A student has a sample of water collected from a river. The student wants to find out whether the river water is a pure substance or a mixture. Explain how the student could use melting point data to determine whether the river water is pure or impure, and what results would indicate each conclusion.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Heat the river water sample and record the temperature at which it changes state.
1. If the sample is pure, it will freeze (or melt) at a single, sharp temperature of 0 °C.
2. If the sample is impure, the freezing (or melting) will occur over a range of temperatures, not a single point.
3. Compare the measured temperature (or temperature range) to the known melting point of pure water.
4. A sharp match at 0 °C indicates a pure substance; a spread or deviation indicates an impure mixture.
1. If the sample is pure, it will freeze (or melt) at a single, sharp temperature of 0 °C.
2. If the sample is impure, the freezing (or melting) will occur over a range of temperatures, not a single point.
3. Compare the measured temperature (or temperature range) to the known melting point of pure water.
4. A sharp match at 0 °C indicates a pure substance; a spread or deviation indicates an impure mixture.
Examiner tips
- Use the word ‘freeze’ or ‘melt’ and state the exact temperature 0 °C for pure water.
- Show that a range of temperatures means impurities are present.
- Mention that a single sharp temperature is the key criterion for purity.
Common mistakes
- Confusing melting point with boiling point; the question asks for melting/freezing.
- Saying the sample is pure if it freezes at any temperature, not specifically 0 °C.
- Failing to note that a range of temperatures indicates a mixture.
Mark scheme (4 marks)
- Heat the river water sample and measure/record the temperature at which it melts/freezes (or measure its boiling point / melting point)
- A pure substance melts/boils at a single, sharp (specific) temperature
- If the river water is impure (a mixture), it will melt/boil over a range of temperatures rather than at a single temperature
- Compare the measured melting point to the known melting point of pure water (0 °C) — if it matches sharply, the sample is pure; if it deviates or spans a range, the sample is impure
Key terms in this question
melting point · pure substance · mixture
Related
- All Eduqas GCSE Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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