A student notices that their repeated measurements of the boiling point of a salt solution give values that are all slightly higher than the accepted boiling point of pure water, and that the values are all very close to each other. Explain what these results tell the student about the accuracy and repeatability of their measurements.

OCR GCSE Physics A: Gateway Science (J249) — P9 Practical skills · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A student measures the boiling point of a salt solution three times and records: 101.8 °C, 101.9 °C, and 101.7 °C. The accepted boiling point of pure water at standard pressure is 100.0 °C.

Model answer (4 marks)

The three measurements are very close to each other (101.8, 101.9 and 101.7 °C), so the experiment is repeatable – the values are within a small range and show good precision.
The mean value (101.8 °C) is about 1.8 °C higher than the accepted boiling point of pure water (100.0 °C). Therefore the results are not accurate – they do not match the true value.
The consistent offset from the accepted value indicates a systematic error, such as the presence of dissolved salt.
Because adding salt raises the boiling point (boiling point elevation), the higher temperature is expected for a salt solution and is not an error in the measurement itself.

Examiner tips

  • Show the mean and the small spread to prove repeatability. Explain the difference from 100.0 °C as a lack of accuracy. Mention systematic error and the effect of salt. Use the term "boiling point elevation" to justify the higher value.

Common mistakes

  • Confusing accuracy with precision – saying the results are accurate because they are close to each other. Forgetting to calculate the mean or to state the spread. Ignoring the effect of salt and claiming the higher temperature is an error in the method.

Mark scheme (4 marks)

  1. The results are repeatable because the values are all very close to each other / there is only a small range between the measurements.
  2. The results are not accurate because they are all far from / do not match the accepted value (100.0 °C).
  3. The consistent difference from the accepted value suggests a systematic error in the investigation.
  4. The higher boiling point is expected / not an error because dissolving salt in water raises its boiling point, so the accepted value for pure water does not apply here.

Key terms in this question

accuracy · repeatability

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