Overall Examination Verdict
The October/November 2024 Cambridge IGCSE Chemistry (0620) examination provided a robust, well-calibrated assessment. The papers tested both fundamental recall and higher-level application. Across Paper 21 (Extended Multiple Choice), Paper 41 (Extended Theory), and Paper 61 (Alternative to Practical), examiners rewarded precise chemical nomenclature, structural clarity, and disciplined mathematical execution.
Where the Marks Were Won and Lost
A substantial proportion of marks in Paper 41 were concentrated in Organic Chemistry and Electrochemistry. Candidates who mastered the full displayed linkages of condensation polymers (polyamides / proteins) and standard molar volume calculations scored heavily. In Question 4(a)(iii), calculating the volume of \(\text{NO}_2\) gas required chaining \(M_r\) calculation, mole ratios \((4:2)\), and molar volume conversion at r.t.p. \((24\,000\text{ cm}^3)\).
Key discriminator marks were won on:
- Electrolysis half-equations: Writing the oxidation of hydroxide ions at the anode \(4\text{OH}^- \rightarrow 2\text{H}_2\text{O} + \text{O}_2 + 4\text{e}^-\) in aqueous copper(II) sulfate.
- Avogadro constant calculations: Determining total hydrogen atoms in \(44.0\text{ g}\) of methyl propanoate \(\text{CH}_3\text{CH}_2\text{COOCH}_3\), requiring multiplication across 8 hydrogen atoms per molecule to yield \(2.41 \times 10^{24}\).
- Graphing & Experimental Logic (Paper 61): Accurately identifying two intersecting straight lines of best fit for continuous precipitation, extending to the origin and plateauing at \(4.6\text{ cm}^3\).
Examiner Pitfalls & Common Misconceptions
Examiners highlighted frequent stumbling blocks where preventable marks were dropped:
- Organic Linkages: Omission of continuation bonds or showing incorrect valencies around nitrogen and carbonyl carbon in polyamide repeating units.
- Incomplete Qualitative Analysis: Failing to distinguish between \(\text{Fe}^{2+}\) and \(\text{Cr}^{3+}\) when adding excess sodium hydroxide vs. ammonia, or forgetting that \(\text{Fe(OH)}_2\) oxidizes to brown on standing.
- Reactivity vs. Inert Electrodes: Confusing observations when switching from graphite (inert) to active copper electrodes in aqueous \(\text{CuSO}_4\) electrolysis.
Preparation & Strategic Guidance
Future candidates should place high revision value on stoichiometry drills, ionic half-equations, and full displayed organic mechanisms. For Paper 6, practice ruler-drawn best-fit lines with distinct regimes (increasing slope and horizontal plateau) and ensure six-point experimental designs clearly detail extraction, separation, and outcome evaluation.