8.3 Vapor and Partial Pressures The pressure exerted by the vapor of a substance is known as the vapor pressure of the substance. Vapor pressure explains why we can smell some liquid substances such as nail polish remover and even some solids like black licorice. The vapor pressure of a substance depends first, on its chemical structure and second, on the temperature of the substance. At a given temperature, there is a maximum value the vapor pressure can reach. Every liquid possesses a vapour pressure. As shown in Table 8.3.1, the vapour pressure of a liquid, in this case water, increases with an increase in temperature. When the vapour pressure reaches atmospheric pressure, the liquid begins to boil.

Vapor Pressure

Intermolecular Forces and Boiling Point The reason different liquids have different boiling points is related to the attractive forces between the particles of different liquids. Intermolecular forces (IMFs), including London dispersion forces, dipole-dipole forces, and hydrogen bonds, were discussed in sections 6.5 and 7.2. Stronger IMFs result in fewer molecules (or atoms) having enough energy to overcome the attraction and escape into the gas phase.

Dalton’s Law of Partial Pressures

Calculation of Partial Pressures