An indicator is a substance that changes colour depending on whether a solution is acidic or alkaline. You need to recall the colours of three of them.
- Litmus is red in acid and blue in alkali. In a neutral solution it is purple.
- Methyl orange is red in acid and yellow in alkali.
- Phenolphthalein is colourless in acid and pink in alkali.
Litmus is often used as paper, so you can dip it in a solution or hold it in a gas. Methyl orange and phenolphthalein are used as solutions, a few drops at a time, which makes them common choices for showing when an acid has been neutralised by an alkali.