An aqueous solution of sodium chloride freezes below 273K. Explain the lowering in freezing points of water with the help of a suitable diagram.
We know that freezing point of pure water is 00C or 273K. At 00C water exist as ice. At freezing point the solid and liquid phases have the same vapour pressure. Now, the freezing point of pure water is fixed. Because the vapour pressure of pure water is fixed. But, if we add a non-volatile solute such as salt in water , the freezing point of the pure water is lowered as the vapour pressure of the water is decreased due to presence of non volatile solute particles at the surface of the water.That is water containg salt is freeze at lower temperatures.This is known as depression of freezing point in presence of non-volatile solute as shown below: