Which process is defined as the transition of a solid directly into a gas?

The transition of a solid directly into a gas is known as sublimation. During sublimation, a substance bypasses the liquid phase and converts directly from a solid state to a gaseous state. This phenomenon occurs under specific conditions of temperature and pressure, where the solid particles acquire enough energy to overcome their intermolecular forces and enter the gaseous phase without first becoming a liquid.

An everyday example of sublimation is the process of dry ice (solid carbon dioxide) transforming directly into carbon dioxide gas at room temperature. This process can often be observed when dry ice is exposed to air and appears to "smoke."

In contrast, other processes such as melting refer to the transition from a solid to a liquid, evaporation describes the process where a liquid turns into a gas, typically at the surface and at any temperature below its boiling point, and condensation is the change from a gas to a liquid. Each of these processes involves different states of matter and energy transformations, highlighting the unique nature of sublimation as a direct transition from solid to gas.