Which of the following method is used for the purification of water?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Reverse osmosis
In osmosis, solvent flows through a semipermeable membrane from the pure solvent into the solution. If a pressure larger than the osmotic pressure is applied on the solution side, the flow is reversed and pure solvent is squeezed out of the solution through the membrane. This is reverse osmosis; it is used for the desalination of sea water and the purification of drinking water, with membranes such as cellulose acetate that let water through but hold back the dissolved ions.
Why the other options are incorrect
Option (1) Ebullioscopic technique — Incorrect. Measurement of the elevation of boiling point, used to find molar masses; it does not purify water.
Option (3) Osmosis — Incorrect. Water flows into the more concentrated solution, so ordinary osmosis dilutes the salt water instead of separating pure water from it.
Option (4) Crystallization — Incorrect. Used to purify solids by separating them from a solution, not to obtain pure water on a large scale.
NCERT (2026-27 edition):
Reverse Osmosis and Water Purification – “The direction of osmosis can be reversed if a pressure larger than the osmotic pressure is applied to the solution side. That is, now the pure solvent flows out of the solution through the semi permeable membrane. This phenomenon is called reverse osmosis and is of great practical utility. Reverse osmosis is used in desalination of sea water.”