Answer – Sublimation is the phase transition where a solid directly converts into its gaseous state without first becoming a liquid.

Explanation:

Usually when the solid state of a substance is heated, it first melts to become a liquid. It is only with further heating that it vaporizes. However, in the case of sublimation, the intermediate liquid state is bypassed, and the solid transforms directly into gas. 

(If the change is due to a chemical reaction where a solid substance is being transformed into a different gaseous substance, it is not called sublimation. Sublimation is a physical change where the substance remains the same throughout, undergoing only a transformation in its physical state.)

Sublimation occurs when a substance is heated while the surrounding pressure is below its triple point. (The triple point is the lowest pressure at which a substance can exist as a liquid.) The higher the triple point of a substance is, the easier it is for it to sublimate rather than melt. 

A well recognized example of sublimation is that of dry ice. What is known as dry ice is essentially solid carbon dioxide. It has a high triple point, which means that the surrounding pressure needed for the existence of liquid carbon dioxide is quite high (about 5 times the normal atmospheric pressure). Thus, it sublimates directly into gas from its solid state. 

Among the uses of sublimation are the separation and purifying of substances, and printing. 

The opposite of sublimation is deposition, where a gas directly transforms into a solid.

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