Solubility and Solubility Product Principle

Solubility:

When a sparingly soluble ionic substance is dissolved in any solvent, it dissociates into positively charged cations and negatively charged anions. The extent of solubility of the ionic substance depends on different factors like temperature, pressure, presence of common ions etc. The ability of a ionic substance to dissolve in any solvent is called solubility.

Solubility product principle:

Solubility product of a sparingly soluble electrolyte is defined as the product of its ions in its saturated condition at 25 ºC.  The value of solubility product of a sparingly soluble salt is always constant at given temperature. For example the solubility product of AgCl at 25ºC is 1.6 × 10-10 ºC. It represents the level at which a solute dissolves in solution.

Let us consider a sparingly soluble electrolyte AB that dissolves very slightly in water. It dissociates into cations and anions.

$$\ce{AB<=>A+ + B-}$$

At first, AB dissociates to give A+ and B. As the concentration of A+ and B is increased, the backward reaction initiates and the reaction comes under equilibrium.   When the solution is saturated at 25ºC, the concentration of AB,  A+ and Bremains constant. Now,we can apply the law of mass action to this equilibrium system.

$$K =  \frac{[A^+][B^-]}{[AB]}$$

$$or, K  \times [AB] = [A^+][B^-]$$

At saturated condition in 25ºC,

$$K  \times [AB] = K_{sp}$$

$$\therefore K_{sp} = [A^+][B^-]$$

The above expression defines the solubility product principle.

Application of solubility product principle:

1. It is used for the precipitation of:

  • Chlorides of group I cations
  • Sulphides of group II cations
  • Hydroxides of group III cations
  • Carbonates of group V cations

2. It has wide range of application in qualitative analysis.

3. This principle is used for the purification of common salt.

When the solubility product of given salt is less than its ionic product value, then precipitation occurs. This is called solubility product principle.

Ionic Product:

When the temperature of solubility is greater or less than 25 ºC, then the product of its ion is called ionic product.

[A+][B] = Ionic product

We know that if the product of concentration of ions present in any solution at a given temperature is less than Ksp, the solution is called unsaturated and more solute can be dissolved in the solution. Thus,

  • When solubility product is greater than ionic product, the solution is unsaturated.
  • When solubility product is equal to ionic product, the solution is saturated.
  • When ionic product is greater than solubility product, the solution is unsaturated.

Difference between ionic product and solubility product:

Solubility Product      Ionic Product
Solubility product of a sparingly soluble electrolyte is defined as the product of its ions in its saturated condition at 25o C. It is the product of the concen-tration of ions  in unsaturated condition.
It is applicable to only saturated solutions It is applicable to all types of solutions of any concentration.
It has a constant value for an electrolyte at a constant temperature Its value is not constant and varies with change in concentration.

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