The properties of solutions which depend on the number of solute particles and are independent of their chemical identity are called colligative properties. These are lowering of vapour pressure, elevation of boiling point, depression of freezing point and osmotic pressure. The process of osmosis can be reversed if a pressure higher than the osmotic pressure is applied to the solution. Colligative properties have been used to determine the molar mass of solutes. Solutes which dissociate in solution exhibit molar mass lower than the actual molar mass and those which associate show higher molar mass than their actual values. Quantitatively, the extent to which a solute is dissociated or associated can be expressed by van’t Hoff factor i. This factor has been defined as ratio of normal molar mass to experimentally determined molar mass or as the ratio of observed colligative property to the calculated colligative property.
Assertion (A): Blood cells swell when they are placed in water containing less than 0.9% (mass/volume) sodium chloride.
Reason (R): Across a semipermeable membrane, solvent molecules flow from a solution of lower concentration to a solution of higher concentration.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
The fluid inside a blood cell has the same osmotic pressure as a 0.9% (mass/volume) sodium chloride solution. The cell membrane lets water through, so water moves between the cell and the outside liquid by osmosis.
Assertion (A): Blood cells swell when they are placed in water containing less than 0.9% (mass/volume) sodium chloride. — True. Water flows into the cells and they swell.
Reason (R): Across a semipermeable membrane, solvent molecules flow from a solution of lower concentration to a solution of higher concentration. — True. This is the direction of osmosis: from the dilute side to the concentrated side.
Why R explains A?
The outside water has less than 0.9% salt, so it is the more dilute liquid, and the cell fluid is the more concentrated one.
By R, water moves into the cells, which is why they swell.
NCERT (2026-27 edition):
1.6.4 Osmosis and Osmotic Pressure – “The flow of solvent from dilute solution to the concentrated solution across a semipermeable membrane is due to osmosis. The important point to be kept in mind is that solvent molecules always flow from lower concentration to higher concentration of solution.”
“…When placed in water containing less than 0.9% (mass/ volume) salt, blood cells swell due to flow of water in them by osmosis.”