Corrosion

The chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties is called corrosion.

The corrosion causes damage to buildings, ships and many other articles especially made of iron. During corrosion, metal gets changed into its oxide, sulphide, carbonate etc.

The most common example of corrosion is rusting of iron.

1.0Rust

Iron corrodes readily when exposed to moisture and gets covered with a brown flaky substance called rust. It is called rusting of iron.

Rust is hydrated Iron (III) oxide [Fe₂O₃.2H₂O]

2.0Some more examples of corrosion

(i) Copper reacts with moist carbon dioxide in the air and slowly loses its shiny brown surface and acquires a green coating of basic copper carbonate.

(ii) Silver articles become black after sometime when exposed to air.

3.0Rancidity

The oxidation of oils or fats in food, resulting in a bad taste and bad smell is called rancidity. It is caused due to prolonged exposure of food in the air. Oxygen present in air oxidizes fats / oils present in food and forms volatile substances, which have bad odour.

Prevention of Rancidity

(i) Rancidity can be prevented by adding antioxidants to foods containing fats and oils. Antioxidants are reducing agents which when added to food, the food do not get oxidised easily and hence do not turn rancid easily.

Common Antioxidants

(a) BHA (Butylated Hydroxy Anisole)

(b) BHT (Butylated Hydroxy Toluene)

(i) Vitamin-E (tocopherol) and vitamin-C (ascorbic acid) are the two naturally occuring antioxidants.

(ii) Rancidity can be prevented by packaging fat and oil containing foods in the presence of nitrogen gas.

(iii) It can be retarded by keeping food in the refrigerator.

(iv) It can also be retarded by storing food in airtight containers.

(v) It can be retarded by storing foods away from light.

4.0Crystallization

Crystallization is the solidification of atoms or molecules into a highly structured form called a crystal. Usually, this refers to the slow precipitation of crystals from a solution of a substance.

The process of obtaining crystals of pure substances from their saturated solution is called crystallisation. Substances like sugar, salt, urea and copper sulphate can be obtained by crystallisation.

Crystallisation is an example of physical change because it is reversible and involves only changes in shape and size. For example, copper sulphate is a blue-coloured crystal that is obtained by heating the aqueous solution of copper sulphate.

5.0Also Read

What is Air Pollution?	Solution And Its Components	Treatment of Water
Oxygen	Colloidal Solution	Separation of The Gases of The Air
Benzoyl Peroxide	Separating The Components of A Mixture	Physical and Chemical Changes of Materials