Exam Focus

Very High Frequency

What Examiners Ask About Purity

• "Explain what is meant by a pure substance" (1 mark) — Say: contains only one type of element or compound
• "Describe how you could test the purity of aspirin" (3 marks) — Measure melting point, compare to literature value (135 °C), pure = sharp melting point at correct temperature
• "Explain why impurities lower the melting point" (2 marks) — Disrupts regular structure, requires less energy to break apart
• "What is a formulation? Give an example" (2 marks) — Designed mixture with each component in a measured quantity; e.g. medicine, paint, petrol
• Data interpretation — You may be given a table of melting point data and asked to identify which sample is most pure (narrowest range, closest to literature value)

AO2 Application Tips

• When given melting point data, always compare range AND temperature to the pure value
• A sample melting over 5+ degrees is definitely impure
• One degree range could still be impure — look for the word "sharp"

Edexcel 1CH0: Examined in Paper 2 (1CH0/2). Edexcel tests purity using melting/boiling point data — you must be able to calculate percentage purity and interpret purity graphs. In Edexcel-style questions, the command word "Suggest" appears frequently — use your chemistry knowledge to apply to an unfamiliar context.