A solid can turn directly into a gas without ever becoming a liquid, bypassing the typical step-by-step progression of phase changes. Matter commonly exists in three basic phases: solid, liquid, and gas. Under specific conditions, however, a solid can bypass the liquid phase entirely and transition straight into its gaseous form. This direct change is a physical phase change, demonstrating that the intermediate liquid state is not always required.
The Specific Phase Transition
The scientific term for a solid transitioning directly into a gas, skipping the intermediate liquid phase, is sublimation. Sublimation is an endothermic process, meaning it requires the absorption of heat energy from the surrounding environment. The energy absorbed is known as the latent heat of sublimation, and it provides enough kinetic energy for the molecules to overcome the attractive forces holding them in the solid lattice. This energy input allows the molecules to escape into the vapor phase without first loosening enough to flow as a liquid.
The Mechanics of Skipping the Liquid State
The occurrence of sublimation is determined by a specific relationship between a substance’s temperature and the surrounding pressure. All substances have a unique point on their phase diagram, called the triple point, where the solid, liquid, and gas phases can coexist in thermodynamic equilibrium. Sublimation occurs at conditions of temperature and pressure that are below this triple point. When a solid is heated at a pressure below its triple point pressure, the liquid phase cannot form. The heat energy absorbed by the solid’s molecules causes them to vibrate with enough intensity to break free and instantaneously enter the gas state. For a substance like carbon dioxide, this triple point pressure is about five times the normal atmospheric pressure, which is why solid carbon dioxide transitions directly to gas at standard pressure.
Everyday Examples of Sublimation
The most common and dramatic example of this process is dry ice, the solid form of carbon dioxide (\(\text{CO}_2\)). At standard atmospheric pressure, dry ice sublimates rapidly, turning directly into \(\text{CO}_2\) gas without melting into a liquid first. This property is used to create dense, low-lying fog effects for special events. Sublimation also occurs with water ice and snow under certain weather conditions; on cold, dry days, ice or snow can slowly disappear below freezing temperatures as water molecules transition directly from solid crystals into water vapor. Furthermore, solids like iodine crystals, when gently heated in a lab, turn into a distinctive purple vapor without passing through a liquid state.
The Opposite Reaction
The reverse process of sublimation also exists, where a substance transitions directly from the gas phase to the solid phase without forming a liquid. This process is known as deposition. Deposition also occurs at temperature and pressure conditions below the substance’s triple point. A simple, everyday instance of deposition is the formation of frost. When water vapor in the air comes into contact with a surface that is below the freezing point, the gaseous water skips the liquid phase and deposits directly as solid ice crystals. This is how the delicate, crystalline patterns of frost are created.