Sublimate is the product of sublimation, a phase transition where a substance moves directly from its solid state to its gaseous state, bypassing the liquid phase. This transition is endothermic, meaning it requires the absorption of heat energy. The term “sublimate” can also be used as a verb to describe this action. Sublimation is distinct from melting and evaporation because the solid never turns into a fluid before becoming a vapor.
How Sublimation Skips the Liquid Phase
The ability of a substance to skip the liquid phase depends on a balance of energy and pressure conditions. To transition from a solid, molecules must gain enough thermal energy to overcome the intermolecular forces holding them in a rigid lattice structure. In a typical phase change, this energy allows molecules to move as a liquid before escaping as a gas.
Sublimation occurs when molecules gain sufficient energy to jump directly into the gas phase. This happens when the solid’s vapor pressure is higher than the surrounding atmospheric pressure at a given temperature. Vapor pressure is the pressure exerted by a substance’s vapor phase in equilibrium with its solid or liquid phase.
For this direct transition to be energetically favorable, the intermolecular forces within the solid must be relatively weak. The energy absorbed, known as the latent heat of sublimation, allows particles to break free from the solid’s attractive forces. This direct leap bypasses the specific conditions required for the formation of a stable liquid.
Everyday Examples and Practical Uses
One of the most widely recognized examples of sublimation is dry ice, which is solid carbon dioxide (\(\text{CO}_2\)). At standard atmospheric pressure, dry ice does not melt into liquid \(\text{CO}_2\); instead, it changes directly into carbon dioxide gas, which is why it is called “dry” ice. This occurs because the pressure required for liquid \(\text{CO}_2\) to exist is about five times greater than the pressure at sea level.
Another common example involves water ice and snow at temperatures below freezing. Over time, a layer of snow or ice can disappear without ever turning into liquid water, particularly in conditions of low humidity and high winds. Solid materials like iodine crystals and naphthalene, the primary component in some mothballs, also exhibit noticeable sublimation at room temperature, releasing purple vapor or a distinct scent, respectively.
Sublimation is a process employed in the method of freeze-drying, or lyophilization, in food and pharmaceutical industries. This technique involves freezing a material and then placing it under a vacuum to encourage the frozen water to sublime directly into water vapor.
By removing the water content through sublimation, the process avoids the heat-induced damage that conventional drying methods cause, preserving the original structure, flavor, and nutritional value of foods. In pharmaceuticals, freeze-drying is employed to stabilize sensitive biological products, such as vaccines and certain drugs, by maintaining their potency and extending their shelf life.
The Science of Deposition and the Triple Point
The reverse process of sublimation is called deposition, where a gas changes directly into a solid without forming a liquid. This occurs when gas molecules lose thermal energy and slow down sufficiently to be captured by intermolecular forces, forming a solid structure. A familiar example is the formation of frost, where water vapor deposits directly onto a cold surface as ice crystals.
Sublimation and deposition are best understood by examining a substance’s phase diagram, a pressure-temperature graph that maps the conditions under which a substance exists as a solid, liquid, or gas. The solid-gas boundary on this diagram represents the conditions where sublimation and deposition occur.
A single point on this diagram, known as the triple point, is where the lines for all three phases—solid, liquid, and gas—meet. The triple point is the only specific combination of temperature and pressure at which all three phases can coexist in thermodynamic equilibrium. Sublimation can only occur at pressures and temperatures that fall below the triple point of a substance.
For \(\text{CO}_2\), the triple point is significantly above standard atmospheric pressure, which explains why solid \(\text{CO}_2\) must sublime at normal conditions. For water, the triple point is very close to standard conditions, which is why ice typically melts to a liquid first.