A student investigates how the density of different liquids varies. She measures the mass and volume of five different liquid samples and records her results. Describe how the student should ensure her results are valid and reliable, and explain how she should identify and deal with any anomalous results.

Edexcel A-Level Physics (9PH0) — 1.1 Practical and investigative skills · Describe · 5 marks · View as Markdown

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

A student is planning an investigation to measure the density of five different liquids using a measuring cylinder and a balance. She intends to use her results to draw a conclusion about how density varies between the liquids.

Model answer (5 marks)

1. Control variables that could affect the measurements, e.g. keep the temperature constant and use the same volume of liquid each time.
2. Repeat each measurement at least three times and calculate a mean to improve reliability.
3. Use suitable equipment with adequate resolution – a balance that measures to 0.1 g and a measuring cylinder with small graduations.
4. Identify an anomalous result as one that does not fit the pattern of the other values or lies well outside their range.
5. Exclude the anomalous value from the mean, then repeat the measurement to confirm whether it is a genuine outlier or a one‑off error.

Examiner tips

  • Use the exact wording from the mark scheme – e.g. "control all variables" and "repeat each measurement".
  • Show the five steps in order and use bullet points or numbering to make the answer clear.

Common mistakes

  • Failing to mention the need for a suitable resolution on the balance and cylinder.
  • Including the anomalous result in the mean instead of excluding it.

Mark scheme (5 marks)

  1. She should control all variables that could affect the measurements, such as temperature or the volume of liquid used each time.
  2. She should repeat each measurement (at least three times) and calculate a mean to improve reliability.
  3. She should use appropriate measuring equipment with a suitable resolution, e.g. a balance measuring to at least 0.1 g and a measuring cylinder with small graduations.
  4. An anomalous result is one that does not fit the pattern of the other results / lies well outside the range of the other values.
  5. Anomalous results should not be included when calculating the mean; she should repeat the measurement to check whether it is a genuine outlier or a one-off error.

Key terms in this question

density · anomalous result · valid · reliable

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