Sublimation (chemistry)

Sublimation of an element or compound is a transition from the solid to gas phase with no intermediate liquid stage. Sublimation is an endothermic phase transition that occurs at temperatures and pressures below the triple point (see phase diagram).

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 pass directly from a solid into the gaseous state. This can occur if the atmospheric pressure exerted on the substance is too low to stop the molecules from escaping from the solid state.

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

Examples

Some substances (such as zinc and cadmium) will sublime 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 sublimes at atmospheric pressure—a block of solid CO₂ (dry ice) at room temperature and at atmospheric pressure will turn into gas without becoming a liquid. Iodine is another example of a substance that produces fumes on gentle heating. In contrast to CO₂, though, it is possible to obtain liquid iodine at atmospheric pressure by controlling the temperature at just above the melting point of iodine. Snow and other water ices also sublime, although more slowly, at below-freezing temperatures. This phenomenon, used in freeze drying, allows wet cloth to be hung outdoors in freezing weather and retrieved later in a dry state. Naphthalene, a common ingredient in mothballs, also sublimes easily. Arsenic can also sublime at high temperatures. Sublimation requires additional energy and is an endothermic change. The heat of sublimation (also called enthalpy of sublimation) can be calculated as the enthalpy of fusion plus the enthalpy of vaporization.

Other substances, such as ammonium chloride, appear to sublime because of chemical reactions. When heated, ammonium chloride decomposes into hydrogen chloride and ammonia in a reversible reaction:

:NH₄Cl → HCl + NH₃

ublimation purification

. [King, R. B. Organometallic Syntheses. Volume 1 Transition-Metal Compounds; Academic Press: New York, 1965. ISBN 0444426078.]

Uses

"Frost-free freezers" work by having a fan and air circulation inside the freezer. The sub-zero temperature combined with the air circulation that keeps the air arid significantly accelerates the sublimation process. This keeps freezer walls and shelves free of ice, although ice-cubes will continually sublime.

"Dye sublimation" is also often used in color printing on 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 transferred to T-shirts, hats, mugs, metals, puzzles and other surfaces.

In the "Fast-Freeze, Deep-Etch" technique, samples (for example, tissue samples) are rapidly frozen in liquid nitrogen and transferred to a vacuum device in which surface ice is sublimed. This effectively etches the sample surface, revealing the preserved 3D structure of the hydrated material. A rotary shadowed surface replica can then be obtained via electron microscopy.

Sublimation is also used to create freeze-dried substances, for example tea, soup or drugs in a process called "lyophilization", which consists in freezing a solution or suspension and heating it very slowly under medium to high vacuum - specifically, a pressure lower than the vapor pressure of the solvent at its melting point. This can be well under the melting point of water if there are organic solvents or salts in the sample being freeze-dried. The resulting solid is usually much easier to dissolve or resuspend than one that is produced from a liquid system, and the low temperatures involved cause less damage to sensitive or reactive substances.

Historical usage

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 core alchemical processes.

ee also

* Phase diagram
* Heat of sublimation:

References