Overall Difficulty Verdict
The June 2025 Chemistry (9620) examination series presents a balanced but mathematically demanding challenge. Units 1 and 2 maintain a very high standard of chemical analysis and structural logic, while Unit 3 extends into high-level physical chemistry calculations (including precise pH buffers and saturated hydroxide masses) and complex transition metal coordination chemistry. Students with strong quantitative mastery and precise drawing techniques will excel, whereas those relying on rote memorisation will find the application-heavy questions extremely challenging.
Where the Marks are Won and Lost
High-scoring students secured substantial marks on standard mechanisms—such as the nucleophilic substitution of halogenoalkanes and the elimination of alcohols—and thermodynamic cycle calculations. However, significant marks were lost in the Time of Flight (TOF) mass spectrometry calculation due to incorrect mass conversions (failing to convert \(69\text{ g mol}^{-1}\) to kilograms per atom using Avogadro's constant). Similarly, in Unit 3, the multi-step calculation of saturated strontium hydroxide mass from pH was a major discriminator, where minor rounding errors or failing to divide the hydroxide ion concentration by two for \(\text{Sr(OH)}_2\) proved costly.
Examiner Pitfalls and Misconceptions
Examiners highlighted several recurring areas of weakness:
- State Symbols: Frequently omitted in precipitation and dissolution equations (e.g., magnesium sulfate dissolving or Period 3 oxides reacting with oxygen), costing straightforward marks.
- Kc Notation: Students continue to use round brackets instead of square brackets for concentration in equilibrium expressions, which is penalised.
- Graph Extrapolations: In Unit 1's calorimetry question, failure to draw two distinct lines of best fit and extrapolate precisely to the fourth minute (time of mixing) prevented students from achieving the correct temperature rise of \(28.5\text{ }^{\circ}\text{C}\).
Strategy and Prediction
To master upcoming series, students must focus on integrating physical chemistry calculations with practical techniques. Ensure that all standard practical steps—such as utilising anti-bumping granules, choosing indicators based on pH ranges, and recognizing the function of a catalyst's support medium—are deeply understood. Our prediction for the next series suggests a likely increase in focus on Transition Metal ligand substitution thermodynamics, rate equations, and complex organic synthesis pathways, which were lighter in this series.