# A student is investigating how the volume of oxygen produced changes over time when hydrogen peroxide decomposes in the presence of a catalyst. Describe how the student should carry out a fair test to compare the effect of two different catalysts on the rate of this reaction, and explain why controlling variables is important in this investigation.

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

> Hydrogen peroxide solution decomposes to form water and oxygen. The rate at which oxygen is produced can be used as a measure of the rate of reaction. A student wants to compare how effectively manganese(IV) oxide and iron(III) oxide act as catalysts for this decomposition.

## Mark scheme (4 marks)

1. Keep one variable the same (e.g. volume/concentration of hydrogen peroxide, mass/surface area of catalyst, temperature) to make the test fair
2. Change only the independent variable — the type of catalyst used (manganese(IV) oxide or iron(III) oxide)
3. Measure the dependent variable in the same way for both catalysts — e.g. measure the volume of oxygen collected at regular time intervals
4. Controlling variables is important because it means any difference in the rate of reaction can be attributed to the catalyst (independent variable) alone / ensures results are valid and a fair comparison can be made

## Key terms

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

## 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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-is-investigating-how-the-a4984df7) · Published by Druglandscape Ltd.