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Thursday, 29 January 2015

The Process Of Sublimation

the proses of sublimation


Sublimation is the movement of a body from solid to gaseous state without passing through the liquid state. This transformation takes place with the purchase of heat and is therefore an endothermic process.
The sublimation occurs in those substances in which the vapor pressure equals the value of atmospheric pressure before reaching the melting temperature.

The solids that sublime (all are molecular solids with low binding energy), can also merge provided during the heating of their vapor pressure is maintained consistently higher than the pressure of sublimation: iodine so heated in a closed container melts at 113.7 ° C (temperature at which the saturated vapor pressure of the iodine that is about 90 mmHg).

The process of bublimazione and subsequent condensation of the vapor, similar to the distillation of liquids, is often used for separating a species that sublimates from other species that do not sublimate.
The phenomenon of the sublimation of solids is matched to that of the evaporation of liquids; in both cases, in fact, surface molecules of a condensed phase change to the state of steam.

But while in the evaporation of the liquid may form steam even within the liquid mass (boiling phenomenon), the similar phenomenon does not occur in the sublimation. This is due to the fact that in liquids the molecules are mobile and assogettate to convection and the heat supplied from the outside is rapidly distributed among all the molecules constituting the liquid mass. In solids instead of the surface molecules that absorb heat will transmit only a small portion within the solid, and, if the bonds between the molecules of the solid are weak, they may be found to have sufficient energy to pass directly from the solid state to the state of steam: in sublimation, and then, the steam is generated only at the surface, and not within the solid.

Examples of substances that can sublimate: iodine, naphthalene, camphor, solid carbon dioxide (also known as dry ice).