Sublimation is the direct phase transformation from the solid to the gaseous phase, bypassing the liquid state. For example, solid carbon dioxide, or dry ice, sublimes to gaseous carbon dioxide at room temperature and pressure. This transition occurs at the solid–gas phase boundary and is not observed above the triple point. In simple sublimation, the substance is heated in a beaker covered with an upturned watch glass as a secondary surface. The pure substance is deposited on the watch glass, while impurities are left behind. Lyophilization, or, freeze-drying is the low-temperature dehydration of heat-sensitive materials with applications in chemistry, biology, and food preservation. Here, the sample is frozen, and placed in a vacuum. When the temperature is raised, the ice sublimes to water vapor, yielding a cryo-desiccated product.