# A student is investigating how the concentration of hydrochloric acid affects the pH of the solution. She measures the pH of five different concentrations of hydrochloric acid using a pH meter and records her results. Describe how the student should carry out this investigation to ensure her results are valid and reproducible, and explain one safety precaution she should take.

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

> A student wants to find out whether increasing the concentration of hydrochloric acid produces a lower pH reading. She has access to a pH meter, a range of hydrochloric acid solutions of different concentrations, distilled water, and standard laboratory glassware.

## Mark scheme (5 marks)

1. Use the same volume of hydrochloric acid solution each time (control variable)
2. Rinse the pH meter (or probe) with distilled water between each measurement to avoid contamination / cross-contamination
3. Take repeat readings (at least three) for each concentration and calculate a mean
4. Change only the concentration of hydrochloric acid (independent variable) and keep all other variables constant
5. Wear eye protection (safety goggles) because hydrochloric acid is corrosive / irritant and could damage eyes

## Key terms

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

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-is-investigating-how-the-ec36d327) · Published by Druglandscape Ltd.