CBSE Class 12 · 2003 · Delhi Compartment · SolutionsShort Answer

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An aqueous solution of sodium chloride freezes below 273 K. Explain the lowering in freezing point of water with the help of a suitable diagram.

Solution

A substance freezes when the vapour pressure of its liquid equals that of its solid.
Dissolved NaCl lowers the vapour pressure of water, so the solution's vapour-pressure curve lies below that of pure water.
The curve of ice is unchanged; the solution's curve now intersects the ice curve at a temperature lower than 273 K.
Hence the solution freezes below 273 K, the depression being

Δ T_{f} = T_{f}^{\circ} - T_{f}

(see figure).

Structure Vapour pressure vs temperature: the solution's curve cuts the ice curve below 273 K (depression of freezing point) — Vapour pressure vs temperature: the solution's curve cuts the ice curve below 273 K (depression of freezing point)

CBSE Class 12 Board · 2003 · Delhi Compartment · Subjective

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