What is the phase transition from a solid directly to a gas called?

The phase transition in which a solid changes directly into a gas is called sublimation. The substance skips the liquid phase entirely.

Dry ice is a familiar example: solid carbon dioxide turns into carbon dioxide gas at ordinary atmospheric pressure instead of melting into a liquid. Solid iodine can also visibly sublime when heated, producing a violet vapour. In everyday conditions, ice can slowly sublime as well, which helps explain why frozen food loses moisture over time.

The reverse process is deposition, also called desublimation, in which a gas changes directly into a solid. Frost forming from water vapour is a common example. Condensation instead changes a gas into a liquid, while evaporation changes a liquid into a gas.

Sublimation is a physical change, not a chemical reaction. A candle’s wax burning into new substances is combustion, even though gases are produced. On a phase diagram, sublimation occurs along the boundary between solid and gas, below the substance’s triple point.