A salt is a compound of metallic and non-metallic elements or radicals. It is formed by the action of: (1) an acid on an alkali or base (a base is a compound of a positive, i.e., a metallic element, or a group of them called a radical, and OH, the hydroxyl group); (2) an acid and a metal; (3) an acid and a salt.

In forming a salt, the hydrogen of the acid is replaced by the metal or metallic radical. If there is an excess of acid - that is, if the base is only sufficient to combine with part of the hydrogen - only part of the hydrogen is replaced by the metal.

To illustrate: When sulphuric acid and sodium hydrate (NaOH) are mixed, the first action is as follows:

H 2SO 4

+

NaOH

=

HNaSO 4

+

H 2O

Sulphuric Acid

Sodium Hydrate

Acid Sodium Sulphate

Water

The second step in the change is:

HNaSO 4 + NaOH - Na 2SO 4 + H 2O

If excess sulphuric acid is used, NaOH may be formed immediately. The salt formed at first is called an acid salt. If all the hydrogen were replaced it would be called a normal salt. Normal salts have no effect on blue or red litmus paper.

One of the principal sodium salts is sodium carbonate, often caned soda ash, and is represented by the formula Na 2CO 3.

Soda crystals, or sal soda, are made by dissolving soda ash in hot water, and allowing the clear liquid to cool. Crystals then form, having the composition of Na 2CO 3.10H 2O. Soda crystals contain over 60% of water, do not dissolve as readily as does soda ash, and are, therefore, not economical to buy.