Edexcel IGCSE Chemistry 4CH1: Frequent mistakes

Pearson Edexcel Principal Examiner Feedback for Chemistry 4CH1 highlights recurring errors in mole calculations, terminology, precision and practical work. Avoiding these can improve your grade.

Mole calculations and ratios

Limiting reagent

Students frequently struggle with mole ratio problems. A common error is giving incorrect mole quantities without considering which reactant is limiting.

Fix: Find moles of each reactant. Compare with ratio. Limiting reactant determines maximum product.

Mole ratio application

Not using the balanced equation correctly. Wrong ratio (e.g. 1:1 when it should be 2:1).

Fix: Write balanced equation. Use correct mole ratio. Check units (mol, g, dm³).

Compound and bonding terminology

Bonds between atoms

Some candidates lose marks by stating bonds form between “two different molecules” rather than “two different atoms”.

Fix: Covalent bonds: between atoms. Ionic: between ions. Intermolecular forces: between molecules.

Incomplete names

Giving incomplete compound names—e.g. “iron(II) oxide” instead of “hydrated iron(III) oxide” for rust.

Fix: Hydrated = with water. Rust = hydrated iron(III) oxide. Check oxidation state.

Numerical precision

Significant figures

Many candidates fail to round answers to the correct number of significant figures or omit required notation (e.g. minus signs in powers).

Fix: Check question: “to 3 significant figures”. Standard form: 5 × 10⁻³ not 5 × 10⁻3 (minus sign).

Burette readings

Recording burette readings incorrectly—omitting trailing zeros (e.g. 25.00 not 25) or writing readings in wrong order.

Fix: Record to 2 decimal places. Initial and final. Use correct order. Units (cm³).

Experimental understanding

Gas volume calculations

Misunderstanding gas volume calculations—failing to add volumes from different parts of apparatus together.

Fix: Total volume = sum of relevant parts. Check units. Use correct gas volume relationships.

Reaction rate and diffusion

Incorrectly linking diffusion rates to reaction kinetics. Diffusion is one factor; concentration, temperature, catalyst also matter.

Fix: Rate depends on: concentration, temperature, surface area, catalyst. Diffusion can be rate-limiting but isn’t the only factor.

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Based on Pearson Edexcel IGCSE Chemistry 4CH1 Principal Examiner Feedback (2022–2023).