SALTS

Salts are formed when acids react with bases. Salts are ionic compounds and solutions of salts conduct electricity.

The aqueous solutions of most salts are neutral (pH = 7),  but some salts produce acidic or basic solutions when dissolved in water due to their hydrolysis. Hydrolysis is degradation of a compound by the action of water.

The salts of strong acids and strong bases give neutral solutions (pH = 7). Examples: Sodium Chloride and Potassium Sulphate. The salts of strong acid and weak bases give acidic solutions (ph < 7). Example: Ammonia Chloride. The salts of weak acids and strong bases give basic solutions (pH >7). Example: Sodium carbonate.

Some Salts Used in Everyday Life

Common Salt: Common salt (sodium chloride or NaCl) is obtained from sea water by the process of evaporation. Rock salt (large crystals of common salt) is mined from underground deposits just like coal. Rock salt was formed when the seas dried up due to evaporation thousands of year ago.

Common salt is used:

• For making useful chemicals like sodium hydroxide, sodium carbonate, sodium bicarbonate, hydrochloric acid, hydrogen, chlorine and sodium metal.
• To improve the flavour of food. It is required for the proper functioning of the nervous system, movement of muscles, and generation of hydrochloric acid in the stomach for digestion of food.
• As a preservative in pickles, and for preserving meat and fish.
• In the manufacture of soap.
• In cold countries to melt ice which gets deposited on the roads.

Three useful products are obtained by the electrolysis of sodium chloride solution (also called brine). Sodium hydroxide, chlorine and hydrogen. The process of electrolysis of sodium chloride solution is called chlor-alkali process.

Chlorine is a disinfectant and is used to sterilize drinking water supply and the water in swimming pools. It is used in the manufacture of bleaching powder and hydrochloric acid. It is used to make plastic, such as polyvinyl chloride (PVC), pesticides, chlorofluorocarbons (CFCs), chloroform, carbon tetrachloride, paints, dyes and solvents for dry cleaning (like trichloroethane).

Hydrogen is used in the manufacture of ghee or margarine, hydrochloric acid, ammonia and methanol. Liquid hydrogen is used as a furl for rockets.

Sodium hydroxide and chlorine combine to form sodium hypochlorite (NaClO) which is a bleaching agent.

Washing Soda

Washing soda is chemically sodium carbonate containing 10 molecules of water of crystallisation (Na₂CO₃.10H₂O). Anhydrous sodium carbonate (Na₂CO₃) is known as 'soda ash'. Washing soda has detergent (or cleaning) properties. It attacks dirt and grease to form water soluble products, which are washed away with water. It is also used for removing permanent hardness of water. It is used in the manufacture of glass, soap, paper and compounds such as borax.

Baking Soda

Baking soda is chemically sodium hydrogen carbonate or sodium bicarbonate. It is used as an antacid. It is sometimes added for faster cooking of food. Baking powder, used for making cakes, bread, etc., is a mixture of baking soda and a mild acid, like tartaric acid of citric acid. As long as baking powder is dry, baking soda, NaHCO₃, and tartaric acid do not react. When is mixes with water present in the dough (for bread or cake), reaction occurs between NaHCO₃ and the acid to produce carbon dioxide gas, which makes the cake or bread soft and spongy.

Without the baking powder the cake obtained is hard and small in size. If baking soda is used instead of baking powder, then Na₂CO₃ formed during baking will give a bitter taste to the cake. Tartaric acid present in baking powder nertralises Na₂CO₃ to give sodium tartarate which has a pleasant taste.

Baking soda is used in fire extinguishers. When the knob of the extinguisher is pressed, sulphuric acid gets mixed with NaHCO₃ to produce carbon dioxide gas, which forces a stream of liquid to fall on the burning substance. Carbon dioxide itself forms a blanket around the burning substance and cuts off its supply of air, thus extinguishing the fire.

Bleaching Powder

Bleaching powder is chemically calcium oxychloride, CaOCl₂. It is also called chloride of lime. The real bleaching agent present in bleaching powder is chlorine. Bleaching agent is a substance which removes colour from coloured substances and makes them colourless. Bleaching powder is used for bleaching cotton and linen (textile industry), and wood pulp (paper industry). It is used for disinfecting drinking water supply, and for making wool unshrinkable. It is also used as an oxidising agent.

Plaster of Paris

Plaster of Paris (P.O.P.) is calcium sulphate hemihydrate (half hydrate), CaSO₄.1/2H₂O. It was initially made by heating gypsum (CaSO₄,.2H₂O), which found in Paris. It is a white powder, which sets into a hard mass on wetting with water due to its conversion to gypsum. Plaster of Paris is used for setting fractured bones in the right position. It is used for making casts in dentistry. It is also used as a fire-proofing material. It is also used for making walls and cellings smooth before painting, and for making toys, decorative materials, chalks, and casts for statues.

Water of Crystallisation

Some salts contain a few water molecules as a part of their crystal structure. This water in known as water of crystallisation. Salts which contain water of crystallisation are called hydrated salts.  For example, CuSO₄.5H₂O, FeSO₄.7H₂O, CaSO₄.2H₂O and Na₂CO₃.10H₂O.

The shape of the crystals and in some cases, their colour, is due to the presence of water of crystallisation. Hydrated salts lose their water of crystallisation on strong heating to give anhydrous salts. On strong heating, blue copper sulphate crystals (CuSO₄.5H₂O) lose their water of crystallisation to give anhydrous copper sulphate (CuSO₄). When water is added to anhydrous copper sulphate, it gets hydrated and turns blue.