Le Chatelier and freeze-drying

[malcolm]

what causes sublimation at low temperatures?

[bharatiyedu]

Sublimation of an element or substance is a conversion between the solid and the gaseous phases of matter, with no intermediate liquid stage. Sublimation is a phase transition that occurs at temperatures and pressures below the triple point

 

At normal pressures, most chemical compounds and elements possess three different states at different temperatures. In these cases the transition from the solid to the gaseous state requires an intermediate liquid state. However, for some elements or substances at some pressures the material may transition directly from solid to the gaseous state. Note that the pressure referred to here is the vapor pressure of the substance, not the total pressure of the entire system.

 

The opposite of sublimation is deposition. The formation of frost is an example of meteorological deposition.

 

 

Sublimation and the Elements

 

Some materials (such as zinc and cadmium) will sublimate at low pressures and thus may be a problem encountered in high-vacuum applications.

 

Carbon dioxide is a common example of a chemical compound that sublimates at atmospheric pressures (in other words when you set it on a table, it turns into CO2) (see dry ice). Iodine is another example. Snow and other water ice also sublimate, though more slowly, at below-freezing temperatures. This allows wet cloth to be hung outdoors in freezing weather and retrieved later in a dry state, and is also the process used in freeze drying. Naphthalene, which is a common ingredient in mothballs, also sublimes slowly. Sublimation requires additional energy and is an endothermic change.

 

Uses of Sublimation

 

Dye-sublimation is also often used to print full-color images onto a variety of substrates, including paper. A small heater is used to vaporize the solid dye material, which then solidifies upon the paper. As this type of printer allows extremely fine control of the primary color ratios it is possible to obtain a good quality picture even with relatively low printer resolution, as compared to other printer types of similar resolution. Standard black and white laser printers are capable of printing on plain paper using a special Transfer Toner containing sublimation dyes which can then be permanently heat transfered to T-shirts, hats, mugs, metals, puzzles and other surfaces.

 

In alchemy, sublimation typically refers to the process by which a substance is heated to a vapor, then immediately collects as sediment on the upper portion and neck of the heating medium (typically a retort or alembic). It is one of the 12 most important alchemical processes.