During which process does a solid change directly to a gas?

Sublimation is the process in which a solid transitions directly into a gas without passing through the liquid phase. This phenomenon typically occurs under specific conditions of pressure and temperature. A common example of sublimation is dry ice (solid carbon dioxide) turning into carbon dioxide gas; as it warms up, it skips the liquid state entirely and becomes gas.

The process is quite different from condensation, which involves the change of a gas into a liquid, or evaporation, which refers to the conversion of liquid into gas. Melting is the change of a solid into a liquid, not into a gas. Therefore, sublimation is uniquely defined as the direct transformation from a solid state to a gaseous state, making it the correct answer. Understanding the conditions that favor sublimation can also highlight the uniqueness of this phase transition compared to others.