What is the definition of solubility?
Solubility is defined as “The total amount of solute per cubic decimeter (dm3) of the solvent at the particular temperature”.
“The amount of one substance that could dissolve in another to form a saturated solution under specified conditions of the temperature and pressure”.
It is measured in two units:
- Physical Unit ( g / dm3 or g / lit)
- Chemical Unit (moles / dm3 or moles / lit)
Solubility Product (Ksp)
When a saturated solution of sparingly soluble salt in the contact with undissolved salt, an equilibrium is established between the dissolved ions and the ions in the solid phase of the undissolved salt.
Application Of Solubility Product (Ksp)
Ksp is used to determine whether a precipitate should form from a solution of known ionic concentration. The value of the solubility product represents the equilibrium condition between the dissolved ions and undissolved solids phase of solute i.e. when a solution is saturated.
When the product of the ionic concentration is equal to the product, a saturated solution is said to exist. If the ionic product is less than the solubility product, the solution is not saturated i.e. more amount of solute can go into the solution. If the ionic product is larger than the product, the solution is said to be supersaturated and the excess of should precipitate as to restore the equilibrium condition. For example, the solubility of CaSO4 is 2.4 x 10-5 moles / dm3.
CaSO4 would form precipitate so as to reach of 2.4 x 10-5 moles / dm3.
If on the other hand
[Ca2+] [SO42-] < 2.4 x 10-5 moles / dm3
If the solution is not saturated and more solute could go into the solution till the product become equal so, precipitates not form.