Saturated and unsaturated solutions

• A solution, in which more quantity of solute can be dissolved without raising its temperature, is called an unsaturated solution.

Or

• A solution which has less than the maximum amount of solute present in it at a given temperature is unsaturated.
• A solution, in which no more solute can be dissolved at that temperature, is called a saturated solution.
• A maximum of 32 grams of potassium nitrate can be dissolved in 100 grams of water at a temperature of 20^oC.
• So, a saturated solution of potassium nitrate at 20^oC contains 32 grams of potassium nitrate dissolved in 100 grams of water.

Effect of temperature on the solubility of salt in water:

• If a saturated solution at a particular temperature is heated to a higher temperature, then it becomes unsaturated.
• This is because the solubility of solute increases on heating and more of solute can be dissolved on raising the temperature of the solution.

SOLUBILITY:

• The maximum amount of a solute that can be dissolved in 100 grams of the solvent to form a saturated solution at a given temperature is known as the solubility of that solute in that solvent.
• Solubilities of salts in solvents are affected by change in temperature.
• While expressing the solubility of a substance, we have to specify the temperature also.

Effect of temperature on the solubility of Salts:

• The effect of temperature depends upon the heat energy changes which accompany the process of dissolution of salt in the solvent.
• If heat energy is absorbed during the process of dissolution it is an endothermic process.
• If heat energy is evolved or released in the process, it is of exothermic nature.

Effect of temperature on endothermic dissolution process

• Salts like sodium chloride, potassium chloride, sodium nitrate, ammonium chloride etc. dissolve in water with the absorption of heat i.e. the process is endothermic.
• The solubility of these salts in water increases with increase in temperature.

Effect of temperature on endothermic dissolution process

• Only few salts like lithium carbonate, sodium carbonate monohydrate, cerium sulphate etc. dissolve in water with the evolution of heat i.e. the process is exothermic.
• The solubility of these salts in water decreases with increase in temperature.
• A maximum of 32 grams of potassium nitrate can dissolved in 100 grams of water 20^oC; therefore, the solubility of potassium nitrate in water is 32 grams at 20^oC.
• A maximum of 36 grams of sodium chloride (common salt) can be dissolved in 100 grams of water at 20^oC, therefore, the solubility of sodium chloride (or common salt) in water is 36 grams 20^oC.
• While expressing the solubility of a substance, we have to specify the temperature also.

Sample Problem

• 12 grams of potassium sulphate dissolves in 75 grams of water at 60^oC. What is its solubility in water at that temperatue?

Solution:

• Here we have been given that 75 grams of water dissolves 12 grams of potassium suphate.
• We have to find how much potassium sulphate will dissolve in 100 grams of water.

Now,

• 75 g of water dissolves = 12 g of potassium sulphate
• So, 100 g of water will dissolve = of potassium sulphate

= 16 g of potassium sulphate

• Thus, the solubility of potassium sulphate in water is 16 g at 60^oC.

Effect of Temperature and Pressure on Solubility:

• The effect of temperature and pressure on the solubility of a substance is as follows:

1. The solubility of solids in liquids usually increases on increasing the temperature; and decreases on decreasing the temperature.
2. The solubility of solids in liquids remain unaffected by the changes in pressure.
3. The solubility of gases in liquids usually decreases on increasing the temperature.
4. The solubility of gases in liquids increases on increasing the pressure; and decreases on decreasing the pressure.