Top 10 IGCSE Chemistry MCQ Mistakes (2022-2025)
The most common Multiple Choice mistakes from Cambridge Examiner Reports. These errors appear year after year - learn to avoid them.
Why MCQ mistakes matter: Multiple choice papers (11-13 for Core, 21-23 for Extended) are worth 30% of your grade. The same conceptual errors appear every year. This guide shows exactly what examiners see students getting wrong - and how to fix it.
The Top 10 MCQ Mistakes
Electrolysis Products
Appears every year: 2022, 2023, 2024, 2025What the Syllabus Expects:
Predict products of electrolysis of molten and aqueous solutions. Understand electrode reactions.
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 above hydrogen → hydrogen discharged instead
- ✓ Molten = pure compound, aqueous = dissolved in water
Ionic vs Covalent Bonding
Appears every year: 2022, 2023, 2024, 2025What the Syllabus Expects:
Describe ionic bonding as electron transfer, covalent bonding as electron sharing. Know properties of each.
What Goes Wrong:
- • Thinking potassium iodide 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 (like silicon dioxide) with simple covalent molecules
- • Not recognising that intermolecular forces are different from covalent bonds
Example Question:
Q: Which substance contains only covalent bonds?
WRONG CHOICES
Iron(III) oxide (ionic)
Argon (monatomic - no bonds)
CORRECT CHOICES
Methane CH₄
Chlorine Cl₂
Why: Metal + non-metal = ionic. Non-metal + non-metal = covalent. Noble gases like argon 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)
- ✓ Silicon dioxide = giant covalent (high melting point like diamond)
Isotopes & Atomic Structure
Appears every year: 2022, 2023, 2024, 2025What Goes Wrong:
- • Not knowing that isotopes have same protons but different neutrons
- • Using atomic number instead of mass number for Mr calculations
- • Forgetting that hydrogen-1 has NO neutrons
- • Confusing electron shells with outer shell electrons
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
Appears every year: 2022, 2023, 2024, 2025What Goes Wrong:
- • Confusing endothermic (takes in heat) with exothermic (releases heat)
- • Thinking breaking bonds releases energy (it requires energy!)
- • Misreading energy diagrams - not understanding which direction shows ΔH
- • Confusing a state change requiring heat with a reaction that releases heat
Example Question:
Q: Which process is endothermic?
WRONG CHOICES
Combustion
Neutralisation
Respiration
CORRECT CHOICES
Photosynthesis
Thermal decomposition
Melting/Evaporation
Why: Endothermic = takes in energy from surroundings (feels cold). Exothermic = releases energy (feels hot).
Remember:
- ✓ EXO = EXit = energy exits = releases heat
- ✓ ENDO = ENter = energy enters = absorbs heat
- ✓ Breaking bonds = requires energy (always)
- ✓ Making bonds = releases energy (always)
Redox & Oxidation States
Appears every year: 2022, 2023, 2024, 2025What Goes Wrong:
- • Identifying the metal that is reduced rather than the substance that causes reduction
- • Choosing products as the oxidising/reducing agent instead of reactants
- • Not knowing that combustion and rusting both require oxygen
- • Confusing gain/loss of oxygen with gain/loss of electrons
Example Question:
Q: In the reaction Mg + CuO → MgO + Cu, what is the reducing agent?
WRONG CHOICES
Cu (product)
MgO (product)
CuO (oxidising agent)
CORRECT CHOICE
Mg (reducing agent - it gets oxidised)
Why: The reducing agent is the substance that causes reduction (and gets oxidised itself). Mg removes oxygen from CuO.
Remember (OIL RIG):
- ✓ Oxidation Is Loss (of electrons or gaining oxygen)
- ✓ Reduction Is Gain (of electrons or losing oxygen)
- ✓ Reducing agent = gets oxidised (loses electrons)
- ✓ Oxidising agent = gets reduced (gains electrons)
Mole Calculations & Ratios
Appears every year: 2022, 2023, 2024, 2025What Goes Wrong:
- • Assuming 1:1 ratios when the equation shows different ratios
- • Not identifying the limiting reagent correctly
- • Using wrong values for relative atomic mass
- • Forgetting to multiply by molar ratio when calculating products
Example Question:
Q: Given 2Mg + O₂ → 2MgO, what mass of MgO forms from 4.8g of Mg?
Method:
1. Moles of Mg = 4.8 ÷ 24 = 0.2 mol
2. From equation: 2 mol Mg → 2 mol MgO (1:1 ratio)
3. Moles of MgO = 0.2 mol
4. Mass = 0.2 × 40 = 8.0g
Remember:
- ✓ Always write the balanced equation first
- ✓ Moles = mass ÷ Mr
- ✓ Use molar ratios from balanced equation
- ✓ Limiting reagent = runs out first
Acids, Bases & pH
Appears every year: 2022, 2023, 2024, 2025What Goes Wrong:
- • Confusing weak acids with dilute acids
- • Thinking lower pH = weaker acid (opposite - lower pH = more acidic)
- • Not knowing that ethanoic acid reacts similarly to mineral acids
- • Forgetting that partial dissociation defines a weak acid
Example Question:
Q: 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
Appears every year: 2022, 2023, 2024, 2025What 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): gas → petrol → naphtha → kerosene → diesel → fuel oil → bitumen
- ✓ Test for alkene: decolourises bromine water
Periodic Table Trends
Appears every year: 2022, 2023, 2024, 2025What Goes Wrong:
- • Thinking the Periodic Table is ordered by mass number (it's atomic number)
- • Confusing Group VII (7) = 7 outer electrons
- • 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
- ✓ Halogen reactivity decreases down the group: F > Cl > Br > I
- ✓ More reactive halogen displaces less reactive one
Units & Measurement
Appears every year: 2022, 2023, 2024, 2025What Goes Wrong:
- • Incorrectly linking units to measuring apparatus
- • 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 has a sharp, fixed melting point
Remember:
- ✓ Pure substance = sharp melting/boiling point
- ✓ Mixture = melts/boils over a range
- ✓ 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
- ✓ Identify oxidising vs reducing agents from REACTANTS
- ✓ Use molar ratios from balanced equations