Chemistry

Tests to distinguish between ferrous and ferric salts
(i) When a ferrous salt solution is treated with a solution of sodium hydroxide, a greenish precipitate of ferrous hydroxide is obtained.
FeSO₄ + 2NaOH → Fe(OH) + Na₂SO₄
When a ferric salt solution is treated with a solution of sodium hydroxide, a brown precipitate of ferric hydroxide is produced.
Fe₂(SO₄)₃ + 6NaOH → 2Fe(OH) + 3Na₂SO₄
(ii) The ferrous salts are generally green-coloured, whereas the ferric salts are brown in colour.
Rusting :
When iron is exposed to moist air, a reddish-brown coating of a mixture of ferric oxide (Fe₂O₃) and ferric hydroxide ((Fe(OH₃)) is deposited on the surface of the metal. This reddish-brown coating is known as rust, and this process is known as rusting. Thus, the slow conversion of iron into a mixture of Fe₂O₃ and Fe(OH) by water and atmospheric oxygen is known as rusting.
Rusting of iron is an oxidation reaction that occurs due to the attack of water and oxygen. It has been found that rusting does not take place in air-free water. It also does not occur in presence of oxygen alone. Both water and oxygen are essential for rusting. Thus, the following conditions are necessary for rusting:
(i) Presence of oxygen or air
(ii) Presence of water or moisture
The process of rusting is continuous. The strength of iron decreases gradually and finally the metal is destroyed completely.
Prevention of Rusting :
Iron can be prevented from rusting by keeping it out of contact with air and water, and also by converting it into an alloy. This can be achieved in the following ways.
1. By covering the surface of iron with grease, paint, varnish, enamel, etc.
2. By galvanizing iron: a thin coating of zinc is deposited on the surface of the iron object. This is done by electroplating. Since zinc does not corrode on exposure to air, zinc metal prevents iron from rusting.
3. By coating the surface of the iron object with chromium, tin, nickel, or aluminium. These metals resist corrosion. Hence, they protect iron from rusting.
4. By converting it into an alloy with chromium and nickel. This alloy is called stainless steel.
Uses of Iron :
1. Iron is used in making household utensils and equipments.
2. Wrought iron and cast iron are largely used in the manufacture of locomotives, railway lines, springs, tubes, etc.
3. Iron finds wide application in house construction, e.g., in the reinforcement of roofs and other parts of buildings.