Study Notes: Solubility of Solids in Liquids

 1. What is a Solution?

A solution is a homogeneous mixture of two or more components.

• Homogeneous means the composition and properties are uniform throughout the mixture.

• The component present in the largest quantity is called the solvent.

• The physical state of the solution is determined by the solvent.

• All other components in the mixture are called solutes.

Example: In a sugar-water solution, water is the solvent and sugar is the solute.

2. Types of Solutions Covered

In this unit, we focus on binary solutions, i.e., those containing only two components – one solute and one solvent.

3. Solubility of a Solid in a Liquid

Not all solids dissolve in all liquids.
Some examples:

• Sodium chloride (NaCl) and sugar dissolve readily in water.

• Naphthalene and anthracene do not dissolve in water but dissolve in benzene.

Rule of Solubility:

"Like dissolves like"

• Polar solutes dissolve in polar solvents.

• Non-polar solutes dissolve in non-polar solvents.

4. Dissolution and Crystallisation

When a solid solute is added to a solvent:

• Some solute dissolves and increases the solute concentration. This process is called dissolution.

• At the same time, dissolved solute particles may recombine and settle back as solid. This process is called crystallisation.

Over time, the system reaches a dynamic equilibrium:

                               Solute (solid)+Solvent (liquid)⇌Solution

At dynamic equilibrium, the rate of dissolution equals the rate of crystallisation, and the concentration of solute in solution becomes constant (under given temperature and pressure).

5. Saturated and Unsaturated Solutions

• A saturated solution is one where no more solute can dissolve at a given temperature and pressure.

• An unsaturated solution is one where more solute can still dissolve at the same temperature and pressure.

• A saturated solution is in dynamic equilibrium with the undissolved solute.

• The concentration of solute in a saturated solution represents its solubility

6. Factors Affecting Solubility

(i) Nature of Solute and Solvent

Solubility depends on:

• Type of bonding

• Intermolecular forces

• Polarity

(ii) Temperature

• Solubility of solids is greatly affected by temperature.

• According to Le Chatelier’s Principle:

  • If dissolution is endothermic (ΔH_sol > 0), solubility increases with temperature.

  • If dissolution is exothermic (ΔH_sol < 0), solubility decreases with temperature.

Example: Most salts dissolve better in warm water (endothermic dissolution).

(iii) Pressure

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

• Reason: Solids and liquids are incompressible; hence, they are not affected by pressure changes.

8. Summary

• A solution is a homogeneous mixture of solute and solvent.

• Solubility depends on the nature of the solute and solvent, temperature, and pressure.

• Temperature affects solubility greatly, especially depending on whether the process is endothermic or exothermic.

• Pressure has negligible impact on solid-liquid solubility due to incompressibility of solids and liquids.

• A saturated solution is in dynamic equilibrium and defines the solubility of the solute at specific conditions.