Why Does Water Have a Low Vapor Pressure?

Water has a low vapor pressure due to the strong intermolecular forces between its molecules. The type of chemical bonding between water molecules is referred to as hydrogen bonding.

Water is the most commonly occurring molecule on Earth, comprising around 70 percent of the planet's oceans and approximately 65 percent of the human body. Water exists in nature in three different states: solid as ice, liquid as water and gas as steam. This characteristic is due to the unique properties of water in terms of boiling and freezing points, viscosity, cohesion, surface tension, heat of vaporization and vapor pressure.

On the atomic level, one molecule of water is composed of two hydrogen atoms that are covalenty bonded to one oxygen atom. When one molecule of water chemical combines with another water molecule, an intermolecular force through hydrogen bonding is produced between the two molecules. This type of bonding is the strongest among all forms of intermolecular forces.

In a state of equilibrium, vapor pressure pertains to the pressure exerted by a vapor on the liquid from which it evaporated. Vapor pressure forms an indirect correlation with intermolecular forces, where increasing forces result to an increase in vapor pressure and decreasing forces result to a reduction in vapor pressure. The powerful hydrogen bonds between water molecules induce a very low vapor pressure, which is only 0.03 atmosphere at 77 degrees Fahrenheit.