The process by which a substance changes directly from a solid to a gas is sublimation.
During sublimation, particles escape from a solid without first becoming liquid. It is an endothermic phase change: heat supplies the energy needed to overcome attractions in the solid. The reverse process, gas changing directly into a solid, is deposition, also called desublimation.
Dry ice is the familiar example. At ordinary atmospheric pressure, solid carbon dioxide sublimes at about −78.5 °C, producing visible fog when its cold gas condenses water vapour from the air. Ice can also sublime, especially at low pressure; freeze-drying uses this principle to remove water from frozen foods and medicines.
A phase diagram explains why pressure matters. Carbon dioxide’s triple point is about 5.1 atmospheres and −56.6 °C, so liquid carbon dioxide cannot exist at ordinary atmospheric pressure. Iodine can produce violet vapour when gently heated. Sublimation is a physical change, not a chemical reaction, and it should not be confused with evaporation, which is liquid-to-gas change.