A student investigates the specific latent heat of vaporisation of water. She heats water in an insulated container until it boils steadily, then records the mass of water that turns to steam over a fixed time interval. She repeats the experiment three times and notices that her three values for the mass of water evaporated are slightly different each time. Explain why repeating the experiment improves the reliability of her result, and describe what she should do with her three mass values to obtain the best estimate of the true mass evaporated.

OCR A-Level Physics B: Advancing Physics (H557) — 1.1 Practical skills assessed · Explain · 5 marks · View as Markdown

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

The student's three recorded mass values for water evaporated are close together but not identical.

Model answer (5 marks)

Repeating the experiment allows the student to identify anomalous (outlier) results, reducing the influence of random error. She should check for and discard any anomalous value that is clearly different from the others before calculating a mean. Then she should calculate the mean (average) of the remaining valid values. This mean gives a more reliable estimate of the true mass evaporated because it is based on more than one measurement.

Examiner tips

  • Use the word ‘average’ or ‘mean’ when describing the calculation.
  • Mention that discarding outliers improves accuracy.
  • Explain that more data reduces random error.
  • Show the calculation steps if asked to compute the mean.

Common mistakes

  • Using the median instead of the mean when the mark scheme specifies mean.
  • Including an outlier in the average.
  • Failing to explain why repetition reduces random error.

Mark scheme (5 marks)

  1. Repeating the experiment allows the student to identify anomalous (outlier) results
  2. Repeating reduces the effect of random error on the final result
  3. She should check for and discard any anomalous result (one that is clearly different from the others) before calculating a mean
  4. She should calculate the mean (average) of the remaining valid values
  5. This mean value gives a more reliable estimate of the true mass evaporated because it is based on more than one measurement

Key terms in this question

specific latent heat of vaporisation · reliability

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