2 Solubility

Solubility

• Solubility of a substance is the maximum amount of the substance that can be dissolved in a specified amount of a solvent at a given temperature.

Factors on which solubility depends:

• • Nature of solute and solvent
  • Temperature
  • Pressure

Solubility of a Solid in a Liquid

When a solute is dissolved in a solvent, the following dynamic equilibrium is established.

Solute + Solvent    Solution

This equilibrium follows Le Chatelier’s Principle.

Effect of temperature:

According to Le Chatelier’s principle, in a nearly saturated solution, if the dissolution process is −

• • Endothermic (Δ_solH > 0), then the solubility will increase with the increase in temperature
  • Exothermic (Δ_solH < 0), then the solubility will decrease with the increase in temperature

Effect of pressure:

• • Pressure has no significant effect on the solubility of solids in liquids.

Reason − Solids and liquids are negligibly affected by pressure as they are highly incompressible.

Solubility of a Gas in a Liquid

• • The solubility of gases in liquids increases with the increase in pressure.
  • Henry’s law − The solubility of a gas in a liquid is directly proportional to the pressure of the gas.

Or

The partial  pressure of a gas in  vapour  phase  (p) is  directly  proportional  to the mole fraction of the gas (x) in the solution,

i.e.                               p = K_Hx

Where, K_H = Henry’s law constant

The plot of the partial pressure of HCl vs. its mole fraction in a solution of cyclohexane is given below.

Some important applications of Henry’s law

(i) Bottles are sealed under high pressure to increase the solubility of CO₂ in soft drinks and soda water.

(ii) Henry’s law states that the solubility of gas increases with an increase in pressure. Therefore, when a scuba diver dives into the sea, the increased sea pressure causes the nitrogen present in air to dissolve in his blood in great amounts. As a result, when he comes back to the surface, the solubility of nitrogen again decreases and the dissolved gas is released, leading to the formation of nitrogen bubbles in the blood. This results in the blockage of capillaries and leads to a medical condition known as ‘bends’, which are painful and dangerous to life. Hence, the oxygen tanks used by scuba divers are filled with air and diluted with helium to avoid bends.

(iii) The concentration of oxygen is low in the blood and tissues of climbers or people living at high altitudes. This is because at high altitudes, the partial pressure of oxygen is less than that at ground level. Low blood oxygen causes climbers to become weak and prevents them from thinking clearly. These are symptoms of ‘anoxia’.

Effect of temperature

Solubility of gases in liquids decreases with the increase in temperature.