Top 10 IGCSE Chemistry MCQ Mistakes
Ten multiple-choice mistakes that cost IGCSE Chemistry students easy marks, with the fix for each.
Why MCQ mistakes matter: Multiple choice papers (11-13 for Core, 21-23 for Extended) are worth 30% of your grade. This guide shows ten easy ways to lose marks, and how to avoid each one. Paper 1 (Core) tests Core content only; anything marked (Extended) is for Paper 2 only.
The Top 10 MCQ Mistakes
Electrolysis Products
What the Syllabus Expects:
All students: name the products at each electrode for molten lead(II) bromide, concentrated aqueous sodium chloride and dilute sulfuric acid, and predict them for any molten binary compound. Extended only: predict products for other aqueous halides and copper(II) sulfate, and write half-equations.
What Goes Wrong:
- • Thinking sodium metal forms during electrolysis of aqueous NaCl (it doesn't - hydrogen forms instead)
- • Confusing which electrode is the anode vs cathode
- • Not knowing that cations go to the cathode and anions go to the anode
- • Forgetting that in aqueous solutions, water can be discharged instead of the ion
Example Question:
Q: What forms at the cathode during electrolysis of aqueous sodium chloride?
WRONG CHOICE
Sodium metal
CORRECT CHOICE
Hydrogen gas
Why: In aqueous solution, hydrogen from water is discharged instead of sodium because sodium is too reactive. Sodium metal only forms from molten NaCl (no water present).
Remember:
- ✓ Cathode is negative → attracts positive ions (cations)
- ✓ Anode is positive → attracts negative ions (anions)
- ✓ In aqueous solutions: metals more reactive than hydrogen stay as ions, so hydrogen forms at the cathode; at the anode, concentrated chloride gives chlorine and dilute sulfuric acid gives oxygen
- ✓ Molten = melted, with no water, so the metal forms; aqueous = dissolved in water, so H⁺ and OH⁻ ions from water are also present
Ionic vs Covalent Bonding
What the Syllabus Expects:
Ions form by electron transfer; an ionic bond is the strong attraction between oppositely charged ions. A covalent bond is a shared pair of electrons. Know the properties of each.
What Goes Wrong:
- • Thinking calcium chloride has covalent bonds (it's ionic - metal + non-metal)
- • Believing solid ionic compounds conduct electricity (ions can't move when solid)
- • Confusing giant covalent structures (diamond, graphite; Extended also silicon(IV) oxide) with simple covalent molecules
- • (Extended) Not recognising that intermolecular forces are different from covalent bonds: melting a simple molecular substance breaks only the weak forces between molecules
Example Question:
Q: Which substance contains only covalent bonds?
WRONG CHOICES
Magnesium oxide (ionic)
Neon (single atoms, no bonds)
Potassium bromide (ionic)
CORRECT CHOICE
Ammonia, NH₃ (two non-metals sharing electrons)
Why: Metal + non-metal = ionic. Non-metal + non-metal = covalent. Noble gases like neon have no bonds.
Remember:
- ✓ Metal + Non-metal = IONIC (electron transfer)
- ✓ Non-metal + Non-metal = COVALENT (electron sharing)
- ✓ Ionic conducts when molten or dissolved (ions free to move)
- ✓ Diamond and graphite = giant covalent (very high melting points); Extended: silicon(IV) oxide too
Isotopes & Atomic Structure
What Goes Wrong:
- • Not knowing that isotopes have same protons but different neutrons
- • Using the proton number instead of the relative atomic mass, Ar, to work out Mr (O is 16, not 8; Cl is 35.5)
- • Forgetting that hydrogen-1 has NO neutrons
- • Counting all electrons instead of outer-shell electrons (sodium, 2,8,1, has 1)
Example Question:
Q: Which atom is an isotope of ¹²C?
WRONG CHOICE
¹⁴N (different element - 7 protons)
CORRECT CHOICE
¹⁴C (same element - 6 protons, different mass)
Why: Isotopes have the SAME number of protons (same element) but DIFFERENT number of neutrons (different mass number).
Remember:
- ✓ Atomic number = protons = defines the element
- ✓ Mass number = protons + neutrons
- ✓ Neutrons = mass number - atomic number
- ✓ Isotopes: same protons, different neutrons
Endothermic vs Exothermic
What Goes Wrong:
- • Confusing endothermic (takes in heat) with exothermic (releases heat)
- • (Extended) Thinking breaking bonds releases energy (it takes in energy)
- • Misreading reaction pathway diagrams: if the products are lower than the reactants, energy is given out (exothermic)
- • Calling melting or evaporation exothermic: both take in energy; freezing and condensing give energy out
Example Question:
Q: Which process is endothermic?
WRONG CHOICES
Combustion
Neutralisation
Respiration
CORRECT CHOICE
Thermal decomposition of calcium carbonate (heat must be supplied all the time)
Why: Endothermic = takes in energy from surroundings (feels cold). Exothermic = releases energy (feels hot). Combustion, neutralisation and respiration all give energy out.
Remember:
- ✓ EXO = EXit = energy exits = releases heat
- ✓ ENDO = ENter = energy enters = absorbs heat
- ✓ (Extended) Breaking bonds = requires energy (always)
- ✓ (Extended) Making bonds = releases energy (always)
Redox & Oxidation States
What Goes Wrong:
- • (Extended) Naming the substance that is reduced when asked for the reducing agent (the one that removes oxygen)
- • (Extended) Choosing products as the oxidising/reducing agent instead of reactants
- • Misreading Roman numerals: iron(III) oxide is Fe₂O₃ (iron +3); iron(II) oxide is FeO (iron +2)
- • Mixing up the definitions: oxidation is GAIN of oxygen but LOSS of electrons (the electron definitions are Extended)
Example Question:
Q: In the reaction PbO + H₂ → Pb + H₂O, which substance is oxidised?
WRONG CHOICES
Pb (a product)
H₂O (a product)
PbO (it loses oxygen, so it is reduced)
CORRECT CHOICE
H₂ (it gains oxygen to become H₂O)
Why: Oxidation is gain of oxygen, and H₂ gains oxygen. Extended: H₂ is also the reducing agent, because it removes oxygen from PbO; PbO is the oxidising agent.
Remember (OIL RIG):
- ✓ Oxidation = gain of oxygen; Extended: loss of electrons (OIL)
- ✓ Reduction = loss of oxygen; Extended: gain of electrons (RIG)
- ✓ (Extended) Reducing agent = gets oxidised (loses electrons)
- ✓ (Extended) Oxidising agent = gets reduced (gains electrons)
Mole Calculations & Ratios
What Goes Wrong:
- • Assuming 1:1 ratios when the equation shows different ratios
- • (Extended) Not identifying the limiting reactant correctly
- • Using wrong Ar values: Cl is 35.5, and O₂ is 32, not 16
- • Forgetting to multiply by molar ratio when calculating products
Example Question:
Q (Extended): C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. What mass of carbon dioxide forms when 4.4 g of propane burns completely?
Method:
1. Moles of C₃H₈ = 4.4 ÷ 44 = 0.10 mol
2. From equation: 1 mol C₃H₈ → 3 mol CO₂ (1:3 ratio)
3. Moles of CO₂ = 0.10 × 3 = 0.30 mol
4. Mass = 0.30 × 44 = 13.2 g (a 1:1 guess gives 4.4 g)
Remember:
- ✓ Always write the balanced equation first
- ✓ Moles = mass ÷ Mr
- ✓ Use molar ratios from balanced equation
- ✓ Paper 1 (Core) uses no moles: scale masses in simple proportion. Extended: the limiting reactant runs out first
Acids, Bases & pH
What Goes Wrong:
- • (Extended) Confusing weak acids with dilute acids
- • Thinking a lower pH means less acidic (the opposite: pH 2 is more acidic than pH 5)
- • Not knowing that ethanoic acid reacts similarly to mineral acids
- • Mixing up indicator colours: methyl orange is red in acid and yellow in alkali; thymolphthalein is colourless in acid and blue in alkali
Example Question:
Q (Extended): Which statement about weak acids is correct?
WRONG CHOICES
Has a low concentration
Fully dissociates in water
Has pH of 1
CORRECT CHOICE
Only partially dissociates in water
Remember:
- ✓ Strong acid = fully dissociates (HCl, H₂SO₄, HNO₃)
- ✓ Weak acid = partially dissociates (CH₃COOH, H₂CO₃)
- ✓ pH 1-6 = acidic, pH 7 = neutral, pH 8-14 = alkaline
- ✓ Concentration ≠ strength
Organic Chemistry
What Goes Wrong:
- • Confusing alkanes (CₙH₂ₙ₊₂) with alkenes (CₙH₂ₙ)
- • Not knowing the order of petroleum fractions
- • Confusing ethane with ethene (similar names, different compounds)
- • Thinking alkanes undergo addition reactions (they do substitution)
Example Question:
Q: A hydrocarbon has formula C₄H₁₀. What type of compound is it?
WRONG CHOICE
Alkene (would be C₄H₈)
CORRECT CHOICE
Alkane (fits CₙH₂ₙ₊₂)
Remember:
- ✓ Alkane = CₙH₂ₙ₊₂ (saturated, single bonds only)
- ✓ Alkene = CₙH₂ₙ (unsaturated, C=C double bond)
- ✓ Petroleum fractions (increasing boiling point): refinery gas → petrol → naphtha → kerosene → diesel oil → fuel oil → lubricating oil → bitumen
- ✓ Test for alkene: aqueous bromine turns from orange to colourless; with an alkane it stays orange
Periodic Table Trends
What Goes Wrong:
- • Thinking the Periodic Table is ordered by mass number (it's atomic number)
- • Forgetting that group number = outer-shell electrons (Group VII has 7)
- • Misidentifying which halogen displaces which in reactions
- • Not knowing that helium has 2 outer electrons, not 8
Example Question:
Q: Chlorine water is added to potassium bromide solution. What happens?
WRONG CHOICE
No reaction (bromide cannot be displaced)
CORRECT CHOICE
Orange/brown colour forms (bromine displaced)
Why: Chlorine is more reactive than bromine, so it displaces bromine from its compound.
Remember:
- ✓ Periodic Table ordered by atomic number (protons)
- ✓ Group number = outer-shell electrons for Groups I to VII; Group VIII noble gases have a full outer shell (helium 2, the others 8)
- ✓ Halogen reactivity decreases down the group: F > Cl > Br > I
- ✓ More reactive halogen displaces less reactive one
Units & Measurement
What Goes Wrong:
- • Picking the wrong apparatus: a burette or volumetric pipette for accurate liquid volumes, a gas syringe for gas, a measuring cylinder only for rough volumes
- • Not recognising that pure substances have sharp melting/boiling points
- • Confusing relative atomic mass with actual mass in grams
- • Struggling with negative temperature values on thermometers
Example Question:
Q: How do you know if a substance is pure?
WRONG CHOICE
It melts over a range of temperatures
CORRECT CHOICE
It melts at one sharp temperature that matches the known value
Remember:
- ✓ Pure substance = sharp melting/boiling point
- ✓ Mixture = melts/boils over a range; an impurity lowers the melting point and raises the boiling point
- ✓ Relative atomic mass is a ratio, not a mass in grams
- ✓ Volume: cm³ or dm³, Mass: g, Temperature: °C
Final Exam Tips
Before the Exam
- ✓ Review ionic vs covalent bonding rules
- ✓ Practice electrolysis predictions
- ✓ Memorise organic homologous series
- ✓ Know the reactivity series
During the Exam
- ✓ Read ALL options before choosing
- ✓ Check units in calculations
- ✓ (Extended) Identify oxidising vs reducing agents from REACTANTS
- ✓ Use molar ratios from balanced equations
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