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Sublimation is the transition of a substance from a solid to a gas without passing through the liquid phase. Sublimation is an endothermic(absorbs energy) phase transition that occurs at temperatures and pressures below a substance’s triple point in its phase diagram. The triple point is where the temperature and pressure where all three phases of a substance coexist in thermodynamic equilibrium.
At normal pressure most chemical compounds and elements possess three different states each at a different temperature. With these, the transition from the solid to the gaseous state requires an intermediate liquid state. All solids which possess an appreciable vapor pressure at a certain temperature can sublimate in air. Some substances sublimate easier than others. Examples would be carbon and arsenic. With these elements sublimation is easier than evaporation, because the pressure of their triple point is very high, and it is difficult to obtain them as liquids. Sublimation requires more energy since it is an endothermic change. The enthalpy of sublimation(heat of sublimation) can be calculated as the enthalpy of fusion plus the enthalpy of vaporization. Every process in nature has an opposite or reverse process. The reverse process of sublimation is deposition. Frost would be an example of this process.
Sublimation is frequently used to purify a compound. During this process a solid is placed in a sublimation container and heated under vacuum. In these conditions the solid vaporizes and condenses as a purified compound on a cooler surface.
Snow and ice will sublimate below their melting point. This is why wet cloth can be hung outdoors in freezing weather and will still dry. Freeze drying is a process where an item(usually food) is dehydrated then frozen and its water is allowed to sublimate under reduced pressure or vacuum. Sublimation has been harnessed to benefit science and everyday life.
We have written many articles about sublimation for Universe Today. Here’s an article about the crooked Southern polar cap of Mars, and here’s an article about comets as the probable cause of Earth’s water.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.