Indicators are dyes that change colour depending on whether they are in an acid or an alkali. There are many different types of indicator. Here are a few of them.

Universal Indicator
Universal is the most often used indicator in school labs. It not only tells you whether the solution is acid or alkali, but it also tells you how weak or strong the acid or alkali is.
Using universal indicator and a chart (see below), you can found out the pH (percentage of Hydrogen) of an solution.
Acids have a pH between 1 and 6. 1 is a very strong acid and 6 is a weak acid.
Alkalis have a pH between 8 and 14. 14 is a very strong alkali and 8 is a weak alkali.
If the pH is 7, the solution is neutral. It is not an acid or an alkali. Water is an example of a neutral solution.



Litmus Indicator
Litmus is good for working out definitely whether something is an acid or an alkali. If the litmus goes red, the solution is an acid. If it goes blue, the solution is an alkali. If it does not change colour, then the solution is neutral.