The molecules (and atoms) of liquids and solids are constantly in motion. When they have enough energy, they may, upon reaching the surface, escape into the surrounding environment as a gas. Vapor pressure refers to the partial pressure of this gas at equilibrium. That is the pressure exerted only by the particular gas of interest, as measured in a closed container. The closed container is required to reach equilibrium. In the open air, the gas disperses freely, and does not reach equilibrium.
Vapor pressure is essentially a measure of how easily a substance enters the gas phase. If you have ever heard the term volatile, or volatility, you were likely talking about a substance that evaporates easily (such as ether or gasoline). Volatile substances have high vapor pressures.
One factor that affects vapor pressure (and volatility) is weight. In general, chemicals with lower molecular weights can enter the gas phase more easily. This makes sense, since given equal amounts of energy, the smaller molecule will travel faster, and can escape into the air (or vacuum) more easily.
Temperature is also a factor which influences vapor pressure. Higher temperatures provide more energy, allowing molecules to move faster. As a result, more molecules escape as gas, shifting the equilibrium towards the gas phase and raising the vapor pressure.
Intermolecular attractions (particularly hydrogen bonding) act to lower vapor pressure. It is because of this that water (which forms hydrogen bonds) has a lower vapor pressure than propane – a molecule with a higher molecular weight. In general, non-polar molecules (like hydrocarbons) have the weakest interactions, and are more volatile as a result.
The presence of other chemicals influences vapor pressure in a small way. If there is a mixture rather than a pure substance, the vapor pressure of each substance is reduced. The reduction is simply proportional to the relative amount of each substance present (calculated using the fraction). In a mixture that was 90% water and 10% sugar, the water’s vapor pressure would be 90% of what it would be for pure water. (Sugar’s would be 10% of what it normally is as well, but sugar has such a low vapor pressure to begin with that we would probably ignore it.)
It is convenient to predict relative vapor pressure by using the boiling point. Boiling point is influenced by many of the same factors (though not temperature). As a result, substances with lower boiling points are expected to have higher vapor pressures.
Vapor pressure is an important part of the world we live in. Without it, the air would be bone dry (no humidity) and we couldn’t smell gas leaks or women’s perfume. Indeed, there wouldn’t be any air in the first place. Thank goodness then, not only that it exists, but that it is so easily understood.
