Cambridge IGCSE Chemistry 0620: Frequent mistakes

Cambridge Principal Examiner Reports for Chemistry 0620 highlight recurring misconceptions and exam technique errors. Avoiding these can improve your grade.

Energy and thermochemistry

Endothermic and exothermic confusion

Candidates often confuse temperature changes with energy direction. In endothermic reactions, surroundings get colder (energy absorbed). In exothermic, surroundings get hotter (energy released).

Fix: Endothermic = temperature decreases (absorbs). Exothermic = temperature increases (releases).

Where does energy come from?

In exothermic reactions, energy comes from bond formation (products have stronger bonds). In endothermic, energy is needed to break bonds.

Fix: Exothermic: bond making releases energy. Endothermic: bond breaking absorbs energy.

Electrolysis

Products at electrodes (aqueous solutions)

A recurring error: expecting sodium metal at the cathode during electrolysis of aqueous sodium chloride. In aqueous solutions, hydrogen forms at the cathode if the metal is more reactive than hydrogen.

Fix: For aqueous solutions: metals above hydrogen in reactivity series → H₂ at cathode. Only less reactive metals (e.g. Cu, Ag) form at cathode.

Anode products

Candidates confuse which ion is discharged. For dilute sulfuric acid, oxygen forms at the positive electrode (from OH⁻ or water).

Fix: Learn the electrolysis rules: concentrate on anion discharge at anode; cation at cathode.

Separation techniques

Chromatography

Common errors: solvent level above the baseline (dissolves spots); wrong solvent choice; not allowing solvent to run sufficiently.

Fix: Solvent must be below baseline. Spot must not be submerged. Use appropriate solvent for the mixture.

Filtration

Selecting wrong filter paper or solvent; not dissolving the solid first before separation.

Fix: Dissolve first if needed. Choose correct filter size. Know when to use filtration vs. crystallisation.

Chemical knowledge gaps

Metal reactions

Misidentifying metals from their reactions with water, dilute acids, or oxide–carbon reduction. Confusion between reactivity series order.

Fix: Learn reactivity series. Know which metals react with water (K, Na, Ca, Mg); with acid (above H); with carbon (below C).

Terminology

Confusing burning vs. combustion (both correct for fuel + O₂); conductor vs. insulator; element vs. compound.

Fix: Use precise terms. Combustion = reaction with oxygen. Conductors = allow current (metals, graphite, molten ionic).

Polymers and bonding

Not understanding polymer structures; carbon bonding patterns (e.g. alkanes, alkenes); addition vs. condensation polymerisation.

Fix: Alkenes: C=C. Addition polymers: monomers add without losing small molecules. Condensation: loses H₂O or similar.

Calculations and data

Precision

Overlooking significant figures or decimal places required in the question. Rounding too early in multi-step calculations.

Fix: Give final answer to required precision. Show working to full calculator accuracy, then round at end.

Not using given information

Candidates sometimes ignore data in tables or diagrams. Marks are lost when answers could be taken directly from the question.

Fix: Read all parts of the question. Use the resource.

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Based on Cambridge IGCSE Chemistry 0620 Principal Examiner Reports (2019–2025).

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