Indicators

Lemon juice and vinegar taste sour: they contain acids. Baking soda tastes bitter and feels soapy: it is a base. But in a lab you must never taste anything to find out. Instead we use an indicator: a substance that changes colour (or smell) in an acid or a base.

Litmus is the best-known indicator. It is a purple dye made from lichen. An easy way to remember what it does:

• ABR: Acids turn Blue litmus Red.
• BRB: Bases turn Red litmus Blue. (B for base, B for blue.)

Many indicators are in your kitchen. Turmeric is one. Spill some dal or curry on a white shirt and wash it with soap: the yellow stain turns reddish-brown, because soap is basic. Rinse it with plenty of water and it turns yellow again. Red cabbage leaves and the coloured petals of flowers such as Hydrangea, Petunia and Geranium also change colour.

Chemists also use synthetic (made) indicators:

Some indicators change smell, not colour. These are olfactory indicators. Onion, vanilla essence and clove oil lose their smell in a base. Why does this matter? A student who cannot see can still test a solution: by smell.