A mother liquor is the part of a solution that is left over after crystallization. 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 such methods as vacuum filtration and centrifugal separators. 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.
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—Elizabeth Stuart Phelps (1844–1911)