Sublimation And Distillation

The processes of sublimation and distillation are very closely allied; in practice, indeed, no hard and fast line is drawn between them.

There are three ways in which the vaporisation of a stable substance may take place: (1) The substance is liquid under ordinary conditions; when heated it boils at a definite temperature depending on the pressure.

(2) The substance is solid; when heated it melts at a definite temperature; when further heated it boils like an ordinary liquid.

(3) The substance is solid; when heated it does not melt, but volatilises at a definite temperature depending on the pressure.

So, also, condensation of vapour may take place in three ways: (1) There may be liquefaction only.

(2) Liquefaction may first take place, and then solidification.

(3) There may be direct passage from the gaseous to the solid state. The term "distillation" is applied to vaporisation and subsequent condensation of the first kind. It should also be applied to vaporisation and condensation of the second kind since it is really liquid which is converted into vapour and which is first formed by condensation.

The term "sublimation" ought, perhaps, only to be employed when the changes of state are of the third kind, but in that case it would rarely be used. Under certain conditions, however, a substance, when heated, may first melt and then boil, but, on cooling, it may pass directly from the gaseous to the solid state. The process is then usually termed sublimation.

Triple Point

The majority of substances may, in fact, undergo either distillation or sublimation according to the pressure under which the vaporisation and condensation take place. This may be best understood by help of the diagram, Fig. 83, in which temperatures are measured as ordinates and pressures as abscissae.

The curve AC represents the boiling points of a substance, the curve ab its volatilising (subliming) points, and the curve ad its melting points under different pressures. The three curves intersect each other at the point a, called by James Thomson the "triple point." The temperature at A, measured vertically from the horizontal axis oy, gives the melting point of the substance under the pressure (triple point pressure) represented by the horizontal distance of a from the vertical axis ox ; it is practically the same as the melting point under atmospheric pressure - generally a small fraction of a degree lower, but in the case of ice 0.0075° higher. The point a also represents both the volatilising point of the solid and the boiling point of the liquid under the triple point pressure.

If a solid substance is heated under a pressure greater than the triple point pressure, its temperature rises until the melting point is reached ; then, after fusion, the temperature again rises until the liquid reaches its boiling point. The melting point and boiling point under any given pressure are indicated by the intersection of a vertical line representing that pressure with the curves ad and ac respectively.

If, however, the pressure is lower than the triple point pressure, the solid substance cannot be melted, but, when heated it volatilises without previous fusion at a temperature given by the intersection of the vertical line of constant pressure with the curve ab. [Although a solid cannot be melted when the pressure is lower than the triple point pressure, a liquid, under certain conditions, may be cooled below its freezing point without undergoing solidification ; it may, indeed, be made to boil at a temperature lower than the freezing point. This is indicated by the dotted line ae which is simply a continuation of the curve ac]

The triple point pressure differs very greatly for different substances : in the vast majority of cases it is far lower than the atmospheric pressure, and therefore, under ordinary conditions, true sublimation rarely takes place.

The following table gives the melting points, the boiling points under normal pressure, and the triple point pressures of a few sub-stances. Complete data have not been obtained for many others.

Fig. 83.

Table 85

The triple point pressure of carbon dioxide is over five atmospheres ; hence when the liquid is allowed to escape from a cylinder in which it has been stored under pressure, that portion of it that escapes vaporisation solidifies at once and cannot be melted again in an open vessel, but gradually volatilises at a temperature of about -78°.

Arsenic and arsenious oxide volatilise without melting when heated under atmospheric pressure, and carbon hexachloride volatilises if the pressure is slightly lowered but melts and boils if the pressure is slightly raised. Water cannot exist as a stable liquid under a pressure lower than 4.6 mm. Below this pressure ice cannot be melted but volatilises at a temperature lower than 0° when heat is supplied.