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 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 Litmus||Blue Litmus|
|Acidic solution||Stays red||Turns red|
|Neutral solution||Stays red||Stays blue|
|Alkaline solution||Turns blue||Stays blue|
The table shows the colour of methyl orange and phenolphthalein in solutions of different pH: