Sublimation is the process when a gas changes directly into a solid without becoming a liquid first.

Explanation:
Sublimation is a phase transition process in which a substance changes directly from a gas to a solid without passing through the liquid state. This occurs when the pressure and temperature conditions are such that the substance's vapor pressure exceeds its triple point pressure.

Key Points:
1. Phase Transition: Sublimation is a type of phase transition, which is a change in the physical state of matter.
2. Gas to Solid: In sublimation, a gas transforms directly into a solid without going through the intermediate liquid phase.
3. Vapor Pressure: The vapor pressure of a substance is the pressure exerted by its gas phase at a given temperature. For sublimation to occur, the vapor pressure of the substance must be higher than its triple point pressure.
4. Triple Point: The triple point is the unique combination of temperature and pressure at which all three phases of a substance (solid, liquid, and gas) can coexist in equilibrium.
5. Examples: Some common substances that undergo sublimation include dry ice (solid carbon dioxide), mothballs (naphthalene), and iodine crystals.
6. Reverse Sublimation: The reverse process, where a solid transforms directly into a gas without becoming a liquid first, is called deposition.
7. Applications: Sublimation has various applications in different fields. For example, it is used for freeze-drying food, preserving museum artifacts, and creating air fresheners.

In conclusion, sublimation is the process when a gas changes directly into a solid without becoming a liquid first. It is a unique phase transition that occurs when the vapor pressure of a substance exceeds its triple point pressure.