A student investigates how the time taken for a piece of toast to cool down depends on its initial temperature. She heats the toast to different starting temperatures and records the time taken for it to cool to room temperature. Explain how the student could ensure her investigation produces valid and reliable results.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The student uses a thermometer to measure temperature and a stopwatch to measure time. She is concerned that her results may not be trustworthy.
Model answer (5 marks)
1. Use a constant room temperature as a control variable – keep the ambient temperature the same for all trials.
2. Controlling room temperature ensures that any change in cooling time is due only to the initial toast temperature, making the results valid.
3. Perform at least three trials for each initial temperature and record the cooling time each time.
4. Calculate the mean cooling time for each initial temperature and discard any outlier readings that are far from the mean.
5. Choose a suitable range of initial temperatures (e.g. 70 °C to 120 °C) and use enough different values (at least five) to cover the range and give a clear trend.
2. Controlling room temperature ensures that any change in cooling time is due only to the initial toast temperature, making the results valid.
3. Perform at least three trials for each initial temperature and record the cooling time each time.
4. Calculate the mean cooling time for each initial temperature and discard any outlier readings that are far from the mean.
5. Choose a suitable range of initial temperatures (e.g. 70 °C to 120 °C) and use enough different values (at least five) to cover the range and give a clear trend.
Examiner tips
- Use the word ‘control variable’ explicitly. Show the calculation of a mean and explain why outliers are removed. Mention the need for a sufficient number of different initial temperatures.
Common mistakes
- Failing to mention room temperature as the control variable. Not calculating a mean or ignoring anomalous data. Using too few initial temperatures or too few trials per temperature.
Mark scheme (5 marks)
- Identify and control a relevant control variable
- Explain why controlling that variable makes the results valid
- Repeat measurements / take repeat readings at each starting temperature
- Use the repeated values to calculate a mean / identify and exclude anomalous results
- Use an appropriate range of starting temperatures / take a sufficient number of different values of the independent variable
Key terms in this question
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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