# Solubility Product (Ksp)

## What is Solubility Product, Ksp?

The solubility product is a kind of equilibrium constant and its value depends on temperature. Ksp usually increases with an increase in temperature due to increased solubility.

Solubility is defined as a property of a substance called solute to get dissolved in a solvent in order to form a solution. The solubility of ionic compounds (which disassociate to form cations and anions) in water varies to a great deal. Some compounds are highly soluble and may even absorb moisture from the atmosphere whereas others are highly insoluble.

## Significance of Solubility Product

Solubility depends on a number of parameters amongst which lattice enthalpy of salt and solvation enthalpy of ions in the solution are of most importance.

• When a salt is dissolved in a solvent the strong forces of attraction of solute (lattice enthalpy of its ions) must be overcome by the interactions between ions and the solvent.
• The solvation enthalpy of ions is always negative which means that energy is released during this process.
• The nature of the solvent determines the amount of energy released during solvation that is solvation enthalpy.
• Non-polar solvents have a small value of solvation enthalpy, meaning that this energy is not sufficient to overcome the lattice enthalpy.
• So the salts are not dissolved in non-polar solvents. Hence for salt to be dissolved in a solvent, its solvation enthalpy should be greater than its lattice enthalpy.
• Solubility depends on temperature and it is different for every salt.

Salts are classified on the basis of their solubility in the following table.

 Category I Soluble Solubility > 0.1M Category II Slightly soluble 0.01M< Solubility<0.1M Category III Sparingly soluble Solubility < 0.1M

## Solubility Product Constant

Suppose barium sulphate along with its saturated aqueous solution is taken. The following equation represents the equilibrium set up between the undissolved solids and ions:

$BaSO_4 \overset{saturated ~solution ~in~ water}{\rightleftharpoons} Ba^{2+} (aq) + SO^{4-} (aq)$

The equilibrium constant in the above case is:

$K$ = $\frac{[Ba^{2+}][SO^{4-}]}{[BaSO_{4}]}$

In case of pure solid substances the concentration remains constant and so we can say:

$K_{sp}$ = $K[BaSO_4]$ = $[Ba^{2+}][SO^{4-}]$

Here $K_{sp}$ is known as the solubility product constant. This further tells us that solid barium sulphate when in equilibrium with its saturated solution, the product of concentrations of ions of both barium and sulphate is equal to the solubility product constant.

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