Final answer:
The change of a solid directly into a gas is called sublimation, which occurs under specific conditions like low pressures or when heating substances such as dry ice or iodine. The heat of sublimation is the energy change that accompanies this process, and the reverse process is deposition.
Step-by-step explanation:
The term that describes the change of a solid substance directly into a gas is sublimation. Sublimation occurs under specific conditions, typically at low pressures or when the heat applied to a solid does not cause it to pass through the liquid phase. An example of sublimation is when dry ice (solid CO₂) is at room temperature and standard pressure, it sublimes to form carbon dioxide gas without becoming liquid. Conditions that facilitate sublimation include reduced atmospheric pressures at high altitudes or heating of a solid like iodine which sublimes to form a vivid purple vapor.
Each substance has a characteristic heat of sublimation, which is an energy change that occurs during the process of sublimation. This change is isothermal, meaning it occurs at a constant temperature. The reverse process of sublimation is called deposition, where a gaseous substance transitions directly into a solid state, bypassing the liquid state. An everyday example of deposition is the formation of frost.