Unit – II B

Solubility of Solids in Liquids:

The concept of Solubility:

  • When a substance (solute) gets dissolved in liquid (solvent) to form solution it means solute-solvent interaction is greater than solute-solute
  • Solubility of a substance is its maximum amount that can be dissolved in a specified amount of solvent. It depends upon the nature of solute and solvent as well as temperature and pressure.
  • Let us consider the effect of these factors in a solution of a solid or a gas in a liquid. When a solid solute is added to the solvent, some solute dissolves and its concentration increases in solution. This process is known as dissolution. Some solute particles in solution collide with the solid solute particles and get separated out of solution. This process is known as crystallisation.
  • A stage is reached when the two processes occur at the same rate. Under such conditions, the number of solute particles going into solution will be equal to the solute particles separating out and a state of dynamic equilibrium is reached. At this stage, the concentration of solute in solution will remain constant under the given conditions, i.e., temperature and pressure.
  • Similar process is followed when gases are dissolved in liquid solvents.
  • Such a solution in which no more solute can be dissolved at the same temperature and pressure is called a saturated solution. An unsaturated solution is one in which more solute can be dissolved at the same temperature.
  • The maximum amount of solute that dissolves completely in a given amount of solvent at constant temperature is called solubility of the solute.

Types of Solutions:

Saturated solution:

  • A saturated solution is defined as the solution that contains a just amount of dissolved solute necessary for establishing equilibrium between dissolved solute and undissolved solid.

Unsaturated solution:

  • An unsaturated solution is defined as a solution in which more solute can be dissolved at the same temperature.

Supersaturated solution:

  • A supersaturated solution is defined as a solution in which excess solute is dissolved than required for formation of a saturated solution.

Factors Affecting Solubility of a Substance:

Nature of solutes:

  • Polar solvent (e.g. water) dissolves polar solute (e.g. NaCl, KCl) and non-polar solvents (CCl4, CS2) dissolve non-polar solutes (e.g. I2, S8).

Effect of temperature on the solubility of solids:

  • The solubility of a solid in a liquid is significantly affected by temperature changes. Consider the equilibrium of solute particles dissolving and crystallising.

Solubility Concept

  • It is a dynamic equilibrium, must follow Le Chatelier,s Principle.
  • If in a nearly saturated solution, the dissolution process is endothermic (ΔsolH > 0), the solubility should increase with the rise in temperature if it is exothermic (ΔsolH < 0) the solubility should decrease.

Effect of pressure:

  • Pressure does not have any significant effect on so the ability of solids in liquids. It is so because solids and liquids are highly incompressible and practically remain unaffected by changes in pressure.
  • Many gases dissolve in water. Oxygen dissolves only to a small extent in water. It is this dissolved oxygen which sustains all aquatic life. On the other hand, hydrogen chloride gas (HCl) is highly soluble in water.
  • Solubility of gases in liquids is greatly affected by pressure and temperature. The solubility of gases increase with increase of pressure.

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