# A student is investigating how the mass of copper sulfate crystals produced changes when different volumes of water are evaporated from a copper sulfate solution. Describe how the student could make this a fair test and explain why controlling variables is important when drawing conclusions from the results.

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

> A student dissolves copper sulfate powder in hot water and then evaporates different volumes of the solution to see how much crystalline copper sulfate forms. She wants to compare her results across several trials.

## Mark scheme (4 marks)

1. Keep one variable the same (e.g. the initial concentration/volume of copper sulfate solution used each time) to make it a fair test
2. Identify a specific control variable that must be kept constant (e.g. starting temperature, same copper sulfate solution concentration, same evaporation method/time)
3. If only one variable is changed at a time, any change in the dependent variable (mass of crystals) can be attributed to that variable
4. If variables are not controlled, you cannot tell which variable caused the change in results / conclusions would not be valid

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-is-investigating-how-the-e7b7db4c) · Published by Druglandscape Ltd.