Boiling commences when the vapor pressure of a liquid or solution equals the external or applied pressure (often atmospheric pressure). Thus, if the applied pressure is reduced, the boiling point of the liquid decreases (see the graph for cinnamyl alcohol in Figure 5.47a). This behavior occurs because a lower vapor pressure is necessary for boiling, which can be achieved at a lower temperature. For example, water produces a vapor pressure of \(760 \: \text{mm} \: \ce{Hg}\) at \(100^\text{o} \text{C}\) (see Figure 5.47b), and so water boils at sea level at \(100^\text{o} \text{C}\). However, if water was subjected to a reduced pressure of only \(100 \: \text{mm} \: \ce{Hg}\), it would boil at roughly \(50^\text{o} \text{C}\) as this is the temperature that it produces a vapor pressure of \(100 \: \text{mm} \: \ce{Hg}\) (see Figure 5.47b). This trend follow the Clausius-Clapyron equation . The dependence of boiling point on applied pressure can be exploited in the distillation of very high boiling compounds (normal b.p. \(> 150^\text{o} \text{C}\)), which may decompose if heated to their normal boiling point. A vacuum distillation is performed by applying a vacuum source to the vacuum adapter of either a simple or fractional distillation (Figure 5.48). When the pressure is lowered inside the apparatus, solutions boil at a lower temperature. \(^{12}\)Data from The Merck Index , 12\(^\text{th}\) edition, Merck Research Laboratories, 1996 . \(^{13}\)Data from J. A. Dean, Lange's Handbook of Chemistry , 15\(^\text{th}\) ed., McGraw-Hill, 1999 , Sect. 5.28.