Acids, Bases and Salts

Acids and Bases:

Curd, lemon juice, orange juice and vinegar taste sour. These substances taste sour because they contain acids. The chemical nature of such substances is acidic. The acidic word acid comes from the Latin word acere which means sour.

The acids in these substances are natural acids. Baking soda is bitter in taste. If you rub its solution between fingers, it feels soapy. Substances like these which are bitter in taste and feel soapy on touching are known as bases. The nature of such substances is said to be basic. Special type of substances are used to test whether a substance is acidic or basic.

These substances are known as indicators. The indicators change their colour when added to a solution containing an acidic or a basic substance. Turmeric, litmus, china rose petals are some of the naturally occurring indicators.

Natural indicators around us:

§ Litmus:

A natural dye the most commonly used natural indicator is litmus. It is extracted from lichens .It has a mauve (purple) colour in distilled water. When added to an acidic solution, it turns red and when added to a basic solution, it turns blue. It is available in the form of a solution, or in the form of strips of paper, known as litmus paper. Generally, it is available as red and blue litmus paper blue litmus paper.

§ China rose indicator:

China rose indicator turns acidic solutions to dark pink (magenta) and basic solutions to green.

§ Turmeric :

They turn basic solution red.

Neutralisation

The reaction between an acid and a base is known as neutralisation. Salt and water are produced in this process with the evolution of heat. The salt may be acidic, basic or neutral in nature.

Acid+Base à Salt+Water (Heat is evolved)

The following reaction is an example:

Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) àSodium chloride (NaCl) + Water (H2O)

Neutralisations In Everyday Life

v Indigestion

Our stomach contains hydrochloric acid. It helps us to digest food. But too much of acid in the stomach causes indigestion. Sometimes indigestion is painful. To relieve indigestion, we take an antacid such as milk of magnesia, which contains magnesium hydroxide. It neutralises the effect of excessive acid.

v Ant sting

The sting of an ant contains formic acid. When an ant bites, it injects the acidic liquid into the skin. The effect of the sting can be neutralised by rubbing moist baking soda (sodium hydrogen carbonate) or calamine solution, which contains zinc carbonate.

v Soil treatment

Excessive use of chemical fertilisers makes the soil acidic. Plants do not grow well when the soil is either too acidic or acidic too basic. When the soil is too acidic, it basic is treated with bases like quick lime (calcium oxide) or slaked lime (calcium hydroxide). If the soil is basic, organic matter is added to it. Organic matter releases acids which neutralises the basic nature of the soil.

v Factory wastes

The wastes of many factories contain acids. If they are allowed to flow into the water bodies, the acids will kill fish and other organisms. The factory wastes are, therefore, neutralised by adding basic substances.

Acid Rain:

As the name indicates the rain containing excess of acids is called an acid rain. The rain becomes acidic because carbon dioxide, sulphur dioxide and nitrogen dioxide (which are released into the air as pollutants) dissolve in rain drops to form carbonic acid, sulphuric acid and nitric acid respectively. Acid rain can cause damage to buildings, historical monuments, plants and animals.