When an ice piece releases vapor, it undergoes a process called sublimation. Sublimation is the direct transition of a substance from a solid phase to a gaseous phase without passing through the liquid phase.
At temperatures below the freezing point of water (0 degrees Celsius or 32 degrees Fahrenheit), ice is in a solid state. However, even at these low temperatures, ice can release vapor molecules into the surrounding air. This happens because some of the ice molecules have enough energy to break free from the solid lattice structure and transition directly into a gas phase.
The process of sublimation occurs due to the difference in vapor pressure between the ice surface and the surrounding air. Vapor pressure is the pressure exerted by the molecules in the gas phase above a solid or liquid. When the vapor pressure of the ice is higher than the partial pressure of water vapor in the surrounding air, molecules from the ice surface escape into the air, resulting in sublimation.
The rate of sublimation depends on several factors, including temperature, humidity, and surface area. Higher temperatures and lower humidity levels generally facilitate faster sublimation. Additionally, a larger surface area of the ice piece allows more molecules to escape, increasing the rate of sublimation.
It's important to note that sublimation is not exclusive to ice; it can occur with other substances as well. For example, solid carbon dioxide (dry ice) can sublimate directly into a gas without going through the liquid phase.