Vacuum Distillation (Distillation Under Reduced Pressure)

Principle: Liquid boils at a lower temperature when external pressure is reduced, preventing thermal decomposition of compounds that decompose at or near their normal boiling points.

Equation: Liquid boils when $$P_{\text{vapor}} = P_{\text{external}}$$.

Examples: Purification of Glycerol (boils at $$563\text{ K}$$ with decomposition, but distills without decomposition at $$453\text{ K}$$ under $$12\text{ mm Hg}$$ pressure); Concentration of spent lye in soap industry.