Sublimation

A chemical process by which certain solid substances are converted by heat directly to a gas without their first passing through a liquid stage. Upon cooling, they turn back into a solid again, also without passing through a liquid stage. Some common examples are the evaporation of snow and ice from sidewalks or streets without first melting, or the evaporation of mothballs (which are composed of naphthalene or para-dichlorobenzene). Generally speaking, it is small, compact rigid, relatively non-polar molecules that will sublime readily, so long as their molecular weight is not high enough to preclude their volatility. Sublimation is used to purify some substances, such as commercial iodine; iodine molecules are readily sublimed, and they can be volatilized directly to a gas, and the vapor cooled back down to a solid on a cooled surface, separating out less easily sublimed impurities.

Some types of colorants (called sublimable dyes) used in heat-transfer printing take advantage of this process.

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