A student is investigating how the concentration of hydrochloric acid affects the rate of reaction with marble chips. Describe how the student should make their investigation a fair test and explain why controlling variables is important when drawing conclusions.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student wants to find out whether using a higher concentration of hydrochloric acid produces gas more quickly when reacted with marble chips. They plan to measure the volume of gas collected over a set time period.
Model answer (4 marks)
1. Keep the mass/size of the marble chips constant for all trials.
2. Keep the volume of acid and the temperature of the reaction constant.
3. Only vary the concentration of hydrochloric acid.
4. Repeat each trial and calculate a mean volume of gas for each concentration.
5. Controlling variables ensures that any change in the rate of gas production is due solely to the concentration of acid, allowing a valid comparison between trials.
2. Keep the volume of acid and the temperature of the reaction constant.
3. Only vary the concentration of hydrochloric acid.
4. Repeat each trial and calculate a mean volume of gas for each concentration.
5. Controlling variables ensures that any change in the rate of gas production is due solely to the concentration of acid, allowing a valid comparison between trials.
Examiner tips
- Use numbered points to show each control clearly. Include a sentence explaining that controlling variables isolates the independent variable. Show that you will repeat trials and calculate a mean to reduce random error.
Common mistakes
- Changing more than one variable at a time. Failing to repeat trials or calculate a mean. Not explaining why controlling variables is important.
Mark scheme (4 marks)
- Keep one variable the same throughout, e.g. the mass/size of marble chips OR the volume of acid OR the temperature
- Only change the independent variable (concentration of hydrochloric acid)
- Controlling variables means any change in results is due to the concentration only / allows a valid comparison to be made
- Repeat the experiment at each concentration and calculate a mean / repeat to identify anomalous results
Key terms in this question
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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