Which process describes when a solid changes directly into a gas?

The process where a solid changes directly into a gas is known as sublimation. During sublimation, the solid phase transitions into the gaseous phase without first becoming a liquid. This phenomenon occurs under specific temperature and pressure conditions, often seen in substances like dry ice (solid carbon dioxide) or ice sublimating into water vapor.

Sublimation is characterized by an increase in entropy, as the particles in the solid become more disordered when they enter the gaseous phase. This process is relevant in various applications, including freeze-drying, where moisture is removed from a product without passing through a liquid phase, helping preserve the structure and composition of the material.

The other processes mentioned involve different phase changes. For example, deposition is the transition from gas to solid, condensation refers to the change from gas to liquid, and melting involves the transition from solid to liquid. Each of these processes has its specific conditions and consequences, thus highlighting the distinct nature of sublimation as a direct solid-to-gas transformation.