Indicators

When an acid is dissolved in water we get an acidic solution. When a base dissolves in water it is an alkali and makes an alkaline solution. If a solution is neither acidic nor alkaline it is neutral. Pure water is neutral, and so is paraffin.

Indicators are substances that change colour when they are added to acidic or alkaline solutions. Litmus, phenolphthalein, and methyl orange are all indicators that are commonly used in the laboratory.

Litmus

Litmus indicator solution turns red in acidic solutions, blue in alkaline solutions, and purple in neutral solutions.

Litmus paper comes as red litmus paper and blue litmus paper. The table shows the colour changes it can make:

Red LitmusBlue Litmus
Acidic solutionStays redTurns red
Neutral solutionStays redStays blue
Alkaline solutionTurns blueStays blue
Blue litmus paper is dipped in a beaker of acid and turns red.
Acids turn blue litmus paper red
Red litmus paper is dipped in a beaker of alkali and turns blue.
Alkalis turn red litmus paper blue

Other indicators

The table shows the colour of methyl orange and phenolphthalein in solutions of different pH:

IndicatorAcidicNeutralAlkaline
Methyl orangeRedYellowYellow
PhenolphthaleinColourlessColourlessPink