Sublimation
The process of sublimation is the transformation of a solid substance into a gaseous phase without first traversing through a liquid phase. Typically, when solid matter changes its physical state as it is heated, it first becomes a liquid. Then as more heat energy is applied, the liquid becomes a gas. When sublimation takes place, however, the solid immediately becomes a gas and no melting occurs. A good example is the sublimation of solid carbon dioxide. Carbon dioxide, the gaseous compound found in our atmosphere that is exhaled during respiration, is a solid at very cold temperatures. Solid carbon dioxide is called dry ice because, at ordinary pressures, it sublimates into a gas without first passing through a liquid phase. The opaque carbon dioxide vapor that emanates from dry ice is very cold compared to room temperature air, and thus flows downward, in a manner opposite of steam. Sublimating solid carbon dioxide is often used to create haunting and mysterious misty effects in theater productions and movies.
Sublimation also occurs with water, the most important inorganic compound for life on Earth. Solid water, or ice, can sublimate very slowly at subfreezing temperatures.
For example, thin layers of snow can be seen to disappear slowly from the sides of roadways in the middle of winter when the temperature does not exceed the freezing point of water. The solid water slowly sublimates into water vapor over time, resulting in snow that vanishes, but does not melt. The same process can be observed at an accelerated pace when frost on car windshields sublimates into water vapor before the windshield warms up from the interior and as the car is moving. The increased air flow over the ice crystals of the frost facilitates their sublimation into water vapor. Frost free refrigerator freezers utilize sublimation of ice crystals to prevent frost formation and ice build up.
Sublimation can occur in reverse as well. Sublimation in reverse is sometimes referred to as deposition. Water vapor forms solid ice as frost on the windows of airplanes as it sublimates, or deposits, directly from gas to solid. Also, water reverse sublimates from air onto very cold surfaces, like dehumidifier coils. Snowflakes also can be formed by reverse sublimation. Amazingly, substances that do not ordinarily sublimate can be made to sublime by changing temperature and pressure. In general, if both temperature and pressure are sufficiently lowered, any solid may sublimate directly into a gaseous state. Under these conditions, the substance cannot exist as a liquid.
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