Sublimation (phase transition)
As sublimation , rarely sublimation (of lat. Sublimis , high in the air located ',' superior '), is the process of immediate transition of a substance from the solid to the gaseous state of aggregation without liquefy before. It is a purely physical process in which the substance remains chemically unchanged.
Under the pressure and temperature conditions under which sublimation occurs, there is no liquid aggregate state, as can be seen in the phase diagram on the right. These conditions are also referred to as sublimation pressure and sublimation temperature, or taken together as the sublimation point . This in turn is part of the sublimation curve of the phase diagram, which is given in the adjacent example by the phase boundary line between solid and gas below the triple point .
The phase change in the opposite direction to sublimation is referred to in technical terms as resublimation or as deposition or desublimation. The resublimation point for pure substances is identical to the sublimation point. In the case of mixtures , it must be noted that both can differ and therefore the direction of the phase transition also plays a role in this case .
If there is a sublimation temperature at normal pressure, this is called the normal sublimation temperature and the substance is tabulated with its value without additionally specifying the sublimation pressure.
- Sublimation in the real sense
- Boron , carbon and arsenic , but also organic compounds such as camphor, are converted directly into gaseous form when heated under normal pressure .
- Dry ice , at -78.5 ° C frozen carbon dioxide is sublimated when heat and directly enters into the gaseous state of aggregation above. Under normal pressure conditions, no liquid such as B. when heating water ice : this is where the term dry ice is derived .
- Sublimation above the triple point
- If the air is sufficiently cold and dry at atmospheric pressure, water also passes directly from its solid state of aggregation into gaseous form ( water vapor ). The reason for this is that the water vapor pressure is a partial pressure of the air and is therefore lower than the atmospheric pressure of the air. This is why this phase transition takes place in the phase diagram for water in the sublimation area below the triple point, even if it appears to occur at atmospheric pressure. Thanks to this effect, damp laundry dries faster in the outside air under the corresponding conditions in frosty conditions than in interior rooms, where the warmer air is already more or less saturated with water vapor.
- Iodine sublimes at normal pressure when heated, whereby this is particularly effective as a violet gas and is therefore often used as a show experiment. In fact, the pressure at the triple point (386.65 K, 12.1 kPa) is far below the ambient pressure (101.315 kPa). However, as long as the temperature is kept below the melting point (113.70 ° C), iodine actually changes from the solid to the gaseous state. In addition, iodine first melts into a deep purple to black liquid.
Confusion with dissociation
When processing materials by means of laser beam sublimation cutting, the sublimation occurs with the high pulse power of the laser. The material does not melt first, but changes directly from the solid to the gaseous state. The result is a very clean cut with no slag or frayed edges.
Sublimation is also an alternative to recrystallization when purifying products in synthetic chemistry. Compared to crystallization , sublimation offers several advantages: The products are often very clean and even the smallest amounts can be conveniently sublimated in a sublimation apparatus. The authors present an example of the application of sublimation of small amounts in the radiochemical field for radiochemical purification of a nuclide in the context of the half-life determination of 79 Se . Sublimation is primarily a laboratory process for cleaning fabrics, e.g. B. for ferrocene and pyrogallol . A disadvantage compared to crystallization from the melt is the comparatively difficult scaling up during sublimation. In the chemical industry, the technical implementation of sublimation as a separation method is therefore of secondary importance. Sublimation is used industrially as a refining process for naphthalene , phthalic anhydride , camphor , anthraquinone , salicylic acid , benzoic acid , uranium hexafluoride and many metals.
Chemical transport reactions are used to purify substances (mostly metals) by means of sublimation. Here, however, the substance to be cleaned is first converted into a secondary substance in chemical reactions, which z. B. has a lower sublimation pressure. Examples of transport reactions are the Mond process or the Van Arkel de Boer process .
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