Vacuum Distillation

distillation performed under reduced pressure. Since a reduction in pressure also reduces the boiling point of substances it means that certain substances which at normal pressures would decompose (break down), can be distilled.

Taken from Dictionary of Science

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Vacuum distillation is a method of distillation whereby the pressure above the liquid mixture to be distilled is reduced to less than its vapor pressure (usually less than atmospheric pressure) causing evaporation of the most volatile liquid(s) (those with the lowest boiling points). This distillation method works on the principle that boiling occurs when the vapor pressure of a liquid exceeds the ambient pressure. Vacuum distillation is used with or without heating the solution.

Taken from Wikipedia