3b3 Indicators

A substance that changes colour depending on the pH is called an indicator.

There are two types of indicator:
Single indicators show only one colour change and are used to decide if the pH is higher or lower than a certain value. This makes them useful for titrations.
Litmus is a common one (red in acid and blue in alkali).
Methyl Orange (red in acid and orange in alkali) and Phenolphthalein (colourless in acid and purple in alkali) are two others that are often used.

Mixed indicators (like Universal Indicator or Full Range Indicator) show a wide range of colours and can be used to decide the pH of a given solution. They are not much use for titration work.