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Sublimation

Sublimation is the change of a solid directly into a gas. The intermediate liquid phase is skipped.

Sublimation typically occurs when certain solids absorb heat. Some notable examples are solid $$\ce{CO2}$$ (known as dry ice) and iodine.

Iodine appears as a blue-black solid at room temperature. Upon heating, iodine crystals begin to sublime into violet fumes.
Iodine appears as a blue-black solid at room temperature. Upon heating, iodine crystals begin to sublime into violet fumes.

Low outside pressure (usually less than atmospheric pressure) often favours sublimation over melting.

Dry ice is one of the few solids that will sublimate at atmospheric pressure. Solid dry ice is spontaneously converted to carbon dioxide gas.

The solid molecules gain kinetic energy as they gain heat, allowing them to vibrate more vigorously.

Once the molecules are heated to a certain threshold, they obtain enough energy to overcome the intermolecular attractive forces. This enables them to move freely and randomly in the gaseous state.

As with melting, sublimation involves the transfer of latent heat. The temperature of the material therefore remains constant during sublimation.