A mother liquor is the part of a solution

Solution

In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...

 that is left over after crystallization

Crystallization

Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...

. It is encountered in chemical processes including sugar refining.

In crystallization, a solid (usually impure) is dissolved in a solvent at high temperature, taking advantage of the fact that most solids' solubilities are higher at higher temperatures. As the solution cools, the solubility of the solute in the solvent will gradually become smaller. The resultant solution is described as supersaturated, meaning that there is more solute dissolved in the solution than would be predicted by its solubility at that temperature. Crystallization can then be induced from this supersaturated solution and the resultant pure crystals removed by a method such as vacuum filtration. The remaining solution, once the crystals have been filtered out, is known as the mother liquor, and will contain a portion of the original solute (as predicted by its solubility at that temperature) as well as any impurities that were not filtered out. Second and third crops of crystals can then be harvested from the mother liquor.