Sublimation is the term that describes the change of state of a material from a frozen form to a gas or visa versa. In sublimation, there is no intermediate liquid phase.
A well-known example of sublimation occurs with dry ice, the frozen form of carbon dioxide. When exposed to air, dry ice changes directly to vapor, which is visible as a cloud immediately above the frozen CO2. In the case of dry ice, the frozen CO2 is energetically more stable as a gas at room temperature than as the frozen solid.
The gaseous tail that develops behind a comet as it approaches the sun is another example of sublimation. Frost and snowflakes are products of a reverse path of sublimation, where water changes directly from the gaseous state to the solid state.
Sublimation has practical applications in forensic science . Forensic analysis of a crime or accident scene often relies on the examination of photographic evidence after the scene has been cleaned. A dye-sublimation printer enables digital pictures to be rendered in print form in a very realistic and detailed fashion, which helps investigators in their analysis.
The basis of a dye-sublimation printer is the vaporization of various colored dyes housed in the printer. The vaporized dyes penetrate the glossy surface of the photographic paper before returning to their solid form. The vapor-to-solid dye sublimation creates a gentle gradation at the edge of each pixel of color, rather than a sudden border between the dye and the paper (as is the case with inkjet type printers). The result is a more realistic image that yields more detail.
Dye sublimation is also used to create digital watermarks on documents. This enables a forensic examiner to differentiate an authentic document from a forgery.
Sublimation can be important in the recovery of compounds that are suspended or dissolved in a fluid or a solid like dry ice. The compounds can be recovered, at least in crude form, by allowing the suspending matrix to sublimate away. This method of recovery is usually gentle, which is advantageous in preserving the chemical structure or even activity of the target drug (i.e., cocaine) or enzyme. Many compounds will sublimate when heated. The effective temperature can be characteristic of the compound and can be measured in a forensic laboratory inexpensively, using a common hot plate.
see also Analytical instrumentation; Exothermic reactions.