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A beaker containing pure water and one containing a solution are placed side by side under a receiver.
 | Fig: Isothermal distillation |
Each liquid emits aqueous vapour into the space. If the, vapour pressures were the same, equilibrium would be established. But the vapour pressure of the water is higher than that of the solution, hence it tends to saturate the space with vapour under a higher pressure than can remain in equilibrium with the solution. Vapour condenses on the latter, and the water is transferred to the solution by isothermal distillation. Equilibrium is reached when all the pure water is evaporated and absorbed by the solution.
Many solid salts, such as potassium carbonate and calcium chloride, become damp on exposure to moist air, and in time liquefy completely. This is called deliquescence. All deliquescent substances are very soluble in water. In moist air, a little saturated solution is formed. Since this is very concentrated, its vapour pressure is less than the partial pressure of aqueous vapour in the atmosphere. Moisture is attracted by the salt, which gradually liquefies to a saturated solution. The latter goes on absorbing aqueous vapour until its dilution is such that the vapour pressure is equal to the partial pressure of water vapour in the air.
Solid substances which attract moisture without liquefaction, such as recently-ignited charcoal, and liquids such as alcohol and sulphuric acid which absorb moisture, are called hygroscopic.
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