Daily life is filled with examples of phase changes -- water freezes into ice cubes, butter melts in a hot frying pan and grass collects dew on a moist summer morning. Vaporization is a phase change in which liquid molecules become gas molecules through evaporation or boiling. Distillation is an intentional process that employs the principles of vaporization to separate the components of a liquid mixture or of a gaseous mixture that has been first cooled into a liquid state.
Every molecule of a liquid substance has a certain amount of kinetic energy associated with its random movement. Evaporation occurs when molecules at the surface of the liquid acquire enough kinetic energy to break free and enter the vapor phase. Boiling is distinct from evaporation because molecules anywhere in a volume of boiling liquid can enter the vapor phase; if they are not at the surface, they move upward as bubbles. Even though evaporation occurs below the boiling temperature of a liquid, increased temperature still causes increased evaporation because higher temperatures lead to more molecules with enough energy to evaporate.
If evaporation occurs in a closed container, vaporized molecules will accumulate above the surface of the liquid. If the liquid is a mixture of two or more compounds, the accumulating vapor will contain more molecules of the compounds that evaporate more readily. In other words, this contained evaporation can separate compounds with a higher boiling point from compounds with a lower boiling point, or from solid compounds that do not evaporate at all. Distillation is the process of collecting this vapor and cooling it so that it returns to the liquid phase.
The Body's Evaporative Cooler
A pervasive example of evaporation is sweat, which is the body's primary mechanism for maintaining proper internal temperature during hot weather. A molecule needs energy to make the transition from liquid phase to vapor phase. When sweat evaporates, it absorbs this energy from the skin in the form of heat and releases it into the surrounding environment, thus gradually decreasing body temperature. Evaporation also increases the amount of moisture in the air, particularly in an enclosed area. Liquid water spread over a large surface area, such as a towel or shallow pan, encourages evaporation and helps to maintain high humidity in places where fresh fruits or vegetables are stored. Plants use a special type of evaporation, known as evapotranspiration, to move water and nutrients from their roots to their leaves.
Salt Water to Fresh Water
Distillation is an effective way to purify liquids or to separate the components of a liquid mixture. An example of purification is distilling potable water from sea water. When sea water is heated to boiling, water molecules rapidly enter the vapor phase while salt (and other non-volatile contaminants) remain in the heated container. This vapor then condenses into drinking water that is much more pure than the original sea water. An example of component separation is the process of refining crude oil, which is a mixture of many distinct hydrocarbon compounds. These compounds have different uses -- for example, some are valuable for fuels such as gasoline while others are better for lubricating oil. These compounds have different boiling points, so oil refineries can employ a specialized distillation procedure to isolate them.
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