3.4 Phase changes  (Page 4/15)

Vapor pressure, partial pressure, and dalton’s law

Vapor pressure is defined as the pressure at which a gas coexists with its solid or liquid phase. Vapor pressure is created by faster molecules that break away from the liquid or solid and enter the gas phase. The vapor pressure of a substance depends on both the substance and its temperature—an increase in temperature increases the vapor pressure.

Partial pressure is defined as the pressure a gas would create if it occupied the total volume available. In a mixture of gases, the total pressure is the sum of partial pressures of the component gases , assuming ideal gas behavior and no chemical reactions between the components. This law is known as Dalton’s law of partial pressures    , after the English scientist John Dalton (1766–1844), who proposed it. Dalton’s law is based on kinetic theory, where each gas creates its pressure by molecular collisions, independent of other gases present. It is consistent with the fact that pressures add according to Pascal’s Principle . Thus water evaporates and ice sublimates when their vapor pressures exceed the partial pressure of water vapor in the surrounding mixture of gases. If their vapor pressures are less than the partial pressure of water vapor in the surrounding gas, liquid droplets or ice crystals (frost) form.

Phet explorations: states of matter—basics

Heat, cool, and compress atoms and molecules and watch as they change between solid, liquid, and gas phases.

Section summary

• Most substances have three distinct phases: gas, liquid, and solid.
• Phase changes among the various phases of matter depend on temperature and pressure.
• The existence of the three phases with respect to pressure and temperature can be described in a phase diagram.
• Two phases coexist (i.e., they are in thermal equilibrium) at a set of pressures and temperatures. These are described as a line on a phase diagram.
• The three phases coexist at a single pressure and temperature. This is known as the triple point and is described by a single point on a phase diagram.
• A gas at a temperature below its boiling point is called a vapor.
• Vapor pressure is the pressure at which a gas coexists with its solid or liquid phase.
• Partial pressure is the pressure a gas would create if it existed alone.
• Dalton’s law states that the total pressure is the sum of the partial pressures of all of the gases present.

Conceptual questions

What is the vapor pressure of solid carbon dioxide (dry ice) at $–\text{78}\text{.}5\text{º}\text{C}$ ?