Sublimation is a phase transition where a substance moves directly from the solid state to the gaseous state, completely bypassing the intermediate liquid state. This process requires the input of energy, typically heat, to provide molecules with enough energy to break free from the attractive forces holding them in the solid structure. Determining a single, simple temperature for sublimation is not possible because the process is heavily dependent on the surrounding pressure.
Sublimation Requires Specific Pressure Conditions
The temperature for sublimation is not fixed because temperature and pressure are inextricably linked in determining a substance’s phase. Every solid has a vapor pressure, which is the pressure exerted by its gaseous form in equilibrium with the solid. Sublimation occurs when the solid’s vapor pressure is greater than the surrounding environmental pressure. If the external pressure is too high, the solid will melt into a liquid instead of transitioning directly to a gas.
For a substance to sublime, the temperature must be high enough to create significant vapor pressure, while the external pressure must be low enough to permit the solid-to-gas transition. The line separating the solid phase from the gas phase on a phase diagram represents the sublimation temperature at any given pressure. This solid-gas line demonstrates that as pressure decreases, the temperature required for sublimation also decreases.
Most solids melt instead of sublime under everyday conditions because standard atmospheric pressure is too high for the solid-gas transition to take precedence. Reducing the pressure significantly allows substances that typically melt to instead sublime. This principle is used in specialized processes, such as freeze-drying, where a frozen substance is placed in a strong vacuum to force the ice to turn directly into water vapor.
Defining the Triple Point
The absolute upper limit for sublimation is defined by the triple point. The triple point is the specific combination of temperature and pressure where the solid, liquid, and gas phases of a substance all coexist in thermodynamic equilibrium. It is the precise point on a phase diagram where the sublimation curve, the melting curve, and the vaporization curve all meet.
The triple point pressure sets the boundary for the sublimation process. If the surrounding pressure is above this value, the substance, when heated, will melt and become a liquid. Sublimation can only occur at pressures below the triple point, which ensures the liquid phase is physically impossible. For example, the triple point of water is \(0.01^\circ\text{C}\) at a very low pressure of \(611.73\text{ Pascals}\).
If ice is heated at the triple point pressure (\(611.73\text{ Pa}\)), it will sublime at \(0.01^\circ\text{C}\). If the pressure is even lower, the sublimation temperature will be slightly lower than the triple point temperature. Conversely, if ice is heated at standard atmospheric pressure (\(101,325\text{ Pa}\)), it must melt into liquid water at \(0^\circ\text{C}\) before it can boil into a gas.
Examples of Sublimating Substances
Solid carbon dioxide, known as dry ice, readily sublimes under normal conditions. Carbon dioxide has a triple point at \(5.1\text{ atmospheres}\) and \(-56.4^\circ\text{C}\). Since standard atmospheric pressure is significantly lower than its triple point pressure, dry ice cannot exist as a liquid at sea level.
At standard atmospheric pressure, dry ice sublimates at a consistent temperature of \(-78.5^\circ\text{C}\) (\(-109.3^\circ\text{F}\)). This fixed temperature is the equilibrium point where the solid transitions directly to the gas phase. This combination makes dry ice useful as a refrigeration agent that leaves no liquid residue.
Water ice also sublimes, often observed in everyday phenomena like the disappearance of snow or ice cubes left too long in a freezer. This slow sublimation occurs below its melting point of \(0^\circ\text{C}\) because the partial pressure of water vapor in the atmosphere is below the triple point pressure.
Iodine is another substance used to demonstrate sublimation, forming a vivid purple vapor when heated. Iodine’s triple point is at \(113.5^\circ\text{C}\) and \(12.07\text{ kilopascals}\). While solid iodine will gradually sublime at room temperature due to its relatively high vapor pressure, it can be melted into a liquid if heated carefully at standard atmospheric pressure above \(113.5^\circ\text{C}\).