Course 6.1Lesson 1 of 3
An acid is sour and turns blue litmus red. A base is bitter and slippery and turns red litmus blue. An indicator is a substance that shows by its colour if a solution is acidic or basic. We never taste a chemical to test it.
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Lemon juice, vinegar, tamarind and curd are sour. Their sourness comes from acids. Soap, baking soda solution and the washing soda that is used to clean clothes feel slippery, and they taste bitter. They are bases.
Never taste or touch a chemical to find out. We need a safe test. A substance that shows the difference by a change of colour is called an indicator.
Litmus is a purple dye made from a lichen. It is used as paper strips, red and blue.
| Indicator | In an acid | In a base |
|---|---|---|
| Blue litmus | turns red | stays blue |
| Red litmus | stays red | turns blue |
| Phenolphthalein | colourless | pink |
| Methyl orange | red | yellow |
| Turmeric | stays yellow | turns reddish brown |
You can try turmeric and the juice of the China rose (hibiscus) flower at home. The turmeric on a white cloth turns red when it meets soap, and it goes yellow again in lemon juice.
You should find a pH, such as , where litmus shows a base but phenolphthalein still looks as if nothing is there.
Some substances change their smell in an acid or a base. These are olfactory indicators. Onion, vanilla essence and clove oil have a smell in an acid, and they lose their smell in a base. This is how you can check a solution without sight.
This program lists the colours of the indicators in an acid and in a base.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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