We all know that if left open all the water in a container will evaporate. (The surroundings keep the water warm so there is always some molecules capable of vaporizing.) What happens to the water if the container is capped.
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the vapor's point of view |
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At first there are no (or very few) gas molecules. |
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molecules vaporize |
now there are gas molecules |
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molecules continue to vaporize at the same rate |
gas molecules being to condense |
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molecules continue to evaporate a same rate |
now more gas molecules are present so rate of condensation increases |
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rate of evaporation is equal to the rate of condensation |
rate of evaporation equals rate of condensation |
Step through the movie with each step on the left |
When the rate of evaporation and the rate of condensation are equal the liquid and the vapor are at equilibrium. There is no noticeable macroscopic change, but there is a great deal still occurring at the molecular level.
What is important about this state....
For a given substance, at a given temperature the concentration of vapor particles over the liquid is a constant.Like every gas, the vapor in the flask exerts pressure against the walls of the flask.
The pressure the vapor molecules exert against the walls of the container is called the vapor pressure, and for a given material at a given temperature it is constant.