Acids and bases around us
Lemon juice, vinegar and tamarind taste sour because they contain acids. The word "acid" comes from the Latin acere, meaning sour.
Substances such as baking soda, soap and lime water feel soapy to the touch and taste bitter. They are bases.
Acid | Found in |
|---|---|
Acetic acid | vinegar |
Citric acid | lemons, oranges |
Lactic acid | curd |
Tartaric acid | tamarind, grapes |
Formic acid | ant sting |
Base | Found in |
|---|---|
Calcium hydroxide | lime water |
Ammonium hydroxide | window cleaner |
Sodium hydroxide / potassium hydroxide | soap |
Magnesium hydroxide | milk of magnesia (an antacid) |
Safety note: never taste or touch an unknown substance to test it. Many acids and bases are harmful. Scientists use indicators instead.
Indicators
An indicator is a substance that changes colour depending on whether it is in an acidic or a basic solution. Solutions that are neither acidic nor basic are neutral. Pure water and common salt solution are neutral.
Litmus
Litmus is the most common natural indicator. It is extracted from lichens and is used as a solution or as litmus paper strips.
An acid turns blue litmus red.
A base turns red litmus blue.
A neutral solution does not change the colour of either.
Turmeric
Turmeric is yellow. In a basic solution it turns red. Acids do not change its colour. This is why a turmeric stain on a white cloth turns red when it is washed with soap, which is basic.
China rose (hibiscus)
A solution made from china rose petals turns acidic solutions dark pink (magenta) and basic solutions green.
Summary of indicator colours
Indicator | In acid | In base |
|---|---|---|
Blue litmus | turns red | no change |
Red litmus | no change | turns blue |
Turmeric | no change | turns red |
China rose | dark pink | green |
Neutralisation
When an acid and a base are mixed in the right amounts, they cancel each other's effect. This is neutralisation:
acid + base → salt + water, and heat is released
Example. Hydrochloric acid reacts with sodium hydroxide to give sodium chloride (common salt) and water.
The mixture becomes warm because neutralisation releases heat.
Neutralisation in daily life
Indigestion: our stomach contains hydrochloric acid. Too much of it causes acidity and pain. An antacid such as milk of magnesia (a base) neutralises the excess acid.
Ant bite: an ant sting injects formic acid. Rubbing on a mild base such as moist baking soda or calamine solution brings relief.
Soil treatment: if soil is too acidic, farmers add quicklime or slaked lime (bases). If it is too basic, they add organic matter (compost), which releases acids.
Factory waste: acidic waste from factories is neutralised with bases before it is released, so that it does not harm aquatic life.