(a) Define ideal solutions. Write one of its characteristics.

(b) .How is the vapour pressure of a solvent affected when a non-volatile solute is dissolved in it ? 

(a)An ideal solution is the solution which obeys Raoult's law exactly over entire range of concentration. Such solutions are formed by mixing the two components which are identical in molecular size, in structure and have almost identical intermolecular forces.
The characteristics of ideal solutions:
(1) It must obey Raoult's law.
(b)  When a non volatile solute is added to a solvent, the vapour pressure of the resulting solution is lower than the vapour pressure of pure solvent.
This is because, the surface area is partly occupied by non volatile solute molecules and partly by solvent molecules. Due to this, the rate of evaporation is lowered.