# Indicator

A dye that changes colour depending on whether a solution is acidic or alkaline. Universal indicator gives a range of colours that match the pH.

Chemistry (school) · Part 06: Acids, bases and salts · Card 4 of 9 · Last updated 2026-10-19

## Analogy

It works like a traffic light for acids: red for acid, green for neutral, purple for strongly alkaline.

## Example

**Universal indicator**

Red/orange: acid
Green: neutral
Blue/purple: alkali

## In practice

Litmus paper turns red in acid and blue in alkali.

## Practice

### Flashcards

- **What colour is litmus in acid and in alkali?** Red in acid and blue in alkali.

## Questions people ask

### What is the difference between litmus and universal indicator?

Litmus has only two colours: red for acid and blue for alkali. It says which side of 7 you are on. Universal indicator is a mixture of dyes that goes through many colours, so it also shows roughly how strong the acid or alkali is.

### Which indicator is best for a titration?

One with a sharp colour change at the end point, such as methyl orange or phenolphthalein. Universal indicator changes gradually through many colours and is hard to judge exactly. A single sharp change lets you tell the moment the acid and alkali have neutralised.

## Related concepts

- [pH scale](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-6/chemistry-school-part-6-concept-3)

Web page: https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-6/chemistry-school-part-6-concept-4
Study this card in the app: https://studyvisually.com/app/learn?category=chemistry-school&card=chemistry-school-part-6-concept-4