Sublimation (phase transition)

Dark green crystals of nickelocene, freshly sublimed on a cold finger.

Sublimation is the transition of a substance from the solid phase to the gas phase without passing through an intermediate liquid phase.The reverse process is also known as sublimation Sublimation is an endothermic phase transition that occurs at temperatures and pressures below a substance's triple point in its 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 (carbon and arsenic) 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 reverse process of sublimation is deposition. The formation of frost is an example of meteorological deposition.

1 Examples
2 Sublimation in physics
3 Sublimation purification
4 Historical usage
5 See also
6 References

Examples

Carbon dioxide

Small pellets of dry ice subliming in air.

Carbon dioxide sublimates readily at atmospheric pressure—a block of solid CO₂ (dry ice) at room temperature and atmospheric pressure. Iodine 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.

Water

Snow and ice sublimate, although more slowly, below the melting point temperature. This phenomenon is used in freeze drying, by hanging wet cloth outdoors in freezing weather to be retrieved dry at a later time. The loss of snow from a snowfield during a cold spell is often caused by sunshine acting directly on the outer layers of the snow. Ablation is a process which includes sublimation and erosive wear of glacier ice.

Other compounds

Camphor being sublimated. Note the white purified camphor on the cold finger, and the dark-brown crude product.

Naphthalene, a common ingredient in mothballs, also sublimates easily. Arsenic can also sublimate at high temperatures. Sublimation requires additional energy and is an endothermic change. The enthalpy of sublimation (also called heat of sublimation) can be calculated as the enthalpy of fusion plus the enthalpy of vaporization.

Various substances appear to sublimate because of undergoing chemical reactions or decomposition. When heated, ammonium chloride decomposes into hydrogen chloride and ammonia in a reversible reaction:

NH₄Cl → HCl + NH₃

Sublimation in physics

Sublimation is the atomic or molecular escape from a solid surface directly into the adjacent gaseous phase of the same element or chemical species. It occurs at temperatures and pressures low enough to prevent the existence of the liquid phase for that element/ chemical species.

Such extremely low temperatures would only allow the gas molecules to directly condense into grains (by extreme short-range cohesiveness among molecules) while the extremely low pressures would only allow the molecules of solid to definitively escape their mutual and collective attractive ranges — as gases exhibit no volume restrictions.

Since the atom/ molecule must break its strong (non-chemical) intermolecular bonds within the solid to escape into the rarefied, non-bonded gas phase, sublimation requires strong local molecular-neighbor out-of-phase (antagonic) oscillations (very short wavelength phonon convergence and overlap) — achievable when the solid absorbs enough energy.

The reverse process to sublimation is the adsorption onto surfaces — e.g. frost formation. Deposition is the technology that brings together (in a laboratory) the conditions that promote the adsorption onto surfaces.

It should be noted that in the above explanation about the temperature and pressure conditions necessary for sublimation to occur, X only allows Y to do Z is equivalent to X allows only Y to do Z, where X, Y, and Z are general statements (they can be any statement). The statement that X only allows Y to do Z can be interpreted to imply X does not help or cause (etc.) Y to do Z — X only allows it — or that when condition X is in place, Y can do nothing but Z. This is like saying that very low temperatures do not help gas molecules condense directly to solid grains — they only let gas molecules do this — or that at very low temperatures, gas molecules can do nothing but condense into grains — i.e. they can't just fly around. This distinction is subtle, but should be considered to avoid misunderstanding the effects of pressure and temperature on sublimation and molecular behavior in general.

Sublimation purification

Sublimation is a technique used by chemists to purify compounds. Typically a solid is placed in a sublimation apparatus and heated under vacuum. Under this reduced pressure the solid volatilizes and condenses as a purified compound on a cooled surface (cold finger), leaving a non-volatile residue of impurities behind. Once heating ceases and the vacuum is removed, the purified compound may be collected from the cooling surface.^[1]^[2]

Historical usage

In alchemy, sublimation typically referred 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.

	To
From	Solid	Liquid	Gas	Plasma
Solid	Solid-solid transformation	Melting/fusion	Sublimation	N/A
Liquid	Freezing	N/A	Boiling/evaporation	N/A
Gas	Deposition	Condensation	N/A	Ionization
Plasma	N/A	N/A	Recombination/deionization	N/A

References

↑ King, R. B. Organometallic Syntheses. Volume 1 Transition-Metal Compounds; Academic Press: New York, 1965. ISBN 0-444-42607-8.
↑ Laurence M. Harwood, Christopher J. Moody. Experimental organic chemistry: Principles and Practice (Illustrated edition ed.). pp. 154–155.