Melting vs. Sublimation

Difference Between Melting And Sublimation

Melting

The melting point is the sharply defined, reproducible temperature at which a pure solid substance, when heated, changes into a liquid. This temperature is the same as the freezing point of the liquid. At the melting point, solid and liquid exist together in equilibrium. Heating this mixture further will not raise its temperature but will cause more solid to melt. Depending upon the substance, the melt may have a greater or lesser density than the solid.

Impurities alter the melting point. In fact, the purity of a sample is often tested by observing the temperature at which it melts. Also, depending upon the substance, the application of pressure may raise or lower the melting point. Thus, pressure lowers the melting point of ice, but raises that of tin.

Sublimation

Sublimation, in chemistry, is the direct passage of a substance from the solid state to that of a gas or vapor and sometimes back to a solid, without ever assuming the liquid form. For example, in the laboratory, iodine can be placed in a deep porcelain dish closed by a round-bottomed flask through which cold water passes as the dish is heated. Iodine vapors are formed that then condense on the bottom of the flask as a solid again. The most familiar sublimates are frost and snow, which form directly from water vapor in the air.

In comparison with operations such as distillation and crystallization, in which solid materials are obtained in a relatively pure state, sublimation is of minor importance in industry. It is carried out in those cases in which the higher temperature required for distilling a liquid would cause decomposition, or in which the use of solvents for crystallization would be accompanied by difficulties. The operation is also useful in putting a substance in the form of a vapor into a gas-phase reaction or in removing a pure product from such a reaction. Substances subjected to sublimation vary from metallic magnesium to organic solids such as anthracene and salicylic acid. Ordinarily a heating pot or a covered pan is used. Large, cooled tubes with scrapers, or quite large chambers with large collecting surfaces that can be individually shaken, are employed to collect the sublimated material.

 

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