# 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.

> OCR GCSE Chemistry A: Gateway Science (J248) — C7 Practical activity skills · Describe · 4 marks

> 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.

## Mark scheme (4 marks)

1. Keep one variable the same throughout, e.g. the mass/size of marble chips OR the volume of acid OR the temperature
2. Only change the independent variable (concentration of hydrochloric acid)
3. Controlling variables means any change in results is due to the concentration only / allows a valid comparison to be made
4. Repeat the experiment at each concentration and calculate a mean / repeat to identify anomalous results

## Key terms

- [fair test](https://www.gradenine.co.uk/glossary/fair-test)

## Related

- [Revision notes for OCR GCSE Chemistry A: Gateway Science (J248)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-is-investigating-how-the-cde85d66) · Published by Druglandscape Ltd.