Evaporation and Boiling

At sufficiently low pressures, there is no liquid phase but only a gaseous and solid phase. This phase change from solid to gas is referred to as sublimation. Note that, at a single point, called the “triple point”, all three phases will exist in equilibrium. This occurs at 0.01 degree Celsius.

Vapor pressure is the pressure at which a gas coexists with its solid or liquid phase. This depends on the substance and its temperature so that an increase in temperature increases the vapor pressure. Partial pressure is the pressure that a gas would create if it occupied the total available volume. According to Dalton’s law of partial pressures, the total pressure is the sum of partial pressures of the different gases in a specific volume. Different gases have pressures that add up to the total pressure of a gaseous system.

EVAPORATION AND BOILING

This topic first gets into the idea of relative humidity. This is the amount of water vapor in the air compared to the maximum possible. At maximum saturation, the relative humidity is 100 percent and there is no possibility of evaporation. Temperature will affect the relative humidity on any given day. At the dew point, the relative humidity is 100 percent, resulting in fog due to condensation of water that stays in suspension. As the temperature raises, there is greater evaporation and things will dry out.

The vapor pressures will increase with temperature because of higher molecular speeds at higher temperatures. At 100 percent humidity, the partial pressure of water is equal to the vapor pressure so no more water can become gaseous. Evaporation will take place at any humidity less than 100 percent. If the partial pressure is less than the vapor pressure, evaporation will take place and if the partial pressure is greater than the vapor pressure, condensation will take place.

Decreased atmospheric pressure will result in a decreased partial pressure of water and lower humidity. This involves a greater evaporation of water from food as would be the case with freeze-drying. Food is subjected to lower pressure in a vacuum as well as lower temperature, which dries out and freezes the food in ways that cannot be done if there wasn’t a vacuum associated with the cooling process.