Explain reverse osmosis and the purification of water.

(N/A) The direction of osmosis can be reversed if a pressure larger than the osmotic pressure $(\Pi)$ is applied to the solution side.
In this case,the pure solvent flows out of the solution through the semi-permeable membrane. This phenomenon is called reverse osmosis.
The pressure required for reverse osmosis is quite high. $A$ common porous membrane used is a film of cellulose acetate placed over a suitable support.
Cellulose acetate is permeable to water but impermeable to impurities and ions present in sea water.
These days,many countries use desalination plants based on this principle to meet their potable water requirements.