# A student is investigating how the mass of a reactant affects the amount of product formed. She dissolves copper sulfate solution in a beaker and adds iron filings to displace the copper. She records the mass of the beaker and its contents at the start and at the end of the reaction. Explain how the student should carry out this investigation fairly, and describe one way she could improve the reliability of her results.

> OCR A-Level Chemistry B: Salters (H433) — 1.1 Practical skills assessed · Explain · 5 marks

> Iron displaces copper from copper sulfate solution. The student wants to find out whether using a greater mass of iron filings produces a greater mass of copper. She has access to a balance, a beaker, iron filings, copper sulfate solution, and standard laboratory equipment.

## Mark scheme (5 marks)

1. Only one variable (mass of iron filings) should be changed whilst all other variables are kept constant — this is a fair test / independent variable identified
2. State a control variable that must be kept constant, e.g. volume/concentration of copper sulfate solution, temperature, or same type/size of iron filings
3. Identify an appropriate dependent variable — the mass of copper produced / the change in mass of the beaker and contents
4. Describe how to use the balance correctly — zero/tare the balance before each measurement OR record mass before and after reaction to calculate the change
5. Describe one way to improve reliability — repeat the experiment (at least three times) and calculate a mean / average result

## Key terms

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

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-is-investigating-how-the-7e4c5dbf) · Published by Druglandscape Ltd.