Acid Base Indicators

We can recognise various food stuff as acids and bases just by their taste.

This is because acids generally have a sour taste.

While bases have a bitter taste.

But it is not always recommended to identify a substance by taste. We, therefore, introduced new substances called indicators.

Indicators are substances which indicate the presence of acids or bases.

They do so by either changing their colour or odour.

Turmeric, a common Indian spice is an example of acid- base indicator.

It turns red in the presence of a base, but does not change its colour in an acid.

Another common indicator is litmus, derived from a plant called lichen.

Acids turn litmus solutions red, while bases turn them to blue.

Litmus solutions can become difficult to handle. So, instead paper strips lined with litmus solution are prepared.

The paper strips can be either blue or red.

Acids turn blue litmus paper red, while bases turn red litmus paper blue.

While indicators such as vanilla and onion change their odour in the presence of acids and bases.

Both vanilla and onion retain their smell in the presence of acid, but lose it in the presence of bases.

These indicators based on change in odour are called olfactory indicators.

Indicators, such as turmeric, litmus, onion and vanilla, found naturally, are called natural indicators

The following card gives a few more examples of natural indicators and their colour change in acids and bases.

Not all indicators used in distinguishing acids and bases are obtained naturally.

Indicators synthesised in laboratories are called synthetic indicators.

Phenolphthalein, a colourless solution, is a common synthetic indicator.

When added to a basic solution, it turns pink.

On adding acid to the same solution, the pink colour starts disappearing.

Further addition of the acid does not affect the colour of solution.

This means phenolphthalein is pink in basic solution, and colourless in neutral and acidic solutions.

Other examples of synthetic indicators are methyl orange and thymol blue.


Indicators indicate the presence of acids and bases by change in colour or odour.

Indicators which change their odour in the presence of acids and bases are called olfactory indicators.

Indicators are classified into natural and synthetic.

The End