CBSE 2025, Set 3

Question

(i) How does sprinkling of salt help in clearing the snow covered roads in hilly areas?

(ii) What happens when red blood cells are kept in 0.5% (mass/vol) NaCl solution? Justify your answer.

(iii) Write an application of reverse osmosis.

Edvaya · Accepted Answer

The correct answer is Subjective. **(i) Salt on snow-covered roads:** Salt (NaCl) dissolves in the thin film of water on snow/ice, forming a solution. This solution has a **lower freezing point** than pure water (colligative property — depression of freezing point). The salt **lowers the freezing point of water** below 0°C (down to about −21°C for concentrated brine), preventing ice from forming and melting existing snow/ice, making it easy to clear. **(ii) RBC in 0.5% NaCl solution:** The normal physiological saline is 0.9% NaCl. A 0.5% NaCl solution is **hypotonic** (less concentrated than the cell contents). **Red blood cells swell up** (and may even burst/haemolyse). **Justification:** Water flows from the hypotonic NaCl solution (lower solute concentration, higher water potential) into the RBC (higher solute concentration) through the semipermeable cell membrane by **endosmosis** (osmosis into the cell). **(iii) Application of reverse osmosis:** **Desalination of sea water** — pressurised sea water is forced through a semipermeable membrane, allowing pure water to pass through while retaining dissolved salts. Used to obtain fresh drinking water from sea water.

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