Question

State the reasons for the following-(3 marks)
(a) Iron articles loose their shine gradually.
(b) Food should be kept in air tight containers.
(c) Apperance of black coating on silver whenever it is exposed to air.

Answer 1

a) Iron reacts with moisture in the air and forms iron oxide or rust which forms a coating over the surface of the iron article. The shiny surface of iron is now covered by this layer of rust and we feel that the metal has lost its shine.

b)Food, especially oily and fatty food can react on contact with air, get oxidised and produce foul and rancid odours. This is called rancidity. To prevent this, food should be kept in airtight containers.

c)Silver reacts with hydrogen sulphide present in the air to form a layer of silver sulphide, which is black in colour. This coating of Silver Sulphide is the black coating that appears on silver when exposed to air. In short, the reason for appearance of the black coating is the corrosion of silver.