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Acids, bases and salts are three main categories of chemical compounds. These have certain definite properties which distinguish one class from the other. The sour taste of many fruits and vegetables, lemon for instance, is due to various types of acids present in them. The digestive fluids of most animals and humans also contain acids. The word 'acid' is derived from a Latin word, which means "sour". The acids we use in the laboratory are stronger acids like hydrochloric acid and sulphuric acid. Strong acids are corrosive and can burn your skin. Bases on the other hand are the chemical opposites of acids. They are bitter in taste and soapy to touch. Sea water and detergents are some examples of substances that are basic. Many bases are oxide or hydroxide compounds of metals. Strong bases can also burn ones skin.
Tasting a substance is not a good way of finding out if it is an acid or a base! Acids and bases can be better distinguished with the help of indicators. Indicators are substances that undergo a change of colour with a change of acidic, neutral or basic medium. Litmus, a purple dye extracted from the lichen plant, is commonly used as an indicator in laboratories. Acids change the colour of litmus solution to red, and bases change the colour of litmus solution to blue. Turmeric is another common household indicator. A stain of turmeric based food spill on a white cloth becomes reddish-brown when soap is scrubbed on it. Soap is basic in nature and changes the colour of the turmeric stain. It turns yellow again when the cloth is washed with plenty of water.
The common indicators used and the colour changes observed are mentioned below:
| Indicator
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Acid
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Alkali
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| Litmus
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Red
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Blue
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| Methyl orange
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Pink
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Yellow
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| Phenolphthalein
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Colourless
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Deep pink
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