Solubility of a substance is defined as mass of the solute in grams which dissolves in 100g of a solvent to form a saturated solution. For example, 100g of water at 25° C can dissolve 36.5 g of sodium chloride to obtain a saturated solution. So, the solubility of sodium chloride at 25° C at 36.5g person 100g of water.
If w g of a solute dissolves in W g of a solvent to obtain a saturated solution at any particular temperature and pressure, then
Solubility of the solute = w/W × 100g / 100g of solvent
The solubility depends on the following factors:
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Temperature:
The solubility of solid solutes in liquids usually increases with a rise in temperature, and decreases with a fall in temperature. The solubility of a gas in a liquid usually decreases with a rise in temperature.
Pressure:
Pressure has very little effect only the solubility of solids in liquids. However, the solubility of a gas in a liquid increases with an increase in the pressure of the gas.
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Nature of solute and solvent:
Polar and ionic compounds dissolve more in polar solvents. For example, the solubility of polar compounds which can form hydrogen bonds with water is usually much higher.