Sublimation of frozen carbon dioxide ("dry ice") is common at the surfaces of comets when they warm up upon approaching the Sun. This is an artist's depiction of sublimation on a comet's surface. Click on image for full size (74 Kb) Image courtesy of NASA.

Evaporation is not quite the correct term to describe what happens to a comet as it approaches the Sun. The correct term is sublimation. The term describes what happens when a frozen material changes to gaseous form. Evaporation describes what happens when a liquid changes to a vapor.

The most common example of sublimation is that of dry ice, which is the common name of frozen carbon dioxide. When dry ice is exposed to the air it begins to sublimate, or change to vapor, before your very eyes. This happens to dry ice because at room temperature the molecules in the ice begin to move so rapidly that they escape the solid and become gases.

When a comet approaches the sun, the comet comes to a region of space where it is warm enough that the ices inside the nucleus become gases. That is when the tails and coma of the comet form.