sublimation
SUBLIMATION
Sublimation is a technique used by chemists to purify compounds. Typically a solid is placed in a vessel which is then heated under vacuum. Under this reduced pressure the solid volatilizes and condenses as a purified compound on a cooled surface, leaving the non-volatile impurities behind. This cooled surface often takes the form of a "cold finger" (shown in the diagram above). Once heating ceases and the vacuum is released, the sublimed compound can be collected from the cooled surface. Usually this is done using a sublimation apparatus.
Sublimation is the process of vaporizing a solid substance and condensing the vapors to again form the solid directly, without passing through an intermediate liquid state. There are very few substances which vaporize without melting, but in all cases of sublimation, the change from the vapor to the solid state, is direct, and without any formation of liquid. The sublimed body is recovered unchanged chemically, but its physical properties are often more or less altered.
Sublimation is influenced by the pressure within the vessel, and is generally carried on under atmospheric pressure only. The process is employed as a means of purification of certain substances, which are heated in closed pans or retorts. In most. eases, the temperature does not exceed a low red heat. Dissociation often occurs in the process.
Sublimation is a phase transition process from a solid to a gas without ever entering an intermediate liquid phase. The ability of any types of solids to sublime depends on the compound’s triple point based on its phase diagram – typically the lower the pressure, the lower the sublimation temperature. (Note that the pressure and temperature of the desired substance must be below its triple point in order to sublime – Figure 1). Solid compounds that can sublime are very rare, for example, solid carbon dioxide (a.k.a. dry ice) can sublime at 1 atm pressure at 78.5 °C. For sublimation to occur, a solid
Sublimation is a technique used by chemists to purify compounds. Typically a solid is placed in a vessel which is then heated under vacuum. Under this reduced pressure the solid volatilizes and condenses as a purified compound on a cooled surface, leaving the non-volatile impurities behind. This cooled surface often takes the form of a "cold finger" (shown in the diagram above). Once heating ceases and the vacuum is released, the sublimed compound can be collected from the cooled surface. Usually this is done using a sublimation apparatus.
Sublimation is the process of vaporizing a solid substance and condensing the vapors to again form the solid directly, without passing through an intermediate liquid state. There are very few substances which vaporize without melting, but in all cases of sublimation, the change from the vapor to the solid state, is direct, and without any formation of liquid. The sublimed body is recovered unchanged chemically, but its physical properties are often more or less altered.
Sublimation is influenced by the pressure within the vessel, and is generally carried on under atmospheric pressure only. The process is employed as a means of purification of certain substances, which are heated in closed pans or retorts. In most. eases, the temperature does not exceed a low red heat. Dissociation often occurs in the process.
Sublimation is a phase transition process from a solid to a gas without ever entering an intermediate liquid phase. The ability of any types of solids to sublime depends on the compound’s triple point based on its phase diagram – typically the lower the pressure, the lower the sublimation temperature. (Note that the pressure and temperature of the desired substance must be below its triple point in order to sublime – Figure 1). Solid compounds that can sublime are very rare, for example, solid carbon dioxide (a.k.a. dry ice) can sublime at 1 atm pressure at 78.5 °C. For sublimation to occur, a solid
References: 1. Fessenden R. J.; Fessenden J. S. Techniques and Experiments for Organic Chemistry, pp.408-409 2. Mayo, D. W.; Pike, R. M.; Forbes, D. C. Microscale Organic Laboratory: With Multistep and Multiscale Syntheses, 5th ed.; pp. 111 – 113. 3. Williamson, K. L. Macroscale and Microscale Organic Experiments, 2nd ed.; pp. 121-122. ANSWERS: 2. It is not as selective as crystalization, it typically requires a vacuum and usually requires that the compound you are trying to separate is volatile, while everything else in the mixture is not volatile. 3.