Chemistry Topic Quiz

The trick here is that no single observation tells you the state on its own - you have to combine all three. "Flows and takes the shape of the container" is true for both gases and liquids, so it alone does not prove gas. The deciding clues are the fixed volume and the fact that it cannot be compressed: both mean the particles are packed closely together. That eliminates a gas, whose particles are spread far apart. But because it still flows and has no shape of its own, it is not a solid either. Close-packed particles that can move past each other describe a liquid. Remember to think about all three particle features - separation, arrangement, and motion - rather than jumping at the first matching property.

On a flat region the heat supplied overcomes the forces between particles so the solid melts, instead of raising the temperature. Heating has not paused; this is melting not boiling; and a constant reading means the average particle energy is constant, not secretly rising.

Don't decide the state from one property alone — use both clues together. "Takes the shape of the container" rules out a solid (whose particles can only vibrate in place), and "volume barely changes when squeezed" rules out a gas (whose particles are far apart and easily compressed). That leaves a liquid: particles are close together (almost incompressible) but disordered, so they can slide past one another and let the substance flow. A common slip is saying a liquid has a "regular arrangement" — that belongs to solids. In a liquid the particles are close-packed but randomly arranged.

Use two clues together: does the substance keep its own shape, and does its volume stay fixed? A solid keeps both its shape and its volume, a liquid keeps its volume but flows to take the container's shape, and a gas spreads out so it has neither a fixed shape nor a fixed volume. The key difference between a liquid and a gas is volume, since a liquid's volume stays the same while a gas fills whatever space it is given.

Diffusion happens because particles move randomly on their own, spreading from where they are crowded to where they are spread out. There is no attractive force pulling them; dissolved particles are not stationary; and random motion, not convection currents, spreads colour even in still water.

Rate $\propto 1/\sqrt{M_r}$, so the heaviest ($M_r=64$) is slowest and the lightest ($M_r=16$) is fastest. Flipping this to make the lightest slowest is wrong, and at the same temperature mass sets the speed rather than all gases spreading equally.

The key is to use all three clues together rather than just one. A fixed boiling point that stays constant while boiling tells you X is pure, so it is not a mixture (mixtures boil over a temperature range and can be separated by physical methods). But X is not an element either, because it breaks down into two different elements, hydrogen and oxygen. The fact that these always appear in the same fixed ratio shows the elements are chemically combined in set proportions. A pure substance of two or more elements chemically combined in a fixed ratio is the definition of a compound. The common trap is to call X a mixture just because it can be split into two elements, but the splitting here needs a chemical change (electricity), not a physical separation, which is exactly what separates a compound from a mixture.

Stirring combines the powders physically, so it is a mixture; the magnet removing the iron shows no bonding. It is not a compound because compounds bond in a fixed ratio, and not an element because two different elements are present.

Protons and neutrons sit in the nucleus while electrons occupy shells, and a neutral atom has equal protons and electrons (23 each). Putting electrons in the nucleus reverses the locations, 51 uses the mass number as protons, and 46 wrongly adds protons and electrons together.

The proton number (atomic number) counts only the protons in the nucleus, while the mass number counts protons and neutrons together. So to find the protons, take the mass number and subtract the neutrons: $39 - 20 = 19$. Be careful not to use the mass number ($39$) as the proton number, and not to use the neutron count ($20$) by mistake. Adding the two figures together has no physical meaning here.

Isotopes share the same proton number but differ in neutrons, so the two atoms with 17 protons each qualify. Matching by equal neutron count or equal mass number is a trap, and atoms with different proton numbers are different elements.

Isotopes react alike because they have the same electrons and electronic configuration, and reactions depend on electrons. Neutron numbers actually differ and play no part, mass numbers differ too, and protons are locked in the nucleus rather than forming bonds.

The metal loses 2 electrons to form a positive ion and the non-metal gains 2 to form a negative ion. Swapping which atom gains or loses is wrong; losing electrons gives a positive (not negative) charge; and a metal reaches a full shell by losing electrons, not gaining them.

The key is that two things must be explained at once: why the melting point is so high, and why conductivity switches on only after melting. A high melting point means strong forces hold the whole structure together, so you need a giant lattice, not separate molecules. The conductivity clue is the classic ionic test: ions can only carry current when they are free to move. In a solid they are trapped in their fixed positions in the lattice, so it does not conduct; once melted, the ions break free and the liquid conducts. Be careful not to grab the "free electrons" idea here, that belongs to metals and would make the solid conduct too. And remember a giant covalent solid contains no ions, so melting it cannot suddenly create charge carriers.

The trick here is to read the clues like a detective rather than recall a definition. The 801 °C melting point tells you the particles are held together very strongly across a giant structure. The conductivity test is the giveaway for ionic: it conducts when molten or dissolved but not when solid. That happens because conduction needs charged particles that can move. In a solid ionic compound the ions are clamped in place in the lattice, so nothing moves and it cannot conduct. Once you melt it or dissolve it in water, the lattice breaks up and the ions are free to drift and carry the current. A metal would already conduct as a solid (its electrons move even in the solid), so that rules metallic out. Small molecules have weak forces (low melting point) and no charge to carry, and giant covalent structures have no free charged particles at all. Only oppositely charged ions held by strong electrostatic attraction explain every line of the data.

Each lithium gives 1 electron and oxygen needs 2, so two lithium atoms per oxygen give two $\text{Li}^+$ and one $\text{O}^{2-}$. One lithium leaves oxygen short, sharing describes covalent bonding, and the metal must lose electrons while the non-metal gains them.

Start from how many outer electrons fluorine already has (7) and how many it needs to copy a noble gas (8 like neon). It only needs one more, so it shares a single pair with the other atom - that one shared pair is the covalent bond. The atom puts one electron into that bond, so its remaining 6 electrons are non-bonding. Six electrons make three pairs, not six, so each fluorine ends up with 1 bonding pair and 3 lone pairs. Check it adds up: 2 shared electrons in the bond plus 6 in the lone pairs equals 8, the full outer shell.

Each nitrogen has 5 outer electrons and needs 3 more, so the atoms share three pairs as a triple bond. One pair leaves them short, a double bond fits oxygen, and the bond is the shared pairs themselves, not forces between molecules.

Nitrogen uses 3 of its 5 outer electrons in bonds, leaving one lone pair to be drawn. Leaving it off ignores those electrons, a fourth bond copies methane, and electrons are shared not transferred in covalent ammonia.

Diamond is hard because every atom is locked in a rigid 3D network of strong covalent bonds, while graphite has layers held by weak forces that slide. Swapping these is the trap, and hardness comes from the bonded network, not from free electrons.

Silicon(IV) oxide is a giant covalent structure where melting breaks many strong covalent bonds, giving a very high melting point. Calling it ionic or metallic is the wrong reason, and treating it as simple molecules would imply weak forces and a low melting point.

In a metal it is the delocalised ('free') electrons that carry the current, not the ions, which stay locked in place. The key is how many of these free electrons each atom gives up. Sodium is in Group I, so each atom donates 1 electron (forming $Na^{+}$). Magnesium is in Group II, so each atom donates 2 electrons (forming $Mg^{2+}$). More electrons in the 'sea' means more charge carriers, so magnesium conducts better. Don't be tricked into thinking the metal ions move, that the bonding is covalent, or that closer packing alone explains it.

Delocalised electrons carry charge, and layers of positive ions slide while metallic bonding still acts. Metals have metallic bonds not covalent ones, the light electrons move rather than heavy ions, and a metal is a giant lattice not separate molecules.

Nitrogen gas is diatomic ($N_2$), the 2- carbonate ion balances 2+ calcium one-to-one as $CaCO_3$, and ammonia is the neutral $NH_3$, not the $NH_4^+$ ammonium ion.

The empirical formula is the simplest whole-number ratio, so 6:12 reduces to 1:2, giving $CH_2$. The molecular formula is unsimplified, dividing by only 2 stops short, and the larger count belongs to hydrogen.

The empirical formula tells you the simplest whole-number ratio of atoms, but the molecular formula tells you the actual number of each atom in one molecule. To get from one to the other, work out the mass of the empirical unit ($CH_2 = 12 + 2 = 14$), then see how many of those units fit into the relative molecular mass: $42 \div 14 = 3$. So the real molecule is three $CH_2$ units, giving $C_3H_6$. Always finish by checking the masses add back up to the $M_r$ you were given.

Charges must balance, so two $Al^{3+}$ (6+) pair with three $O^{2-}$ (6-) to give $Al_2O_3$. A one-to-one ratio leaves charge unbalanced, swapping the charges onto the wrong ions gives the reverse subscripts, and three-to-six is not reduced to its simplest ratio.

Relative atomic mass is a weighted average, so $(35 \times 0.75) + (37 \times 0.25) = 35.5$. A simple halfway average ignores the abundances, the heavier isotope's mass alone ignores both, and swapping the abundances gives 36.5.

The subscript outside the bracket multiplies both atoms inside, so $40 + 2(16) + 2(1) = 74$. Doubling only the oxygen undercounts the hydrogen, and doubling the calcium too over-counts an atom outside the bracket.

Concentration in g/dm³ tells you how many grams of solute would be present in one full cubic decimetre (1000 cm³) of solution. Here only 250 cm³ holds the 16.0 g, which is a quarter of a dm³, so one whole dm³ would contain four times as much: 64.0 g. The trick is to convert the volume to dm³ first by dividing by 1000, then divide the mass by that volume. Dividing the mass straight by 250 forgets the unit conversion, and multiplying by 0.250 reverses the operation. Turning the mass into moles gives a number you would use for mol/dm³, not for the g/dm³ asked here.

Half a mole holds half of $6.02 \times 10^{23}$, which is $3.01 \times 10^{23}$ molecules. Quoting the constant ignores the 0.50 mol, dividing wrongly makes a part-mole bigger than a mole, and multiplying by three counts atoms, not molecules.

$3.6 \div 18 = 0.20$ mol of water, and with two hydrogens each that is 0.40 mol, giving $2.408 \times 10^{23}$ atoms. Counting molecules forgets the two hydrogens, multiplying by three adds the oxygen, and using 3.6 as moles skips the mass-to-moles step.

Two moles of $Al$ give three of $H_2$, so 0.20 mol gives 0.30 mol, and $0.30 \times 24 = 7.2$ dm³. A one-to-one ratio omits the extra hydrogen, a one-to-three ratio adds too much, and the molar gas volume is 24, not 36.

Electrolysis only happens when there are charged particles (ions) that are free to MOVE. Two things must both be true: the substance is an ionic compound, AND it is molten or dissolved so the ions can travel to the electrodes. The molten ionic substance W ticks both boxes. The non-ionic ones have no ions to discharge, and the solid ionic one has its ions trapped in a fixed lattice, so neither can be electrolysed in the state shown.

Electrons carry charge through the metal wires and ions carry it through the electrolyte, never swapping. The electrolyte has no free electrons, solid wires have no free ions, and reversing the two combines both errors.

The cathode is negative and attracts positive metal ions, so the magnesium electrode is the negative cathode and the chlorine electrode is the positive anode. Naming them the wrong way swaps the electrodes, reversing the charges is also wrong, and the electrolyte is the molten compound, not the metal product.

With inert graphite the anode gives off oxygen, while a copper anode dissolves and loses mass as copper plates onto the cathode. An electrode name is not an observation, copper deposits on the cathode not the anode, and a sulfate solution gives no chlorine and no hydrogen at the anode.

The key is to track where each ion goes and which one is actually discharged. Cations (positive ions) head to the cathode; anions (negative ions) head to the anode. Sodium is too reactive to be deposited from solution, so hydrogen is released at the cathode instead, giving the squeaky pop. Because the solution is concentrated in chloride, chlorine (not oxygen) is given off at the anode, which is why it bleaches the damp litmus. Naming the actual products, rather than just saying "cathode" or "anode", is what the question is checking.

In a molten binary salt the only ions present are the metal cation and the non-metal anion, so each electrode discharges just one of them. Positive metal ions are attracted to the negative cathode where they gain electrons to form the metal, while negative bromide ions move to the positive anode where they lose electrons to form $Br_2$. Remember opposite charges attract, so the cathode collects the cation and the anode collects the anion.

Petrol is a hydrocarbon, so burning it releases carbon dioxide and water. A fuel cell instead reacts hydrogen with oxygen, and hydrogen has no carbon at all, so no carbon dioxide is possible. Hydrogen and oxygen are the fuels going in, not the products coming out, so they are used up rather than released. That leaves water (as vapour) as the only thing in the exhaust, which is why fuel cells are considered cleaner for local air quality.

At the point of use the cell gives out only water, unlike a petrol engine emitting carbon dioxide and other pollutants. Hydrogen is actually harder to store than liquid petrol and is currently costly with few refuelling points, and the cell has no carbon so makes no carbon dioxide to capture.

A rise in temperature means heat flows out of the chemicals into the surroundings, so the reaction is exothermic. Calling it endothermic contradicts the warming, saying heat is taken in reverses the energy flow, and blaming stirring ignores the reaction as the heat source.

Burning methane releases heat, so the reaction is exothermic and $\Delta H$ is negative. A positive sign belongs to endothermic reactions that take heat in, and the value is not zero because there is a net release of energy to the surroundings.

A falling temperature means the surroundings lose heat to the dissolving, so the process is endothermic. Calling it exothermic contradicts the cooling, saying heat is released reverses the energy flow, and blaming cold tap water ignores that the dissolving itself draws energy in.

Think of activation energy as a fixed "energy hurdle" that belongs to the reaction itself, not to the conditions. Heating the gas does not lower the hurdle. What heating does is give the particles more energy on average and spread their energies out, so more of them can clear the hurdle when they collide. That is why the correct choice keeps $E_a$ the same but increases the proportion of particles with energy $\geq E_a$. A common trap is thinking heat only makes particles collide more often, but it also makes a bigger fraction of those collisions energetic enough to react. The only way to actually lower $E_a$ is to add a catalyst.

To classify a reaction from its profile, compare only the start (reactants) and end (products) energy levels, not the height of the central hump. Here the products sit lower than the reactants, so the surplus energy is released to the surroundings, which makes the reaction exothermic. The hump simply shows the activation energy, $E_a$, that must be supplied before the reaction can proceed.

A chemical change makes a new substance, shown when magnesium and acid fizz off hydrogen and the metal is used up. Melting wax and crushing sugar only change state or size, and dissolving salt just spreads it through water and lets it be recovered by evaporation, so each stays physical.

A chemical change makes a new substance, and the best way to be sure is to look for several signs at once that you cannot explain by just mixing or dissolving. A solid that forms and will not dissolve again is a new product, and a gas bubbling off is another new product, so seeing both together (plus the mixture warming up) is convincing. Be careful with single clues: warming up, going colder, a colour change, or a few bubbles can all happen in physical changes too. The biggest giveaway of a physical change is that you can get the starting substance back, like recovering it by evaporating the water.

Crushing spreads the same mass over a bigger surface, so acid particles collide with the solid more often and the rate rises. The substance is not more reactive, the acid's concentration is unchanged, and equal mass spread thinly reacts faster, not at the same rate.

At constant temperature the particles do not speed up, but crowding them into a smaller volume gives more particles per unit volume and more collisions each second. Kinetic energy is unchanged at fixed temperature, only a catalyst lowers activation energy, and the same gas is squeezed, not added to.

A catalyst speeds a reaction and is recovered unchanged, so its mass stays the same. It is not used up or made heavier because it is regenerated each cycle rather than acting as a reactant, and it increases the rate rather than slowing it.

More concentrated acid packs more particles into the same volume, so they collide with the magnesium more often each second. Faster-moving particles describes heating, only a catalyst lowers activation energy, and adding more particles does not make individual ones more reactive.

The steepness of a rate curve tells you how fast the gas is being made, while the height where the curve flattens tells you how much gas is made in total. Raising the temperature makes the particles collide more often and harder, so the reaction is faster and the curve is steeper, but it does not change the amount of reactant, so the final volume is the same. Here experiment 2 is steeper, so it is the hotter run, yet both level off at $50\,\text{cm}^3$.

The double arrow $\rightleftharpoons$ shows a reversible reaction, where products can react to remake the reactants. Releasing energy describes an exothermic change, a single arrow shows a one-way reaction, and the symbol says nothing about needing a catalyst.

At equilibrium both reactions continue at equal rates, so concentrations stay constant. The reactions never stop, the reactants are not used up as in a completed reaction, and constant concentrations need not be numerically equal.

Anhydrous copper(II) sulfate is white and turns blue as it takes in water to form the hydrated salt. Blue to white is the opposite dehydration step, and pink belongs to cobalt(II) chloride, not copper(II) sulfate.

Since the forward reaction is exothermic, heating favours the endothermic reverse, shifting back to the colourless reactants. Speeding products confuses rate with position, only a catalyst leaves position unchanged, and heating does not favour the exothermic side.

Whenever you are given amounts of two reactants, you must check which one runs out first — that is the limiting reactant, and it controls how much product forms. Here, fully reacting the nitrogen would demand 12 mol of hydrogen, but only 9 mol is available, so hydrogen is limiting. Working from the 3:2 hydrogen-to-ammonia ratio in the balanced equation gives 6 mol of ammonia. A common trap is to use the nitrogen amount because it gives a neat number, but that ignores that there is not enough hydrogen to use all the nitrogen.

Setting the neutral compound to zero with $+1$ for potassium and $-2$ for each of four oxygens leaves $+7$ for manganese. A value of $+5$ misses some oxygens, $+4$ ignores the potassium, and a metal bonded to oxygen cannot carry a negative number.

Zinc is oxidised: it loses electrons and its oxidation number rises from $0$ to $+2$. Gaining electrons would lower the number, a fall to $-2$ misjudges the direction, and a rise from losing electrons is oxidation, not reduction.

A reaction is redox when oxidation and reduction occur together, the electrons lost by one being gained by the other. Oxidation alone ignores the electron acceptor, and the processes are simultaneous, with one oxidation always matched by one reduction.

The ion is reduced: it gains an electron and its oxidation number falls from $+3$ to $+2$. Losing an electron would raise the number, a rise to $+4$ misreads adding charge, and a falling number from gaining electrons is reduction, not oxidation.

The copper(II) oxide loses its oxygen to become copper, so it is reduced. Loss of oxygen is reduction, not oxidation, and it is the carbon that gains oxygen to form carbon dioxide, not the copper(II) oxide.

A reducing agent is the substance that pulls oxygen away from something else, and in the process it gets oxygen added to itself (it is oxidised). In $Fe_2O_3 + Al \rightarrow Fe + Al_2O_3$ the aluminium starts as a bare metal and ends up as $Al_2O_3$, so it has gained oxygen and is oxidised, which is exactly what the reducing agent does. The iron oxide is the one being reduced, so it cannot be the reducing agent.

Each of these acid reactions produces a salt, so a salt forms in all three. Gas does not form with the metal oxide, weak acids still react with metals, bases, and carbonates, and these reactions give soluble salts rather than precipitates.

Nitric acid is the acid because it donates a proton (H+) to ammonia. The proton acceptor is the base, accepting a proton is the wrong definition for an acid, and donating electrons confuses proton transfer with electron transfer.

Propanoic acid is weak because it only partially dissociates, leaving most molecules intact. Releasing a few H+ ions does not make it strong, being dissolved in water does not make Y weak, and equal concentration does not equalise strength.

All four are bases because each is a metal oxide or hydroxide, and the soluble ones are also alkalis. A base need not dissolve, only soluble bases are alkalis, and metal hydroxides count as bases too.

In acid, litmus turns red while both thymolphthalein and phenolphthalein stay colourless, gaining colour only in alkali. Blue litmus, pink phenolphthalein and blue thymolphthalein are all alkali colours, not acid ones.

Only the non-metal oxide $SO_2$ is acidic and reddens blue litmus. Sodium oxide dissolves to give an alkali, while magnesium oxide and nickel(II) oxide are metal oxides and so are basic.

Zinc oxide is amphoteric, reacting with both acid and alkali. Calcium oxide and copper(II) oxide are basic and react only with acid, while carbon dioxide is acidic and reacts only with alkali.

Excess insoluble oxide uses up all the acid and is filtered off, leaving the dissolved salt to crystallise from the filtrate. The salt is in the liquid not the residue, skipping filtration leaves impurities, boiling dry ruins crystals, and adding alkali only forms a hydroxide.

Precipitation needs two soluble salts, one supplying lead ions and one supplying iodide ions, so lead(II) nitrate plus potassium iodide works. Nitric acid gives no iodide, solid carbonate keeps lead ions locked up, and the last pair has no lead ions.

To predict a precipitate, work out the two salts you could make by swapping partners, then check the solubility rules for each one. A precipitate only appears if at least one of those new salts is insoluble. Silver chloride is one of the few insoluble chlorides, so mixing a solution containing silver ions with one containing chloride ions produces a white solid. The other three mixtures only ever pair ions into salts that are all soluble (sodium, potassium, ammonium and nitrate salts are always soluble), so they stay as clear solutions.

Rows are periods and columns are groups, so the third row and second column give Period 3 and Group II. Swapping the labels or miscounting the columns gives the wrong placement.

Boiling point rises down the group with widening gaps, so the next member sits well above $+58\,^{\circ}\text{C}$. A very low value reverses the trend, a tiny rise ignores the growing gaps, and an averaged value abandons the trend.

Across a period the elements change from metallic to non-metallic. Melting point and outer-electron count are different properties, and metals on the right with non-metals on the left reverses the real direction.

Don't judge metal vs non-metal by hardness alone. The Group I metals (like sodium and potassium) are soft enough to cut with a knife and float on water because their density is low, yet they are definitely metals. The reliable clues here are chemical: the gas pops with a splint, so it is hydrogen, and red litmus turning blue means the solution is an alkali. A reactive metal added to cold water gives exactly this combination - hydrogen gas plus a metal hydroxide that makes the water alkaline. Watch the litmus carefully: red turning blue means alkaline, so any answer claiming the solution is acidic has the test backwards.

Reactivity increases down Group I because the outer electron is lost more easily, so the reaction with water gets more vigorous. A less vigorous reaction borrows the halogen pattern, melting point actually falls, and the metals stay soft and easy to cut.

Density generally increases down Group I, so rubidium should be denser than potassium despite potassium's small dip below sodium. Reading that dip as a falling trend gives too low a density, reactivity actually rises down the group, and melting point falls rather than rising with mass.

Atoms get heavier down Group VII so density increases, while reactivity decreases from chlorine to iodine. A rising reactivity copies the Group I pattern, and a falling density mistakenly assumes lighter elements lower down.

Chlorine is a yellow-green gas, bromine a red-brown liquid, and iodine a grey-black solid, following increasing density. Swapping the gas and solid ignores that the lightest is the gas and heaviest the solid, and chlorine is the gas, not the red-brown liquid.

Going down Group VII the elements get denser and change from gas to liquid to solid, so each element sits between its neighbours for both properties. Since bromine is between chlorine (a low-density gas) and iodine (a denser solid), its density must lie between the two values and its state must be the in-between one, a liquid. Interpolating gives a liquid of roughly $3$ $\text{g/cm}^3$.

Transition elements form coloured compounds, so a brightly coloured chloride is expected while potassium's is white. Floating and melting in the hand fit soft, light Group I metals, and a violent reaction with cold water describes potassium, not a transition element.

The trick here is that iron is a transition metal, so it does not have one fixed valency, it can be +2 or +3. Which one you get depends on the strength of the substance reacting with it. Chlorine is a powerful oxidising agent, so it pushes iron right up to the +3 state, giving $FeCl_3$. Dilute hydrochloric acid is gentler, it only manages to take iron to +2, giving $FeCl_2$ plus hydrogen. A common slip is to assume iron always behaves the same way, or to mix up which reagent is the stronger oxidiser.

Noble gases are unreactive because their full outer shell gives no tendency to gain, lose or share electrons. Saying there are no outer electrons confuses a full shell with an empty one, reactivity is not about mass or speed, and noble gases are single atoms, not molecules.

Atoms bond in order to get a full (stable) outer shell of electrons. Nitrogen's outer shell ($2,5$) is not full, so nitrogen atoms share electrons in pairs to complete it, giving $N_2$ molecules. Argon's outer shell ($2,8,8$) is already full, so it gains nothing by bonding and simply exists as single atoms. The wrong answers mix up the real reason: it is not about how many inner shells the atom has, not about how heavy or fast the atom is, and certainly not about an empty outer shell. Argon's outer shell is full, not empty, and that completeness is exactly what makes it unreactive and monatomic.

Metals can be hammered into sheets and drawn into wires because their atom layers slide without breaking. Shattering describes brittle non-metals, while low melting points and poor conductivity are non-metal traits, unlike the high-melting, conducting metals.

A metal with a dilute acid always gives a salt plus hydrogen, so zinc gives zinc chloride and hydrogen. Oxygen names the wrong gas, an oxide forms with oxygen not acid, and a hydroxide forms with water not acid.

Cables need conductivity to carry current and low density to stay light over long spans. Conductivity alone misses why aluminium beats better-conducting copper, dismissing conductivity forgets the cable's job, and aluminium is actually less dense than copper, not more.

Copper is ductile, so it can be drawn into thin wires and bent around corners without snapping. Low density and corrosion resistance suit aluminium uses, not shaping wire, and a brittle wire would crack rather than bend.

Think of pure copper as neat layers of identical marbles - they slide over each other easily, so the metal is soft. Brass mixes in zinc atoms, which are a slightly different size. Those odd-sized atoms get in the way and stop the layers from sliding smoothly, so the metal becomes harder. This is the key idea behind alloys: an alloy is a mixture of atoms, not a new compound, and not a surface coating like galvanising. Watch out for the tempting wrong answers about "stronger bonds" or "forming a compound" - the hardness comes from the disrupted layers, not from any new chemical bonding.

Silver and copper atoms differ in size, distorting the layers so they can no longer slide, making the alloy harder. An alloy is a mixture, not a compound; easier sliding gives softer metals; and delocalised electrons explain conduction, not hardness.

Alloys are harder and stronger than pure metals, ideal for resisting load. An alloy is a mixture not a pure substance; heat conduction is irrelevant to strength; and softer metals would deform under weight.

The series runs sodium > magnesium > zinc > copper. Starting with copper reverses the order; placing magnesium before sodium, or zinc before magnesium, swaps neighbours from their fixed order.

Zinc is more reactive than silver, so it displaces silver, which coats the zinc as the zinc dissolves. A more reactive metal does displace a less reactive one; silver staying dissolved reverses the reactivity; and hydrogen bubbles belong to metal-acid reactions.

Only metals above hydrogen react with dilute acid to give hydrogen; silver sits below hydrogen, so it gives no bubbles. Iron reacts slowly, while zinc and magnesium react readily.

Reactivity is judged by how easily a metal reacts, and reaction with cold water is the strongest sign of all, so the metal that fizzes in cold water comes first. Among metals that ignore cold water, the ones that still react with dilute acid are more reactive than one that does nothing, and a steady stream of bubbles means a readier reaction than a few slow bubbles. Putting these clues together ranks the metals from the cold-water reactor down to the metal that reacts with neither water nor acid.

Iron rusts only when water and oxygen are both present. Boiled water under oil has neither fresh oxygen nor air; dry air lacks water; and removing carbon dioxide changes nothing, as it was never required.

Zinc is more reactive, so it corrodes sacrificially and protects the exposed iron. Nearby zinc speeding rusting reverses reactivity; protection ending at a scratch ignores the sacrificial effect; and zinc does not supply oxygen.

A barrier covers the metal to keep out water and oxygen, which gloss paint does. Magnesium gives sacrificial protection, drying removes a condition, and acid leaves bare metal exposed.

Gold is so unreactive it does not combine, so it is found native. Calcium is too reactive for carbon reduction; zinc is below carbon so reduced by carbon, not electrolysis; and very reactive potassium is always combined.

Carbon dioxide reacts with hot coke to form carbon monoxide: $C + CO_2 \rightarrow 2CO$. The other equations show iron being reduced, coke burning to carbon dioxide, or carbon monoxide formed directly from carbon and oxygen.

Carbon monoxide gas removes oxygen from iron(III) oxide, leaving molten iron. The solid coke is not the main reducing agent, calcium oxide forms slag with impurities, and heat alone cannot decompose the ore.

Both anhydrous salts react specifically with water: blue cobalt(II) chloride goes pink, and white copper(II) sulfate goes blue, as each becomes hydrated. These are tests for the presence of water, not for carbon dioxide or acids, so reading the change as a gas or acid result misuses the test. A positive result only tells you water is present, not that the liquid is pure, since impurities would not stop the salts from gaining water.

Pure water has fixed melting and boiling points of $0\,^{\circ}\text{C}$ and $100\,^{\circ}\text{C}$, so matching both confirms purity. Impurities would lower the melting point, raise the boiling point and spread the change over a range.

Tap water carries dissolved chloride ions that would react with silver nitrate before the test, giving a misleading result, while distillation removes them. The choice is about chemical impurities, not temperature or boiling point, and distillation removes substances rather than adding silver.

Sewage adds disease-causing microbes, while phosphates trigger eutrophication, where algal growth and decomposition use up dissolved oxygen. The two effects must not be swapped, and this pollution lowers oxygen rather than raising it.

Nitrogen is the element that plant proteins and growth depend on, so a good fertiliser packs as much nitrogen as possible into each kilogram. To compare salts fairly you cannot just count nitrogen atoms or pick the one that "sounds nitrogen-rich" - you have to work out the percentage of the total mass that is nitrogen. Ammonium nitrate carries two nitrogen atoms inside a light formula mass of 80, giving 35% nitrogen, which beats the heavier sulfate, the single-nitrogen chloride, and the potassium-laden nitrate. That high soluble-nitrogen content is exactly why ammonium nitrate is such a widely used fertiliser.

Treat NPK as a checklist: nitrogen, phosphorus, potassium all have to be present. First work out what the given compound already brings. In $(NH_4)_3PO_4$ the $NH_4$ groups give you N and the $PO_4$ gives you P, so two boxes are ticked and potassium is the gap. Then scan the choices for a potassium compound (look for the symbol $K$). Potassium chloride, $KCl$, is the only one that contains potassium, so it is what completes the fertiliser. The trap answers either pile on an element you already have or add an element (calcium) that plants do not get from an NPK blend.

The trick here is to know what changes when a hydrocarbon burns. Combustion takes oxygen out of the air and puts carbon dioxide in, so contaminated air must show oxygen dropping below 21% AND carbon dioxide rising above the tiny 0.04% found in clean air. Only one sample does both. Watch out for the sample where carbon dioxide is raised but oxygen is still 21% — that is a trap, because you cannot make extra CO₂ by burning fuel without using up some oxygen at the same time. The nitrogen figure is a distraction: burning a hydrocarbon does not change the amount of nitrogen.

Greenhouse gases absorb outgoing thermal energy from the warm Earth and re-emit part of it back down, reducing the energy lost to space. Blocking sunlight would cool the surface, there is no heat-releasing reaction, and the gases re-emit rather than permanently store energy.

With too little oxygen the carbon is only partly oxidised, forming carbon monoxide rather than the carbon dioxide made when oxygen is plentiful. Sulfur dioxide comes from sulfur in the fuel, and oxides of nitrogen form in the hot conditions of car engines.

The high engine temperature forces nitrogen and oxygen from the air to combine, so the nitrogen need not be in the fuel. The converter removes the oxides by reaction with carbon monoxide, forming harmless nitrogen and carbon dioxide, not by filtering, adding oxygen, or neutralising.

Sulfur dioxide causes acid rain, not global warming or blood poisoning, which belong to other pollutants. The control measure must be specific, such as low-sulfur fuel or flue gas desulfurisation, rather than a vague 'burn less coal'.

A displayed formula shows every bond as a line, so count both carbon-carbon and carbon-hydrogen bonds: 2 chain bonds plus 8 bonds to hydrogen gives 10. Counting only hydrogen bonds gives 8, and counting carbons or atoms confuses atoms with bonds.

A structural formula removes ambiguity, showing the atoms grouped as an -OH, not a -COOH (no second oxygen or C=O is written). With no written C=C it is saturated, and it is a two-carbon chain, not a nine-atom ring.

Substituting $n=4$ into the alkene formula $C_nH_{2n}$ gives $C_4H_8$ exactly. The alkane formula would need two more hydrogens, and both alcohols and carboxylic acids must contain oxygen, which this formula lacks.

Isomers share one molecular formula but differ in arrangement; the two butenes are both $C_4H_8$ with the C=C in different places. A shorter or saturated chain changes the formula, and a flipped single structure is just the same molecule redrawn.

Don't try to memorise which group goes with this question - work it out from what the compound does. Two clues are given: it turns litmus red (so it's an acid) and it fizzes with a carbonate (acids react with carbonates to give carbon dioxide). The functional group that makes an organic compound behave as an acid is the carboxylic acid group, $-\\text{COOH}$. The biggest trap here is choosing the alcohol group $-\\text{OH}$ just because it also contains oxygen and an H - but alcohols are neutral and do not fizz with carbonates. The C=C double bond and an ether-style $-\\text{O}-$ linkage are not acidic either, so they cannot explain the observations.

The carboxyl carbon needs its C=O double bond and the oxygen needs its bond to hydrogen drawn in full. Two single C-O bonds leave the carbon with only three bonds, omitting the H leaves an oxygen short, and an extra H gives carbon five bonds, which is impossible.

Four carbons give the stem 'butan-', so any 'propan' answer miscounts. Number from the end giving the -OH the lower locant, which is position 2, not 3; it is not on an end carbon, so it is not the 1-ol.

The trick here is that "fuel" and "fossil fuel" are not the same thing. All four substances can be burned to release energy, but only fossil fuels are the finite resources dug or pumped out of the ground that took millions of years to form from dead organisms. Natural gas pumped from underground rock and coal mined from seams both fit that origin. The fermented-crop fuel is renewable because the crops grow back each year, and the gas made by reacting steam with a metal is manufactured in a reaction, not extracted from a buried deposit, so neither is fossil. Always check the ORIGIN of a fuel, not just whether it burns.

Read the formula off the combustion products. One mole of $CO_2$ contains one carbon atom, so the molecule has 1 carbon. Two moles of $H_2O$ contain 4 hydrogen atoms ($2 \times H_2O$), so the molecule has 4 hydrogens. That gives $CH_4$, methane, the main gas in natural gas. The 2 moles of oxygen also fit the balanced equation $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$. Ethane ($C_2H_6$) and propane ($C_3H_8$) would make 2 and 3 moles of $CO_2$, and $C_2H_4$ is an alkene, not a natural gas constituent.

A hydrocarbon contains hydrogen and carbon only. Adding oxygen makes a different family such as an alcohol, carbon alone is an element, and a hydrogen-and-oxygen molecule like water has no carbon.

If something can be split apart just by heating and condensing (distillation), it has to be a mixture, because a pure compound would have a single fixed boiling point and would all turn to gas at once. Crude oil gives several fractions at several temperatures, so it contains several different hydrocarbons. The pattern is simple: small molecules have weak forces between them and boil at low temperatures (gases at the top), while big molecules have strong forces and boil at high temperatures (thick bitumen at the bottom). So different boiling-point ranges = different chain lengths.

A saturated hydrocarbon has only single covalent bonds and holds the maximum hydrogen for its carbons. A double bond instead makes a molecule unsaturated, the shared pairs are covalent not ionic, and only unsaturated molecules react by addition.

Substitution swaps one hydrogen for one chlorine, and the removed hydrogen leaves with the spare chlorine as hydrogen chloride. Simply adding chlorine misses that a hydrogen must leave, and the by-product is hydrogen chloride, not hydrogen gas.

A C=C double bond means the molecule is unsaturated: the carbons are not bonded to as many separate atoms as possible. Saturated molecules have only single C-C bonds, and every carbon still forms four bonds, not three.

The double bond opens and both bromine atoms add on, so addition gives one product, 1,2-dibromoethane. Substitution would form a second product such as HBr, and addition never makes two products.

Cracking needs a high temperature and a catalyst, not a low temperature or water. Balancing atoms, C10H22 minus C8H18 leaves the alkene ethene (C2H4); ethane (C2H6) would not balance.

Fermentation needs yeast at about 30 °C with no air, since air oxidises the ethanol and 90 °C kills the yeast; hydration adds steam to ethene at high temperature and pressure over phosphoric acid. Using ethane has no double bond, and swapping in nickel or yeast confuses the two catalysts.

Fermentation's real plus is that sugar from crops is renewable, while ethene comes from finite crude oil. The ethene route is the continuous one, fermentation gives dilute ethanol needing distillation, and it is slow and relies on yeast.

The metal replaces the acidic hydrogen of COOH, giving sodium ethanoate, CH3COONa. CH3COOH is just the unreacted acid, sodium carbonate is a reactant, and 'ethanate' is not a real carboxylate name.

Both routes oxidise ethanol to ethanoic acid: bacteria use oxygen from air, while the lab uses warm acidified potassium manganate(VII) whose purple colour fades. Souring is bacterial not yeast fermentation, neither is a reduction, and no manganate(VII) colour change occurs in the wine.

An addition polymer is built from lots of small monomers that each contain a C=C double bond. When they join, that double bond "opens up" - it turns into two single bonds that link to the next units along the chain, so the long chain has only single bonds in its backbone. To go backwards from a repeat unit to the monomer, you put the double bond back between the two carbons. The repeat unit here is $-CH_2-CHCl-$ (2 carbons, 3 hydrogens, 1 chlorine), so reforming the double bond gives $CH_2=CHCl$, which has exactly those same atoms. The saturated answers are wrong because a monomer for an addition polymer must be unsaturated, and the answer without chlorine is wrong because it leaves out an atom that is clearly present in the chain.