Solubility

Maximum amount of solute which can be dissolved in a specified amount of solvent at constant temperature is called solubility. Solubility is effected by nature of solute and solvent as well as by temperature and pressure.

1.0Solubility Curves

The graphs which show the variation of solubility with temperature are called solubility curves. With its help, solubility of any solute at a particular temperature can be find out.

2.0Solubility of Solid in a Liquid

Polar solutes are soluble in polar solvent and non-polar solutes are soluble in non-polar solvent due to similar intermolecular forces. 

When solid solutes are dissolved in solvent then following equilibrium exists.

Solute + Solvent Solution

Solubility of solids is affected by temperature and pressure according to Le-Chatelier principle. If dissolution is exothermic then solubitlity decreases with increase in temperature if endothermic then solubility increases with increase in temperature. 

Solubility of solids is not affected by pressure significantly since solids are highly incompressible.

3.0Solubility of Gases in Liquid

Certain gases are highly soluble in water like NH₃, HCl etc. and certain gases are less soluble in water like O₂, N₂, He etc. Solubility of gases is highly effected by pressure and temperature. Increasing pressure solubility increases and increase in temperature solubility decreases since during dissolution of gas, pressure of gas decrease and dissolution of gas is exothermic in nature.

Partial pressure : Partial pressure is defined as if a container filled with more than one gas, each gas exerts pressure. The pressure of anyone gas within the container is called is partial pressure.

4.0Henry's Law 

It can be stated as at a constant temperature

The solubility of a gas in a liquid is directly proportional to the partial pressure of the gas present above the surface of liquid or solution.

or

Mole fraction of gas in the solution is proportional to the partial pressure of the gas over the solution.

or

The partial pressure of the gas in vapour phase (P) is proportional to the mole fraction of the gas (X) in the solution. (This is most common definition)

P = K_HX K_H =  Henry's Constant

Henry's Constant depends on nature of gas and temperature. K_H increases with increase in temperature, therefore solubility of gas decreases.

Application of Henry's Law

(1) Solubility of CO₂ in soft drinks increases at high pressure, therefore, their bottles are sealed under high pressure.

(2) Scuba divers must cope with high concentrations of dissolved gases while breathing air at high pressure underwater. Increased pressure increases the solubility of atmospheric gases in blood. When the divers come towards surface, the pressure gradually decreases. This releases the dissolved gases and leads to the formation of bubbles of nitrogen in the blood. This blocks capillaries and creates a medical condition known as bends, which is painful and dangerous to life. To avoid bends, as well as the toxic effects of high concentration of nitrogen in the blood, the tanks used by scuba divers are filled with air diluted with helium (11.7 % helium, 56.2% nitrogen and 32.1% oxygen).

(3) At high altitudes the partial pressure of oxygen is less than that at the ground level. This leads to low concentration of oxygen in the blood and tissues of people living at high altitudes or climbers. Low blood oxygen causes climbers to become weak and unable to think clearly, symptoms of a condition known as anoxia.

5.0Effect of Temperature

Solubility of gases in liquids decrease with rise in temperature. When dissolved, the gas molecules are present in liquid phase and the process of dissolution can be considered similar to condensation and heat is evolved in this process. We have learnt in the last Section that dissolution process involves dynamic equilibrium and thus must follow Le Chatelier’s Principle. As dissolution is an exothermic process, the solubility should decrease with increase of temperature.

Values of Henry's Constant for Some Selected Gases in Water