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Solid-Dissolved Solid in Equilibrium

If we dissolve a solid phase (say, sugar) in a liquid phase (say, water), we find that the solid phase stops dissolving after a certain stage. The resultant solution is said to be a saturated solution of the given solid in the given liquid. At this stage, un dissolved solid exists in equilibrium with the dissolved solid and can be represented as

un dissolved solid ⇔ dissolved solid

As usual, this is a dynamic equilibrium. The dynamic nature can be demonstrated by adding solute containing radioactive species. After sometime, it is observed that the solution as well as the earlier un dissolved solute acquires radioactive species Fig.

The maximum mass of solid dissolved per unit mass of liquid (sometime also expressed as mass per unit volume of liquid) is known as solubility of the solid in the said liquid. The process of dissolution is associated with the absorption (endothermic) or evolution (exothermic) of heat. The effect of temperature on the solubility depends upon whether the dissolution process is exothermic or endothermic. For exothermic dissolution, the solubility decreases with increase in temperature (for example, anhydrous sodium sulphate), whereas for endothermic dissolution, the solubility increases with increase in temperature (for examples, hydrated sodium sulphate, ammonium nitrite and ammonium nitrate).

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