Phase Transitions: Sublimation and Deposition

It is commonly observed that regular ice melts under ambient conditions, but dry ice does not; instead, dry ice transitions directly into the gas phase. This transition from solid to gas — without passing through the liquid phase — is known as sublimation.

Generally, compounds that sublimate exhibit weak intermolecular forces in the solid state. In dry ice, or solid carbon dioxide, weak dispersion forces exist between CO₂ molecules.

At atmospheric pressure, dry ice remains a solid below negative 78.5 °C. However, at −78.5 °C, the surface molecules acquire enough thermal energy to completely overcome the attractive forces and transform directly into the vapor phase. This is the sublimation point of dry ice.

The amount of energy required to sublimate one mole of a solid is called its molar heat of sublimation or its molar enthalpy of sublimation. As sublimation is an endothermic process, its enthalpy value is always positive.

The reverse of sublimation — that is, the direct transition from vapor to solid — is called deposition. When gas molecules collide with cooler solid surfaces, they lose heat. Multiple collisions result in a significant loss of heat, and the molecules ultimately