# A student adds a few drops of universal indicator to separate samples of dilute hydrochloric acid, sodium hydroxide solution, and sodium chloride solution. Describe and explain the colours observed, and explain what the colour of the sodium chloride solution tells you about the products formed when an acid and an alkali react together.

> Eduqas A-Level Chemistry — 3.9 Acid-base equilibria · Describe and Explain · 5 marks

## Mark scheme (5 marks)

1. Hydrochloric acid turns universal indicator red (or orange/red) because it is acidic / has a pH below 7
2. Sodium hydroxide turns universal indicator purple (or violet/blue) because it is alkaline / has a pH above 7
3. Sodium chloride solution turns universal indicator green because it is neutral / has a pH of 7
4. The neutral pH of sodium chloride solution shows that an acid and an alkali produce a salt (sodium chloride) and water
5. The reaction between an acid and an alkali is called neutralisation

## Key terms

- [universal indicator](https://www.gradenine.co.uk/glossary/universal-indicator)
- [acid](https://www.gradenine.co.uk/glossary/acid)
- [alkali](https://www.gradenine.co.uk/glossary/alkali)

## Related

- [Revision notes for Eduqas A-Level Chemistry](https://www.gradenine.co.uk/learn)
- [How to answer "Describe and Explain" 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-adds-a-few-drops-ec94d9eb) · Published by Druglandscape Ltd.