Question

Write scientific reasons.
a. Lemon or tamarind is used for cleaning copper vessels turned greenish.
b. Generally the ionic compounds have high melting points.
c. Sodium is always kept in kerosene.
d. Pine oil is used in froth flotation.
e. Anodes need to be replaced from time to time during the electrolysis of alumina.

Answer 1

a. Copper vessels turned greenish due to the formation of copper carbonate layer. The citric acid present in the lemon or tamarind neutralises the basic copper carbonate and dissolves the layer. That is why, tarnished copper vessels are cleaned with lemon or tamarind juice to give the surface of the copper vessel its characteristic lustre.

b. In an ionic compound there is strong electrostatic force of attraction between opposite charged ions. To overcome these forces a considerable amount of energy is needed. Therefore, ionic compounds have high melting points.

c. Sodium is a very reactive metal. It is kept in kerosene to prevent it from coming in contact with oxygen and moisture. If this happens, it will react with the moisture present in air and form sodium hydroxide. This is a strongly exothermic reaction and lot of heat is generated.

d. Pine oil is added in the froth flotation method to create froath or bubble so that metal can be purify easily because pine oil prevents the ore from gangue for furthur mixing. Pine oil also acts as best substance for forming froth for the minerals. It also increases the non wettability of mineral particles.

e. In the electrolysis of alumina, graphite rod is used as anode.

• During the electrolytic reduction of alumina, aluminium is produced at the cathode and oxygen gas is evolved at the anode.
• This gas reacts with the graphite rods (anode) and forms carbon dioxide.

Thus, the graphite rods are continuously eroded. Hence, the graphite rods i.e. anodes be replaced from time to time during the electrolysis of fused alumina.