- The vapor pressure of a liquid is the pressure exerted by its vapor when the liquid and vapor are in dynamic equilibrium.If we were to place a substance in an evacuated, closed container, some of it would vaporize. The pressure in the space above the liquid would increase from zero and eventually stabilize at a constant value, the vapor pressure.
Finally, recognize that liquids that aren’t in a closed container still have a vapor pressure. However, the material will eventually evaporate or vaporize (turn into a gas) completely.
- Even though the pressure in our closed container is constant, molecules of the vapor are still condensing into the liquid phase and molecules of the liquid are still evaporating into the vapor phase. However, the rate of these two processes is equal, so there is no net change in the amount of vapor or liquid. This process is called dynamic equilibrium. For this reason, the term equilibrium vapor pressure is sometimes used.Vapor pressure and boiling point have an intimate relationship. The boiling point is the temperature at which the vapor pressure of the liquid equals the external pressure. For example, because the air pressure is lower in a city far above sea level such as Denver, the boiling point of water is lower than in a sea level city such as New York.
Most materials have very low vapor pressures. For example, water has a vapor pressure of approximately 20 torr at room temperature (22 °C = 72 °F). But remember that vapor pressures increase with temperature; water will have a vapor pressure of 760 torr = 1 atm at its boiling point of 100 oC (212 oF).
In general, the higher the vapor pressure of a material at a given temperature, the lower the boiling point. In other words, compounds with high vapor pressures are volatile, forming a high concentration of vapor above the liquid; this can sometimes pose a fire hazard.
||Flammable liquid handling charts and more are available at Safety Emporium.|
- Vapor pressures are discussed at the Thousand Island Secondary School.
- Jim Plambeck discusses vapor pressure.
- Vapor pressures of Volatile Chemicals, an online calculator at the USDA.
- An encylopedia article on vapor pressure at AllRefer.com.
- A video demonstration of the vapor pressure of diethyl ether by Oliver Seely. Requires Real Player.
- The vapor pressure of mixtures is dealt with using Raoult’s Law.
- Vapor Pressure, part of a General Chemistry help tutorial on liquids at Purdue University.