Matter can exist in three primary states: solid, liquid, and gas. A change from one state to another is known as a phase transition, occurring when a substance gains or loses energy, usually in the form of heat. While matter often transitions sequentially (solid to liquid to gas), some substances skip the liquid phase entirely, moving directly between the solid and gaseous states under specific conditions of temperature and pressure.
What Sublimation Is
Sublimation is a specific phase transition where a solid substance changes directly into a gas, completely bypassing the intermediate liquid phase. This transition occurs when the solid absorbs enough energy from its surroundings to overcome the forces holding its particles in a fixed structure. Because it requires an input of energy, sublimation is classified as an endothermic process. This process is possible when the substance’s temperature and pressure are below its triple point, the condition where all three states can coexist.
A classic example of sublimation is dry ice, which is the solid form of carbon dioxide (\(\text{CO}_2\)). At atmospheric pressure, dry ice transforms directly into carbon dioxide gas, producing the visible fog often used in theatrical effects. Another common instance involves the element iodine, where the dark, solid crystals readily turn into a purple gas when gently heated.
The Process of Deposition
The opposite of sublimation is a phase transition called deposition, which is sometimes referred to as desublimation. Deposition is defined as the transition of a substance directly from a gaseous state to a solid state, again skipping the liquid phase. This loss of energy means that deposition releases heat into the surroundings, classifying it as an exothermic process.
The most familiar example of deposition in nature is the formation of frost. When water vapor in very cold air makes contact with a surface below the freezing point, the vapor changes directly into solid ice crystals without first forming liquid water droplets. This process is also how snow forms in clouds when water vapor transitions directly into ice crystals.
Comparing All Six Phase Changes
Sublimation and deposition are two of the six primary phase transitions that substances can undergo between the solid, liquid, and gaseous states. These six changes form three pairs of opposites, with one process in each pair being endothermic (absorbing heat) and the other being exothermic (releasing heat).
- Melting (solid to liquid) and its opposite, freezing (liquid to solid).
- Vaporization (liquid to gas), which includes boiling and evaporation, and its reverse, condensation (gas to liquid).
- Sublimation (solid to gas) and its opposite, deposition (gas to solid).
The transitions that involve an increase in molecular energy—melting, vaporization, and sublimation—are all endothermic processes. Conversely, the transitions that involve a decrease in molecular energy—freezing, condensation, and deposition—are all exothermic processes.