2$ are incorrect because the charges do not balance.
• Melting point is a physical property used to identify a substance, not to measure its chemical reactivity or solubility.

Top Tip: Always use the Roman numeral to determine the metal's charge. Remember: Pure = Sharp melting point!

EXPLANATION:

Why the correct answer (A) is correct:

• Testing for water: Anhydrous copper(II) sulfate is a white powder. When it touches water, it turns blue. This color change is a standard chemical test to identify if water is present in a liquid.
• Chemical Formula: The Roman numeral (II) tells you that the Iron ion has a +2 charge ($Fe^{2+}$). Since a Chloride ion ($Cl^-$) has a -1 charge, you need two chlorides to balance one iron ion, giving the formula $FeCl_2$.

Why other options are wrong:

• Testing Purity: This test only shows if water is there. It cannot tell if the water is pure. To check purity, you must measure its boiling point (100°C).
• $FeCl_3$: This is Iron(III) chloride, where iron has a +3 charge.

💡 Top Tip: Always use the Roman numeral in the name to find the metal's charge! It’s the easiest way to write formulas correctly.

This question tests your knowledge of chemical tests and physical properties.

Why A is correct:

1. The chemical test: Blue anhydrous cobalt(II) chloride paper turns pink when it touches water. This confirms water is present in the sample.
2. The physical test: Pure water boils at exactly 100 °C. Because this sample boils at 104 °C, it must be impure. Impurities (like dissolved salt) always raise the boiling point of a liquid.

Why the others are wrong:

• It is not pure water because the boiling point is not 100 °C.
• It cannot be a different liquid (like pure ethanol) because other liquids would not turn the paper pink.

Helpful Tip: Always distinguish between identity and purity. A chemical test (paper) tells you what is there, while a physical test (boiling point) tells you how pure it is. Only pure substances have sharp, exact boiling points!

EXPLANATION:

Why the correct answer is A: Two tests were performed on the liquid:

1. Chemical Test: Turning blue cobalt(II) chloride paper pink is the standard test for the presence of water.
2. Physical Test: Pure water has a fixed boiling point of exactly 100 °C. Because this sample boils at 103 °C, it contains impurities (like dissolved salts) which raise the boiling point.

Therefore, the liquid is water, but it is impure.

Why other answers are incorrect:

• Any claim that the water is pure is incorrect because pure substances must boil at an exact, specific temperature.
• Any claim that the liquid is not water is incorrect because the cobalt(II) chloride paper specifically confirms water is present.

Helpful Tip: Always remember: Chemical tests (like the paper test) tell you what a substance is, while physical tests (like boiling point) tell you how pure it is.

Why it’s correct: Pure substances have a specific, fixed boiling point. If a solvent boils exactly at its known temperature, it is confirmed to be pure. The Roman numeral (II) represents the oxidation state (the charge) of the metal ion. Therefore, iron(II) indicates that the iron ion has a $+2$ charge ($Fe^{2+}$).

Why others are wrong:

• The (II) does not mean there are two iron atoms in the chemical formula.
• The Roman numeral describes the metal (iron) specifically, not the sulfate ion.
• While color or density can give clues, measuring the boiling point is a standard, precise way to verify the purity of a liquid solvent.

Helpful Tips:

• Roman Numerals: These always indicate the charge of the metal ion, never the number of atoms.
• Purity Check: Remember that impurities cause a substance to melt or boil over a range of temperatures rather than at a fixed point.

Why the correct answer (A) is right:

• Formula: The "(II)" in iron(II) sulfate tells us the iron ion has a 2+ charge (Fe²⁺). The sulfate ion has a 2– charge (SO₄²⁻). These charges balance perfectly in a 1:1 ratio, giving the formula FeSO₄.
• Purpose: Anhydrous copper(II) sulfate is a white powder that turns blue when it touches water. This is a chemical test to confirm the presence of water.

Why the others are wrong:

• Incorrect Formulas: Formulas like Fe₂SO₄ are incorrect because the ionic charges do not balance.
• Purity vs. Presence: Copper(II) sulfate cannot tell you if water is pure; it only shows if water is there. To test for purity, you must check if the liquid boils at exactly 100°C.

Helpful Tip: Remember, "anhydrous" means "without water." It changes color to signal water is present, but it won't tell you what else is mixed in!

Why the correct answer is correct: To identify the liquid as water, we use a chemical test. Anhydrous cobalt(II) chloride turns from blue to pink when water is present. To determine if the water is impure, we look at its boiling point. Pure water boils at exactly 100 °C. Since this liquid boils at 102 °C, the presence of impurities has raised the boiling point.

Why the others are wrong:

• Any option stating the boiling point is 100 °C describes pure water, which contradicts the question.
• While anhydrous copper(II) sulfate is a valid test for water (turning from white to blue), it must be paired with the higher boiling point (102 °C) to prove the liquid is impure.

Tips & Pitfalls:

• Memory Trick: Impurities increase the boiling point but decrease the melting point (like putting salt on icy roads).
• Chemical Tests: Cobalt(II) chloride (Blue $\rightarrow$ Pink) and Copper(II) sulfate (White $\rightarrow$ Blue) only prove water is present, not that it is pure. Only a fixed boiling point proves purity.

Why the correct answer is correct: Pure substances have a specific, fixed boiling point. At standard pressure, pure water boils exactly at 100 °C. Because this sample boils at 102 °C, it means impurities (like dissolved salt or minerals) are present. These impurities interfere with the liquid molecules, requiring more energy (a higher temperature) to turn them into gas.

Why the other answers are incorrect: Any choice suggesting the water is "pure" is wrong because pure water cannot boil above or below its fixed boiling point. Even a small difference of 2 °C is a clear sign that the water is a mixture, not a pure substance.

Helpful Tip:

• Impurities always increase the boiling point and decrease the melting point.
• If you see a range of temperatures (e.g., boiling from 101–103 °C), that is also a sign of an impure substance!

Why A is Correct:

• Boiling Point: Pure substances have a specific, fixed boiling point (e.g., pure water boils at $100^\circ\text{C}$). If water contains impurities, its boiling point increases. Measuring it is a standard test for purity.
• Formula: The Roman numeral (II) tells us the iron ion has a charge of $+2$ ($Fe^{2+}$). Since an oxide ion has a charge of $-2$ ($O^{2-}$), the charges balance exactly $1:1$, giving the formula FeO.

Why Others are Incorrect:

• $Fe_2O_3$: This is the formula for iron(III) oxide, where iron has a $+3$ charge.
• Purity: While boiling water can kill bacteria (making it "safer"), the scientific reason for measuring the exact boiling point is to check for chemical purity.

Top Tip: The Roman numeral in a name tells you the charge of the metal ion, not the number of atoms in the formula!

EXPLANATION:

Why the correct answer is correct: Pure substances have a fixed, "sharp" boiling point. For pure water, this is exactly 100 °C at standard pressure. When a liquid contains impurities, its boiling point increases (this is called boiling point elevation). Because 103 °C is higher than the boiling point of pure water, the liquid is classified as impure.

Why the other answers are incorrect: If the liquid were pure, it would boil at exactly 100 °C. Any deviation from this specific temperature indicates that the substance is a mixture rather than a pure compound.

Helpful Tips & Common Pitfalls:

• Boiling Point: Impurities always increase the boiling point.
• Melting Point: Impurities always decrease the melting point.
• Range vs. Sharp: Pure substances boil at one specific temperature, while impure substances boil over a range of temperatures (e.g., from 103 °C to 105 °C).

EXPLANATION:

Pure substances have a fixed, specific boiling point. For pure water, this is exactly 100 °C (at standard pressure).

Why A is Correct: Impurities (like salt or minerals) act like "anchors," making it harder for water molecules to escape into the air. This requires more energy, which raises the boiling point. Since the student measured 104 °C, the water must be impure.

Why other answers are incorrect: Any option claiming the water is "pure" is wrong. In chemistry, pure substances don't have "flexible" boiling points. Even if 104 °C seems "close," any deviation from 100 °C proves the substance is a mixture.

Helpful Tip: Always remember: Impurities raise the boiling point but lower the melting point.

Common Pitfall: Don't assume the thermometer is broken or the student made a mistake. If a question gives you a specific temperature, use it to determine the substance's purity!

Why A is correct: Pure water has a fixed, exact boiling point of 100 °C. When impurities (like salt or minerals) are dissolved in water, they raise the boiling point. Because this sample boils at 103 °C, it must be impure.

Why the others are wrong: The cobalt(II) chloride test (turning from blue to pink) only confirms that water is present. It does not tell you if the water is pure or contains dissolved solids. Therefore, any statement claiming the water is pure based on this color change is incorrect.

Helpful Tips:

• Purity = Fixed Points: Pure substances have sharp, specific melting and boiling points. If the temperature is different from the standard (100 °C for boiling water), it is impure.
• Presence vs. Purity: Don't confuse a chemical test (which identifies a substance) with a physical test (which checks purity).

EXPLANATION:

Why the correct answer (A) is correct: To identify a substance, we use chemical tests. Turning anhydrous cobalt(II) chloride paper pink proves water is present. However, to check for purity, we must look at physical properties. Pure water boils at exactly 100 °C. Since this liquid boils at 105 °C, the higher temperature proves it contains impurities.

Why the other options are wrong:

• The chemical test (pink paper) only identifies the liquid as water; it does not guarantee it is pure.
• If a liquid boils at any temperature other than 100 °C, it cannot be pure water.
• The liquid is definitely water (due to the paper test), but the 105 °C boiling point confirms it is a mixture.

Top Tip: Always distinguish between identification and purity.

• Chemical tests (like cobalt paper) tell you what a substance is.
• Physical tests (boiling/melting points) tell you how pure it is.

Why A is Correct: A precise melting point acts like a "chemical fingerprint." Pure substances melt at one specific, sharp temperature. Chemists use this for two reasons:

1. Identity: Every pure compound has a unique melting point. By comparing your result to a data book, you can confirm what the substance is.
2. Purity: If a substance is impure, it will melt at a lower temperature and over a wider range (e.g., 110–115°C instead of a sharp 120°C).

Why others are incorrect: Options suggesting that melting point measures solubility or reaction speed are incorrect because melting is a physical change, not a chemical reaction. Options regarding concentration are wrong because melting points identify the substance itself, not how much of it is dissolved in a solvent.

Helpful Tip: Remember: Pure = Sharp & High. Impure = Broad & Low. If your sample melts over a range of more than 2°C, it isn't pure!

To prove a liquid is pure, you must check its physical constants, such as its boiling or melting point.

Why Answer A is Correct: Every pure substance has a unique, fixed boiling point. Pure water boils at exactly 100 °C (at standard pressure). If the liquid boils at a different temperature or over a range (e.g., 102–105 °C), it contains impurities.

Why Other Methods are Incorrect:

• Chemical Tests: Using anhydrous copper(II) sulfate or cobalt(II) chloride only proves that water is present. These tests will show a positive result even if the water is mixed with impurities like salt or sugar.
• Appearance: Being "colorless" or "clear" doesn't mean it is pure; many toxic chemicals look just like water.

Top Tip: Remember the difference: Chemical tests (color changes) show what is there, but physical tests (boiling/melting points) show how pure it is.

Why the correct answer is correct: Pure substances have specific, fixed physical properties. Pure water has an exact boiling point of 100 °C (at standard pressure). If a liquid boils at this precise temperature, it confirms the substance is pure.

Why the other observation is incorrect: Turning anhydrous cobalt(II) chloride from blue to pink is a chemical test that only confirms water is present. It does not check for purity; even "dirty" water or a mixture containing water would cause this color change.

Tips and Pitfalls:

• Chemical tests (like color changes) identify what a substance is.
• Physical tests (like boiling or melting points) are used to check how pure a substance is.
• Common Pitfall: Many students mistake any test for water as a test for purity. Always remember: purity requires a fixed, sharp measurement!

Why Answer A is Correct: In chemistry, the best way to check if a substance is pure is to measure its physical constants, such as its boiling point or melting point. Pure substances have a specific, "sharp" boiling point. Since the boiling point of pure water is exactly 100 °C, reaching this precise temperature confirms the water is free from impurities.

Why the others are incorrect:

• Testing for Presence vs. Purity: Some tests (like anhydrous copper(II) sulfate) only prove that water is present; they cannot tell you if it is pure.
• Mass Changes: Heating a hydrated salt causes the mass to decrease (not increase) as water vapor escapes.
• Color Changes: While the solid changes color (from green to white), this only indicates a chemical change, not the purity of the products.

Top Tip: Remember that purity = fixed physical properties. If a question asks about purity, look for specific melting or boiling points!

Why A is correct:

1. The Chemical Test: Anhydrous cobalt(II) chloride changes from blue to pink only when it reacts with water. This confirms water is present in the sample.
2. The Boiling Point: Pure water boils at exactly 100 °C. Because this liquid boils at 104 °C, it must contain dissolved impurities, which raise the boiling point. Therefore, the sample contains water but is not pure.

Why other answers are wrong:

• It cannot be pure water because the boiling point is not 100 °C.
• It cannot be a different pure liquid because a liquid that isn't water would not turn cobalt(II) chloride pink.

💡 Helpful Tip: Don't confuse presence with purity!

• Chemical tests (like cobalt chloride) prove a substance is there.
• Physical tests (like boiling or melting points) prove if it is pure. Impurities always increase the boiling point.

To choose the correct answer, let's break the question into two parts:

1. The Formula of Iron(II) Sulfate The Roman numeral (II) tells us the iron ion has a 2+ charge ($Fe^{2+}$). The sulfate ion always has a 2- charge ($SO_4^{2-}$). Since the charges are equal and opposite, they cancel each other out in a 1:1 ratio, giving us $FeSO_4$.

2. Testing for Water Anhydrous means "without water." Anhydrous copper(II) sulfate is a white powder that turns blue when it touches water. Therefore, it is used to show that water is present.

Why other options are wrong:

• Incorrect Formulas: Formulas like $Fe_2SO_4$ or $Fe(SO_4)_2$ have unbalanced charges.
• Presence vs. Purity: While this test shows water is present, it cannot tell if the water is pure. To test for purity, you must check if it boils at exactly $100^\circ C$.

💡 Tip: Always look at the Roman numeral for the metal's charge!

Why the correct answer is correct: Pure substances have a fixed, specific boiling point. For water, this is exactly 100 °C (at standard pressure). When a substance is impure (meaning it has other things dissolved in it), its boiling point increases. Because 102 °C is higher than 100 °C, the sample must contain impurities.

Why the other answers are incorrect: Any statement claiming the water is "pure" is wrong because pure water never boils above 100 °C. If a sample boils at a temperature higher or over a range (e.g., 102–105 °C), it is always classified as impure.

Helpful Tips:

• The "Impair" Rule: Impurities increase the boiling point but lower the melting point.
• Common Pitfall: Don’t assume the thermometer is broken! In chemistry problems, a change in boiling point is almost always a test of your knowledge of purity.

EXPLANATION:

To find sulfur’s oxidation state in $\text{FeSO}_4$, focus on the sulfate ion ($\text{SO}_4^{2-}$). Each Oxygen is -2. With four oxygens totaling -8, sulfur must be +6 to result in the overall -2 charge ($+6 - 8 = -2$). Regarding purity, pure water always has a fixed boiling point of exactly 100 °C at standard pressure.

Why other answers are wrong:

• Incorrect Oxidation States: Values like +4 are wrong because that is the state for sulfite ($\text{SO}_3^{2-}$), not sulfate.
• Incorrect Boiling Points: Any temperature above or below 100 °C (e.g., 102 °C) indicates the water is impure.

Helpful Tip: Always check the ion charge first! In neutral $\text{FeSO}_4$, the sum of all oxidation states is 0. Since $\text{Fe}$ is +2 and four $\text{O}$ are -8, the $\text{S}$ must be +6 to balance the formula.

Why A is correct:

• Melting Point: Pure substances have a specific, "sharp" melting point. If a substance contains impurities, it will melt at a lower temperature and over a wider range. Therefore, measuring it confirms the purity of the sample.
• Oxidation State: In chemistry, the Roman numeral in brackets, like (II), specifically tells us the oxidation state (or charge) of the metal ion. So, iron(II) means the iron has an oxidation state of +2.

Why others are wrong: Incorrect options often claim that melting point shows "reactivity" (it doesn’t) or that the (II) means there are "two iron atoms." This is a common mistake; the number of atoms is shown by small subscript numbers (like $\text{Fe}_2$), not Roman numerals.

Tips & Pitfalls:

• Pure = Sharp: Remember that pure solids melt at one exact temperature.
• Roman Numerals = Charge: Always link Roman numerals to the oxidation state of the metal, never the quantity of atoms.

Why A is correct: Ionic compounds must be electrically neutral (the total charge must equal zero). The name "Iron(II)" tells you the iron ion has a +2 charge ($Fe^{2+}$). The phosphate ion has a -3 charge ($PO_4^{3-}$). To balance these, you need three iron ions ($3 \times +2 = +6$) and two phosphate ions ($2 \times -3 = -6$). This results in the formula $Fe_3(PO_4)_2$.

Why others are wrong: Other options typically use the wrong charges (such as treating Iron as +3) or fail to balance the total charge to zero. A chemical formula is only valid if the positive and negative charges perfectly cancel out.

Tips & Pitfalls:

• The Criss-Cross Rule: A quick trick is to swap the charge numbers: the 2 from $Fe^{2+}$ becomes the subscript for phosphate, and the 3 from $PO_4^{3-}$ becomes the subscript for iron.
• Roman Numerals: These indicate the charge of the metal, not the number of atoms!

Why A is correct: All neutral compounds must have a total charge of zero. In one formula unit of $\text{Fe}_3(\text{PO}_4)_2$, the three $\text{Fe}^{2+}$ ions (totaling $+6$) perfectly balance the two $\text{PO}_4^{3-}$ ions (totaling $-6$). Therefore, the sum of all oxidation states always equals zero.

Why the others are wrong:

• It’s not a molecule: Ionic compounds form a large crystal lattice, not individual molecules. Any answer calling it a "molecule" is incorrect.
• Mass vs. Ratio: The formula tells you the ratio of atoms (3:2:8), but not the mass percentage. Iron and Phosphorus have different atomic masses, so their mass proportions won't match their atom counts.
• Purity: A chemical formula only tells you what the substance should be. You cannot determine the purity of a physical sample just by looking at its formula; that requires lab testing.

Tip: Always check if a substance is ionic (metal + non-metal). If it is, avoid any answer choice that uses the word "molecule"!

Why Choice A is correct: In chemistry, a neutral compound must have a total charge of zero. The name Iron(II) sulfate tells us the oxidation state of the iron ion is +2 ($Fe^{2+}$). The sulfate ion ($SO_4^{2-}$) has an oxidation state of -2. When you add them together ($+2 + -2$), the result is zero. This confirms that the correct, balanced formula is $FeSO_4$.

Why the others are wrong: Other options are incorrect because they likely suggest formulas where the charges do not balance (such as $Fe_2SO_4$) or use the wrong oxidation state for Iron (like +3). If the oxidation states don't sum to zero, the formula cannot represent a neutral compound.

💡 Tip: The Roman numeral in the name always tells you the oxidation state of the metal (e.g., II = +2). ⚠️ Pitfall: Don't let the "4" in $SO_4$ confuse you; it refers to the number of oxygen atoms, not the charge!

This question tests your knowledge of chemical tests and purity.

Why A is correct:

1. The Test: Anhydrous cobalt(II) chloride paper changes from blue to pink specifically to show that water is present.
2. The Boiling Point: Pure water has a fixed boiling point of exactly 100 °C. Because this liquid boils at 103 °C, the presence of impurities has raised its boiling point. Therefore, it is water, but it isn't pure.

Why other answers are wrong:

• It isn't pure water because the boiling point is not 100 °C.
• It isn't another liquid (like ethanol) because other liquids would not turn the cobalt(II) chloride paper pink.

Helpful Tips:

• Purity Check: Pure substances have specific, sharp melting and boiling points.
• Impurities: Adding a substance to a liquid always increases its boiling point and decreases its melting point.
• Remember: The cobalt chloride test only proves water exists; it doesn't prove it is pure!

Why A is Correct: Pure water has a fixed, specific boiling point of exactly 100 °C at standard pressure. When a substance is impure (meaning something like salt or sugar is dissolved in it), its boiling point increases. Since this liquid boils at 102 °C, it must contain impurities that have raised the boiling point above the normal level for pure water.

Why other answers are wrong:

• Any claim that the liquid is pure water is incorrect because pure substances always boil at their exact, constant boiling point.
• If an option suggests it is a pure liquid other than water, while possible in theory, chemistry questions usually use "102 °C" to demonstrate how impurities affect water.

Helpful Tips:

• The "Purity Rule": Pure substances have sharp, fixed boiling/melting points. Impure substances boil at higher temperatures and melt at lower temperatures.
• Range: Mixtures often boil over a range of temperatures (e.g., 102–105 °C) rather than one single point.

Why Answer A is correct: Pure water has a fixed boiling point of exactly 100 °C. When a substance (like salt or sugar) is dissolved in water, it becomes impure, which causes the boiling point to increase. Because 103 °C is higher than the standard boiling point of water, we can conclude the liquid is water containing impurities.

Why the others are wrong:

• Pure water: This is incorrect because pure water must boil at exactly 100 °C at standard pressure.
• Other pure liquids: While other liquids have different boiling points, 103 °C is a classic example used in chemistry problems to show "elevated" water temperature due to salt or minerals.

Helpful Tip: Remember the "Impurity Rule":

1. Impurities RAISE the boiling point (it boils at a higher temperature).
2. Impurities LOWER the melting point (it melts at a lower temperature).

Why it’s correct: Pure substances have unique, fixed boiling and melting points. At standard pressure, pure water boils at exactly 100 °C. If the liquid contained impurities (like salt), the boiling point would increase or occur over a range of temperatures rather than a single point.

Why other answers are wrong:

• Appearance (Colorless/Odorless): Many substances, like ethanol or dilute acid, are also clear and odorless.
• Chemical Tests: Tests like turning anhydrous copper(II) sulfate blue only prove that water is present. Even impure salt water would pass this test.
• pH Level: A liquid can be neutral (pH 7) but still contain dissolved solids or minerals.

Helpful Tip: In chemistry, physical constants (boiling/melting points) are the only way to prove purity. Chemical tests only prove the identity of a substance, not how pure it is!

CORRECT ANSWER: The boiling point of pure water is 100 °C.

EXPLANATION:

To determine if a substance is pure, you must check its physical properties, such as its boiling or melting point.

Why it’s correct: Pure substances have a fixed, sharp boiling point. Pure water boils at exactly 100 °C. Because this liquid boils at 103 °C, the boiling point proves the water is impure.

Why other answers are wrong:

• Cobalt(II) chloride is a chemical test. It only proves that water is present (it turns from blue to pink), but it cannot tell you if the water is pure or mixed with something else.
• Any statement suggesting that a boiling point of 103 °C is "normal" for pure water is incorrect; impurities always raise the boiling point.

Tip: Remember this rule: Chemical tests show presence; Physical tests show purity. Impurities always increase the boiling point and decrease the melting point!

Why A is correct: Every pure substance has a specific "fingerprint" boiling point. Pure water always boils at exactly 100 °C (at standard pressure). When impurities (like salt) are dissolved in water, they raise the boiling point. Since this liquid boils at 102 °C, it is an impure mixture, not pure water.

Why the others are wrong:

• Appearance: Just because a liquid is colorless doesn't mean it is pure. Many mixtures, like sugar water, look identical to pure water.
• Precision: In chemistry, "close" isn't good enough. Any temperature other than 100 °C proves the water contains other substances.

Helpful Tip: Pure substances melt and boil at one specific temperature. Impure substances (mixtures) usually boil at a higher temperature and melt at a lower temperature than the pure substance.

Why Answer A is Correct

1. Purity: Pure substances have specific, fixed boiling points. For example, pure water boils at exactly 100°C (at sea level). If the boiling point is higher, it indicates the solvent is not pure because dissolved solutes (like iron(II) sulfate) raise the boiling point.
2. Oxidation States: By definition, the sum of oxidation states for all atoms in a neutral compound (like $\text{FeSO}_4$) must always be zero.

Why Other Answers are Incorrect

• Any option suggesting the sum is not zero (e.g., +2 or -2) is incorrect because chemical formulas represent neutral molecules, not individual ions.
• Boiling point measurements cannot identify which specific chemical is dissolved; they only indicate whether the solvent is pure or contaminated.

Helpful Tip Always remember that "aqueous" means the solvent is water. A common pitfall is confusing the charge of a single ion (like $\text{Fe}^{2+}$) with the total sum of the entire compound.

To show that a substance is pure, we must look at its physical properties, such as its boiling point or melting point. Pure substances have a single, fixed boiling point. For water, this is exactly 100 °C at standard pressure. If the water were impure, it would boil over a range of temperatures or at a different temperature.

Why the other result is incorrect: The cobalt(II) chloride test is a chemical test used only to detect the presence of water. Cobalt(II) chloride turns from blue to pink if any water is present, even if that water is mixed with impurities (like salt or sugar). Therefore, it proves the liquid contains water, but it doesn't prove it is pure.

Helpful Tip: Always remember: Chemical tests (like cobalt chloride or copper sulfate) identify what a substance is, while physical tests (boiling/melting points) identify how pure it is.

Why A is Correct:

1. The Test: Anhydrous copper(II) sulfate turns from white to blue when it touches water. This detects the presence of water.
2. The Formula: "Iron(II)" means the iron ion has a $2+$ charge ($Fe^{2+}$). The phosphate ion has a $3-$ charge ($PO_4^{3-}$). To balance these charges to zero, you need three $Fe^{2+}$ ions ($+6$) and two $PO_4^{3-}$ ions ($-6$), resulting in $Fe_3(PO_4)_2$.

Why Others are Wrong:

• Purity: Chemical tests only show if water is present. To check for purity, you must measure physical properties like boiling point ($100^\circ\text{C}$).
• Incorrect Formulas: Formulas like $Fe_2(PO_4)_3$ would be for Iron(III), and $FePO_4$ doesn't balance the $2+$ and $3-$ charges correctly.

Student Tips:

• Roman Numerals: These tell you the charge of the metal, not how many atoms there are!
• Anhydrous: This means "without water." Only anhydrous salts can be used to test for its presence.

Why A is correct: Pure substances have fixed, sharp boiling and melting points. Pure water boils at exactly 100 °C (at standard atmospheric pressure). If any impurities are dissolved in the water, the boiling point will increase and occur over a range of temperatures rather than at one specific point.

Why other answers are wrong:

• Chemical tests (like turning anhydrous copper sulfate blue or cobalt chloride paper pink) only prove that water is present. They do not prove it is pure—salty sea water would still pass these tests!
• pH tests show that a liquid is neutral, but it could still contain dissolved impurities that don't affect pH.

Helpful Tip: Always remember: Physical properties (boiling/melting points) are used to check for purity. Chemical tests are only used to identify what a substance is. Look for the word "exactly" to spot a purity test!

Why A is correct: Pure water has a fixed, sharp boiling point of exactly 100 °C. When substances are dissolved in water (impurities), they cause the boiling point to increase. Because this sample boils at 103 °C, it is definitely water, but it is impure.

Why other conclusions are incorrect:

• "The sample is pure water": Incorrect because pure water cannot boil above 100 °C at standard pressure.
• "The sample is not water": Incorrect because the cobalt(II) chloride test turned pink, which confirms that water is present.
• "The cobalt test shows it is pure": Incorrect. Chemical tests like cobalt(II) chloride only prove that water exists in the sample; they do not measure purity.

Tips & Pitfalls:

• Physical tests (boiling/melting points) check for purity.
• Chemical tests (cobalt chloride) only check for the identity of the substance.
• Memory trick: Impurities raise the boiling point but lower the melting point!

To prove a substance is pure, you must check its physical properties, such as its boiling point or melting point. Pure substances have a fixed, "sharp" boiling point. Because pure water boils at exactly $100\text{ °C}$ (at standard pressure), this result confirms the liquid contains no impurities.

Why the other result is incorrect: Adding anhydrous cobalt(II) chloride is a chemical test used only to detect the presence of water. It turns pink if any water is present, even if that water is mixed with contaminants like salt or sugar. It identifies the substance as water but does not prove its purity.

Top Tip:

• Chemical tests (like cobalt chloride) identify WHAT a substance is.
• Physical tests (like boiling point) identify how PURE a substance is.
• Common Pitfall: Remember that impurities usually raise the boiling point and lower the melting point of a substance!

EXPLANATION:

Why A is correct: Pure substances have specific, fixed "fingerprints" called melting and boiling points. Pure water boils at exactly 100 °C (at standard pressure). If any impurities (like salt or minerals) are mixed in, the boiling point will rise. Measuring this precise temperature is the only way to confirm the liquid is 100% pure.

Why other answers are wrong: Common chemical tests—such as using anhydrous copper(II) sulfate (which turns blue) or cobalt chloride paper (which turns pink)—only prove that water is present. They cannot tell if the water is pure; even salt water or muddy water would give a positive result for those tests!

Top Tip: To check for purity, always look for fixed physical constants like boiling or melting points. Chemical tests only identify what a substance is, not how clean it is.

EXPLANATION:

Why the correct answer is correct: In chemistry, the best way to prove a substance is pure is to measure its physical constants. Pure substances have a "sharp" or fixed boiling point. Pure water will boil at exactly 100 °C at standard pressure. If the liquid contains impurities (like salt), the boiling point will increase or happen over a range of temperatures.

Why other answers are incorrect: Chemical tests—like turning anhydrous copper(II) sulfate blue—only prove that water is present. They do not prove it is pure; even salty or dirty water would pass these tests. Similarly, being colorless or having a pH of 7 does not prove purity, as many mixtures share these traits.

Tips & Pitfalls:

• Tip: Always look for boiling or melting points to confirm purity.
• Pitfall: Don’t be fooled by "clear" or "colorless." Saltwater is clear and colorless, but it is a mixture, not pure water!

EXPLANATION:

Pure substances have a specific, fixed boiling point. For pure water, this is exactly 100 °C (at standard pressure).

Why A is correct: In chemistry, purity is defined by how closely a substance matches its known boiling or melting point. Because 103 °C is higher than 100 °C, the liquid cannot be pure water. Impurities increase the boiling point because they interfere with the liquid's ability to turn into gas.

Why other options are incorrect:

• Any claim that the liquid is "pure" is wrong; even a small deviation (like 101 °C) indicates it is a mixture.
• Thinking 103 °C is "close enough" to be pure is a common mistake. In science, pure substances must hit their exact target temperature.

Top Tip:

• Pure substances boil at a single, fixed temperature.
• Impure substances (mixtures) boil over a range of temperatures and usually at a higher boiling point than the pure version.

To identify the liquid as impure water, we need two types of evidence:

1. Why A is correct:

• The Chemical Test: Anhydrous cobalt(II) chloride paper is a standard test for water. It turns from blue to pink when water is present. This confirms the liquid is water.
• The Purity Test: Pure substances have specific, fixed boiling points. Pure water boils at exactly 100 °C. The presence of impurities increases the boiling point. Therefore, a boiling point of 102 °C proves the water is impure.

2. Why other options are incorrect:

• Any option stating the liquid boils at 100 °C describes pure water, which contradicts the question.
• If an option says the paper stays blue, it means no water was detected.

💡 Helpful Tip: Remember the difference: Chemical tests (like cobalt chloride) tell you what the substance is. Physical tests (like boiling point) tell you how pure it is!

Why Answer A is correct:

1. The Chemical Test: Blue anhydrous cobalt(II) chloride turning pink is a specific test for the presence of water. This confirms the liquid contains water.
2. The Physical Test: Pure water has a fixed boiling point of exactly 100 °C. Since this liquid boils at 103 °C, it must contain impurities. Dissolved impurities always raise the boiling point of a liquid.

Why other options are wrong:

• It is not pure water because the boiling point is higher than 100 °C.
• It is not a different liquid (like ethanol) because other liquids will not turn anhydrous cobalt(II) chloride pink.

Helpful Tip: Always separate identity from purity:

• Chemical tests (like cobalt paper) tell you what the substance is.
• Physical tests (boiling/melting points) tell you how pure it is. Pure substances have sharp, exact boiling points!

The correct answer is A because iron is a transition metal, meaning it can form ions with different charges (such as $+2$ or $+3$). The Roman numeral (II) specifically tells us that in this compound, the iron ion has an oxidation state (charge) of +2.

Why other answers are incorrect:

• The "(II)" does not mean there are two iron atoms in the formula (that would be shown by a subscript, like $\text{Fe}_2$).
• It does not refer to the charge of the sulfate ion or the whole compound (which is always neutral).

Helpful Tips:

• Roman Numerals = Charge: Always associate the Roman numeral with the positive charge of the metal ion, not the number of atoms.
• Check the Partner: Since sulfate ($\text{SO}_4$) always has a $-2$ charge, the single iron atom must be $+2$ to balance it out!

EXPLANATION:

Why the correct answer (A) is correct:

• Pure Water: Pure substances have specific, fixed boiling points. Pure water boils at exactly 100 °C (at standard pressure). If it boils at any other temperature, it is impure.
• Oxidation State: In the name iron(II) phosphate, the Roman numeral (II) directly indicates that the iron ion has an oxidation state of +2. You can also calculate this: since the phosphate ion ($PO_4^{3-}$) has a -3 charge, three $Fe^{2+}$ ions (+6 total) are needed to balance two $PO_4^{3-}$ ions (-6 total).

Why the other options are wrong:

• Options suggesting a boiling point other than 100 °C describe impure water.
• Options suggesting oxidation states like +3 or +8 are incorrect for the formula $Fe_3(PO_4)_2$.

Tips & Pitfalls:

• The Shortcut: Always check the Roman numeral in a compound's name—it usually tells you the oxidation state of the metal!
• Purity: Remember that "pure" means fixed physical properties (exact boiling/melting points).

Why A is correct: Every pure substance has a specific, fixed boiling point (like a fingerprint). Pure water boils at exactly 100 °C. Because this liquid boils at 102 °C, it must contain dissolved substances (impurities). Impurities typically increase the boiling point of a liquid.

Why other options are incorrect: Any claim that the liquid is "pure" is incorrect because pure substances do not deviate from their fixed boiling point under standard conditions. If it were pure water, the measurement would be exactly 100 °C.

Student Tip:

• Impurities always "spread out" the physical properties: they increase boiling points and decrease (lower) melting points.
• If you see a boiling range (e.g., 102 °C–105 °C) instead of a single number, that is another major sign that the substance is impure.

Why it’s correct: Pure substances have fixed, sharp boiling and melting points. At standard pressure, pure water will always boil at exactly 100 °C. If impurities (like salt) are present, the boiling point will increase and the water will boil over a range of temperatures rather than at one specific point.

Why the others are incorrect:

• Chemical tests (like using anhydrous copper(II) sulfate or cobalt chloride paper) only confirm that water is present. Even sea water or soapy water would give a positive result for these tests.
• pH tests only show if the water is neutral. Many impurities can be dissolved in water without changing its pH.

Top Tip: To remember the difference, think: Chemical tests confirm identity (what the substance is), while Physical tests (boiling/melting points) confirm purity (how clean it is).

EXPLANATION:

Why the correct answer is A: Pure substances have fixed, exact boiling and melting points. At standard pressure, pure water will always boil at exactly 100 °C. If the liquid boils at a different temperature (like 102 °C), it contains impurities.

Why the other answers are incorrect:

• Chemical tests (like turning cobalt chloride paper pink or anhydrous copper(II) sulfate blue) only prove that water is present. They cannot tell if the water is pure; even salt water or muddy water would give the same result!
• pH tests showing a value of 7 only mean the liquid is neutral. Many mixtures, like sugar water, are neutral but are not pure water.

Helpful Tip: Remember: Chemical tests (color changes) show what is there. Physical tests (boiling/melting points) show how pure it is. Impurities usually raise the boiling point and lower the melting point.

Why the correct answer is A: Pure substances have a specific, fixed boiling point. For pure water at standard pressure, this is exactly 100 °C. When impurities (like salt or sugar) are dissolved in water, they interfere with the liquid molecules, making it harder for them to turn into gas. This causes boiling point elevation. Because the sample boils at 102 °C (higher than 100 °C), it must contain impurities.

Why other answers are incorrect:

• "The water is pure": This is incorrect because pure water will not boil above 100 °C under normal conditions.
• "The thermometer is broken": In science questions, we assume the equipment is working unless stated otherwise. The temperature change is a classic sign of impurity.
• "The water is boiling faster": Turning up the heat makes water boil more vigorously, but the temperature stays at its boiling point until all the liquid has evaporated.

Top Tip: Impurities always increase the boiling point and decrease the melting point. Think of how we put salt on icy roads to melt the ice!

Why A is correct: Pure water has a unique "chemical fingerprint"—at standard pressure, it always boils at exactly 100 °C. Because this liquid boils at 103 °C, it cannot be pure. The presence of dissolved substances (impurities) causes boiling point elevation, meaning the liquid must get hotter than 100 °C before it can turn into gas.

Why other answers are wrong: Any choice suggesting the liquid is "pure water" is incorrect because pure substances have fixed, sharp boiling points. If the temperature is not exactly 100 °C, the substance is either impure or a different liquid entirely.

Tips & Pitfalls:

• The "Purity Rule": Pure substances have fixed boiling/melting points. Mixtures boil over a range of temperatures or at different temperatures than the pure version.
• Pitfall: Don't be fooled by the word "colorless." Many impurities (like salt or sugar) are invisible once dissolved but still change the boiling point!

Why the correct answer is correct:

• Pure Water: Pure substances have a single, fixed boiling point. At standard pressure, pure water boils at exactly 100 °C.
• Iron(II) Sulfate: The "(II)" tells you the Iron ion has a 2+ charge ($\text{Fe}^{2+}$). The sulfate ion always has a 2– charge ($\text{SO}_4^{2-}$). To make a neutral compound, one $\text{Fe}^{2+}$ balances one $\text{SO}_4^{2-}$, resulting in $\text{FeSO}_4$.

Why the others are wrong:

• Boiling Point: If a liquid boils over a range (e.g., 98–102 °C), it is an impure mixture, not pure water.
• Chemical Formula: Formulas like $\text{Fe}_2\text{SO}_4$ or $\text{Fe}(\text{SO}_4)_2$ are incorrect because the positive and negative charges do not balance to zero.

💡 Tip: Roman numerals like (II) or (III) show the charge of the metal ion, not the number of atoms in the formula! Always "swap and drop" or balance charges to find the right formula.

Why the Correct Answer is Correct The Roman numeral (II) in iron(II) oxide tells you the iron ion has a charge of 2+. Since the oxide ion always has a charge of 2-, they balance perfectly in a 1:1 ratio, giving the formula FeO. We measure the melting point because pure substances have a specific, sharp melting point. If the sample melts at a lower temperature or over a range, it contains impurities.

Why the Other Answers are Incorrect

• $Fe_2O_3$: This is iron(III) oxide, where iron has a 3+ charge.
• Identify the element: Iron(II) oxide is a compound, not an element.
• Check magnetism: Melting a substance doesn’t test its magnetic properties; you would use a magnet for that!

Helpful Tip Always remember: The Roman numeral tells you the charge of the metal, not the number of atoms in the formula!

Why A is correct:

1. Purity: Pure substances have fixed, "sharp" boiling points. Pure water always boils at exactly 100°C at standard pressure.
2. Oxidation State: The Roman numeral in the name tells you the oxidation state directly. Iron(II) sulfate means the iron ion has an oxidation state of +2.

Why other options are wrong:

• Impurity: If the water contained impurities, it would boil at a higher temperature (above 100°C) or over a range of temperatures.
• Incorrect State: Any option suggesting an oxidation state of +3 is wrong because that would be iron(III) sulfate, a different compound.

Top Tips:

• The "Roman Numeral" Rule: Always look at the brackets! They are a shortcut to finding the metal's charge/oxidation state.
• Pure = Precise: If a question asks for proof of purity, look for a single, exact temperature rather than a range.

EXPLANATION:

The Roman numeral (II) specifically indicates the oxidation state (or charge) of the iron ion. Because iron is a transition metal, it can form ions with different charges (such as $+2$ or $+3$). The (II) tells you that in this specific compound, each iron atom has lost two electrons to become an $Fe^{2+}$ ion.

Why other options are wrong:

• It does not mean there are two iron atoms; the number of atoms is shown by small subscript numbers (like the $_2$ in $H_2O$).
• It is not the atomic number; iron’s atomic number is always 26, regardless of its charge.
• It does not describe the sulfate part of the name.

💡 Helpful Tip: Roman numerals are only used for metals that can form multiple different ions. Always remember: The numeral = the positive charge.

EXPLANATION:

Pure substances have fixed, exact physical properties. Pure water always boils at exactly 100 °C (at standard atmospheric pressure). Because this liquid boils at 103 °C, it must be impure. Dissolved impurities (like salt) act like "anchors," making it harder for water molecules to escape as steam, which raises the boiling point.

Why other answers are wrong:

• Any claim that the liquid is "pure" is incorrect because pure substances do not boil over a range or at temperatures different from their fixed constant.
• If a liquid's boiling point doesn't match the known standard (100 °C), it cannot be pure water.

💡 Helpful Tips:

• Boiling vs. Melting: Impurities increase the boiling point but decrease the melting point (e.g., putting salt on icy roads).
• Common Pitfall: Always check the pressure! While altitude can change boiling points, in most chemistry questions, we assume standard pressure where pure water is exactly 100 °C.

EXPLANATION:

Why Answer A is Correct: Anhydrous cobalt(II) chloride is a specific chemical test used to detect the presence of water. In its "anhydrous" (dry) state, it is blue. When it comes into contact with water, it becomes "hydrated" and changes color to pink. This color change confirms that water is present in the liquid.

Why other answers are wrong: The most common mistake is thinking the liquid is pure water. However, pure water must boil at exactly 100 °C. Because this liquid boils at 102 °C, it is an impure mixture (water with something else dissolved in it). The cobalt chloride test only proves water is there, not that the liquid is pure.

Helpful Tip: Always distinguish between a chemical test (like cobalt chloride), which identifies what a substance is, and a physical test (like boiling point), which identifies if it is pure.

Why the correct answer is correct: Pure substances have a fixed, sharp boiling point. If a liquid is 100% pure water, it will boil at exactly 100 °C and the temperature will remain constant until all the liquid has evaporated. If impurities were present, the liquid would boil over a range of temperatures (e.g., starting at 101 °C and rising).

Why the other options are incorrect:

• Chemical tests (like turning anhydrous copper(II) sulfate blue) only prove that water is present. They cannot tell if the water is pure or mixed with something else.
• Physical properties like being colorless or odorless are not unique to water; many clear liquids (like ethanol) look the same but are not water.

Tip: Always remember the difference between identity and purity. Chemical tests tell you what it is; sharp melting/boiling points tell you how pure it is.

Why Answer A is correct: This question tests two things: the chemical test for water and the effect of impurities.

• The Chemical Test: Cobalt(II) chloride paper turns from blue to pink only when water is present. This confirms the liquid contains water.
• The Boiling Point: Pure water boils at exactly 100 °C. Because this liquid boils at 102 °C, it must contain impurities (like salt), which raise the boiling point.

Why other answers are incorrect:

• If the liquid were pure water, it would boil at exactly 100 °C.
• If the liquid were a different substance entirely (not water), the paper would have stayed blue.

💡 Helpful Tip: Always use two tests to identify a substance! A chemical test (like the paper) tells you what it is, but a physical test (boiling point) tells you if it is pure. Impurities increase the boiling point but decrease the melting point.

The Correct Answer is A because turning blue anhydrous cobalt(II) chloride paper pink is the specific chemical test used to confirm the presence of water.

Why the others are wrong:

• "It is pure water": This is incorrect because pure water has a fixed boiling point of exactly 100 °C. Since this liquid boils at 103 °C, it contains impurities (like salt or sugar) that have raised the boiling point.
• "It is not water": This is incorrect because the color change of the cobalt(II) chloride paper proves that water is present.

Helpful Tips:

• Test for Presence vs. Purity: Cobalt(II) chloride paper only tells you if water is there. To check if water is pure, you must check if it boils exactly at 100 °C or freezes exactly at 0 °C.
• Impurity Rule: Impurities always raise the boiling point and lower the melting point of a substance.

EXPLANATION:

Why it’s correct: Pure substances have fixed, specific boiling points. Pure water boils at exactly 100 °C (at standard pressure). When impurities (like salt or minerals) are added to water, they raise the boiling point. Because this sample boils at 102 °C, it is no longer pure.

Why other answers are wrong:

• Any claim that the water is "pure" is incorrect because pure water does not boil at 102 °C.
• If the atmospheric pressure were lower (like on a mountain), the boiling point would decrease below 100 °C, not increase.

Helpful Tip: Impurities "stretch" the temperature range where a substance stays liquid. They raise the boiling point and lower the melting point. If you see water boiling above 100 °C or melting below 0 °C, it is a sign the sample is impure.

Why the correct answer is A: There are two clues in this question:

1. The Chemical Test: Anhydrous cobalt(II) chloride turns from blue to pink only when it reacts with water. This confirms water is present.
2. The Boiling Point: Pure water boils at exactly 100 °C. Because this liquid boils at 102 °C, it must contain impurities (like salt or sugar), which raise the boiling point.

Why other answers are incorrect:

• It cannot be pure water because the boiling point is not 100 °C.
• It cannot be another liquid (like ethanol) because only water triggers that specific color change in cobalt chloride.

Tips & Pitfalls:

• Presence vs. Purity: Chemical tests (like cobalt chloride) tell you if water is present. Physical properties (like boiling point) tell you if that water is pure.
• Common Pitfall: Students often forget that impurities increase the boiling point and decrease the melting point.

Explanation:

Pure substances have a specific, fixed "fingerprint" boiling point. At standard pressure, pure water always boils at exactly 100 °C. Because this liquid boils at 103 °C, it contains dissolved substances (impurities) that have raised its boiling point. Therefore, it is impure water.

Why other options are wrong:

• "It is pure water": Incorrect, because pure water cannot boil at 103 °C under normal conditions.
• "The volume changed the boiling point": Incorrect. Boiling point is an intensive property, meaning it stays the same whether you have a teaspoon or a gallon of the liquid.

Helpful Tip: Impurities act like "anchors" that make it harder for liquid molecules to escape into the air. This increases the boiling point and decreases the melting point. If a substance doesn't hit its exact "textbook" temperature, it isn’t pure!

Why A is Correct: To find sulfur’s oxidation state in $\text{FeSO}_4$, set the total charge to zero:

• $\text{Fe}$ is $+2$ (given as iron(II))
• $\text{O}$ is $-2$ (standard)
• Calculation: $(+2) + \text{S} + 4(-2) = 0 \rightarrow 2 + \text{S} - 8 = 0$.
• Solving for $\text{S}$ gives +6.

For the water test: Anhydrous (dry) copper(II) sulfate is white. It turns blue when it touches water, confirming water is present.

Why Others Are Wrong:

• Incorrect Oxidation States: Values like $+4$ belong to sulfur in sulfite ($\text{SO}_3^{2-}$), not sulfate ($\text{SO}_4^{2-}$).
• Testing Purity: This chemical test only shows if water exists. It cannot prove water is 100% pure (only a boiling point of $100^\circ\text{C}$ proves purity).

Tips & Pitfalls:

• "Anhydrous" means "without water."
• Pitfall: Don't confuse presence (is it there?) with purity (is it the only thing there?).

EXPLANATION:

Why it’s correct: Iron(II) indicates an iron ion with a 2+ charge (Fe²⁺). The phosphate ion is PO₄³⁻. To make the compound neutral, the charges must balance to zero. By using three Fe²⁺ ions (+6) and two PO₄³⁻ ions (-6), we get the formula Fe₃(PO₄)₂.

Regarding purity, pure substances have fixed, sharp boiling and melting points. Pure water always boils at exactly 100 °C (at standard pressure).

Why wrong answers are incorrect:

• Formulas like Fe₂(PO₄)₃ are incorrect because they use iron(III) instead of iron(II).
• Observations like "colorless" or "pH 7" do not prove purity; only a precise boiling or melting point confirms a substance is 100% pure.

Helpful Tip: Use the "Swap and Drop" method! Take the number from the iron charge (2) and drop it behind the phosphate, and take the phosphate charge (3) and drop it behind the iron. Don’t forget the brackets for the (PO₄)!

Why the correct answer is correct: Iron(II) indicates the iron ion has a $2+$ charge ($Fe^{2+}$). The sulfate ion ($SO_4^{2-}$) has a $2-$ charge. Since the charges are equal and opposite, they cancel out in a 1:1 ratio, giving the formula $FeSO_4$. Regarding purity, pure water boils at exactly 100 °C. A boiling point higher than this (103 °C) proves the water contains dissolved impurities, which raise the boiling point.

Why other options are wrong:

• $Fe_2SO_4$ or $Fe(SO_4)_2$: These formulas are incorrect because the charges of the iron and sulfate ions are not balanced.
• "Pure water": Any option claiming 103 °C indicates pure water is wrong; pure substances have fixed, exact boiling points.

Helpful Tip: The Roman numeral (II) tells you the charge of the metal ion ($+2$). Also, remember: Impurities increase boiling points but decrease melting points.

Why A is correct: The liquid must contain water because anhydrous cobalt(II) chloride paper specifically turns from blue to pink when it touches water. However, the liquid is impure because pure water boils at exactly 100 °C. Since this sample boils at 102 °C, the presence of impurities has raised the boiling point.

Why the others are wrong:

• It is not pure water because its boiling point is not exactly 100 °C.
• It cannot be a liquid other than water because those liquids would not change the color of the cobalt paper.

Helpful Tip: Always distinguish between identification and purity. Use a chemical test (like cobalt chloride paper) to identify what a substance is, but use physical properties (like boiling point or melting point) to check how pure it is. Pure substances have sharp, exact boiling points!

The Correct Answer: A (The liquid is impure water)

Why it’s correct: This conclusion is based on two pieces of evidence:

1. The Chemical Test: Anhydrous cobalt(II) chloride turns from blue to pink only when water is present. This confirms the liquid contains water.
2. The Boiling Point: Pure water boils at exactly 100 °C. Because this liquid boils at 103 °C, it must be impure. Dissolved impurities (like salt) always raise the boiling point of a liquid.

Why other answers are wrong:

• It cannot be pure water because the boiling point is higher than 100 °C.
• It cannot be a different liquid (like ethanol) because other liquids would not turn the cobalt(II) chloride pink.

Top Tip: Pure substances have a fixed, sharp boiling point. If a liquid boils at a temperature different from its known standard, it is almost always impure!