# A student investigates the reaction between dilute nitric acid and sodium hydroxide solution. She adds a few drops of universal indicator to the sodium hydroxide solution before slowly adding dilute nitric acid from a burette. Describe what the student would observe as the acid is added, and explain why neutralisation has occurred when the indicator shows a green colour.

> Pearson Edexcel International GCSE Chemistry (4CH1) — 3.4 Acids, alkalis and salts · Describe · 4 marks

> Neutralisation is the reaction in which an acid and a base react together to form a salt and water. Universal indicator shows a green colour at pH 7.

## Mark scheme (4 marks)

1. The indicator colour changes from purple/blue/violet (in the alkaline sodium hydroxide) towards green as the acid is added
2. When the indicator is green, the solution is neutral (pH 7), meaning the acid and alkali have reacted in exactly the right proportions
3. The hydrogen ions (H⁺) from the nitric acid have reacted with the hydroxide ions (OH⁻) from the sodium hydroxide
4. The products of the neutralisation reaction are a salt (sodium nitrate) and water

## Key terms

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

## Related

- [Revision notes for Pearson Edexcel International GCSE Chemistry (4CH1)](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-investigates-the-reaction-between-be0bf863) · Published by Druglandscape Ltd.