SUPPLEMENTARY TEXT:
Evaporation and Condensation
When enough heat is supplied to a liquid at standard atmosphere, the liquid evaporates to a gas. When heat being removed from a gas, it condenses to a liquid. The heat absorbed in the first process is called the heat of evaporation or vaporization and the heat evolved in the second, the heat of condensation.
At any temperature the heat absorbed when a liquid evaporates equals the heat evolved when a gas condenses to a liquid.
At equilibrium the average transmitted kinetic energies of the molecules in the liquid and gas phases are the same, their potential energies, however, differing. Although strong inter molecule forces hold the molecules in a liquid, the attractive forces between gas molecules are relatively small. Hence, these molecules can collide within the liquid surfaces practically unhindered.
The number of gas molecules striking the surface is directly proportional to the concentration. If they are to remain in the liquid, they must lose potential energy equal to the difference between their binding energies in the gas and liquid phases.
Their loss of energy accounts for the heat of condensation. At the same time, the molecules evaporating from the liquid must absorb an equivalent amount of energy. This effect accounts for the heat of evaporation or vaporization.
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