Indicators of Acids and Bases
An indicator is a substance that changes colour to show whether a solution is acidic or basic. Common ones are litmus (red in acid, blue in base), phenolphthalein (colourless in acid, pink in base) and methyl orange (red in acid, yellow in base).
Common acid–base indicators and their colours
| Indicator | In acid | In base |
|---|---|---|
| Blue litmus | Turns red | Stays blue |
| Red litmus | Stays red | Turns blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
| Turmeric (natural) | Stays yellow | Turns red |
| Red cabbage (natural) | Red / pink | Green / yellow |
Types of indicators
Natural indicators come from plants — litmus (from lichen), turmeric, and red cabbage juice all change colour with acidity. Synthetic indicators like phenolphthalein and methyl orange are made in the lab and give sharper, more reliable colour changes for titrations.
Olfactory indicators (such as onion or vanilla) change smell rather than colour, useful for visually impaired students.
Acids, bases and salts easier to remember, 1-to-1. Try a free chemistry assessment.
Related concepts
Frequently asked questions
Blue litmus turns red in an acid. Red litmus stays red.
Phenolphthalein is pink in a base and colourless in an acid.
A plant-derived substance that changes colour with acidity — for example litmus, turmeric or red cabbage juice.