November 23, 2008

What is vapor pressure?

What is vapor pressure?
If a puddle of water is left on the ground for a day or two, it will dry up because the liquid evaporates. The water does not boil, but individual water molecules gain enough energy to escape from the liquid as vapor. Over a period, an open, small pool of water will dry up in this way. If the liquid is in a closed container, at equilibrium, some molecules are always evaporating and vapor molecules are condensing, so there is no overall change in the system. The vapor (gaseous) molecules that have escape from the liquid state exert a pressure in the surface of the liquid known as the vapor pressure.

When the vapor pressure is high, the liquid evaporates (is vaporized) easily and many molecules exist in the vapor state; the boiling point is low. Conversely, a low vapor pressure indicates that the liquid does not vaporize easily and that there are few molecules existing in the vapor state. The boiling point for these liquids is higher. The liquid boil when the vapor pressure reaches the external pressure.

The vapor pressure increases with the increasing temperature. At higher temperatures, the molecules have more energy and it is easier for them to overcome the forces holding them within the liquid and to vaporize, and so there are many molecules in the vapor states.

The vapor pressure decreases with addition of solutes, such as salt or sugar. In effect, the solutes dilute the water; there are less water molecules (in the same volume)) available for vaporization and thus, there will be fewer molecules in the vapor state and the vapor pressure will be lower. Attraction to the solute also limits evaporation.
What is vapor pressure?

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