Write four important purposes of making alloy.

 Describe ionic compounds on the basis of the following property:

The strong force of attraction between positive and negative ions.

Describe ionic compounds on the basis of the following property:

Solubility of compounds in water.

Describe ionic compounds on the basis of the following property.

Electrical conductivity.

Show the formation of $\mathrm{NaCl}$ from sodium and chlorine atoms by transfer of an electron (s). 

Give reason for the following:

Zinc can displace copper from copper sulphate solution.

Give reason for the following:

Silver articles become black after some time when exposed to air.