Theoretical and physical chemistry by Bigelow Samuel Lawrence 1870-

Author:Bigelow, Samuel Lawrence, 1870- [from old catalog]
Language: eng
Format: epub
Tags: Chemistry, Physical and theoretical
Publisher: New York, The Century co.
Published: 1912-03-25T05:00:00+00:00

Fig. 35.

phase, start condensation which proceeds until the conditions are readjusted and they are represented by some position on the full line of the isotherm. We distinguish then between conditions of what we call stable equilibrium, all of which are represented by the full line of the isotherm, and these others which we can realize, but which a small thing will upset, and which we call metastable equilibria.

There is no contradiction to Gibbs' phase law in these phenomena. When we have a supercooled gas there is but one phase present, not two. The introduction of mere traces of the other phase at once starts a change which does not stop until conditions of stable equilibrium are reached.

Dew. We have many examples of this phenomenon on a large scale. For instance, during a hot simmier day much water evaporates and is held in the atmosphere as water vapor. In the evening, the temperature falls and the air is apt to become supersaturated with water vapor and the moisture precipitates where there is some moisture already, that is, on the surface of the groimd and on vegetation, as dew. Moreover, these objects radiate heat into space and so cool off more rapidly than the air above them; this, of course, increases the deposition of dew. Clouds hinder radiation into space and less dew deposits on cloudy than on clear nights.

Formation of Clouds. Again, after a hot and windless day, dust has settied; it may grow cooler and a supercooled condition may exist at an elevation. If we watch for it, we may frequentiy see a small cloud grow to many times its size. It is surprising how few appear to have observed this common phenomenon. It sometimes happens that the atmosphere becomes so supersatiu'ated that clouds form with extreme rapidity, and we get heavy showers, occasionally what we describe as a cloud-burst.

Superheating. On the other hand, we know that water in a per-fectiy dean flask may be heated above its boiling point. This is only too readily shown, especially when we try to boil solutions of the alkalies which clean a flask or beaker of all grease. The liquid superheats and then, suddenly, a volume of vapor forms which throws out part of the contents. The liquid " bumps:" Globules of water suspended in oil of the same specific gravity have been heated to 145°.

The conditions of superheated liquids are represented by positions on the prolongation of the curve DC below C and above c. These are likewise conditions of metastable equilibriimi, but one phase is present where there should be two and the phase law does not apply.

Small things upset the metastable equilibrium. The presence of the least particle of the vapor prevents such superheating.

Glass beads, platinum scrap, bits of porcelain, sharp edges of almost any sort diminish bumping because they appear to facilitate the formation of the first bubble of vapor. Porous substances such as pumice, even capillary tubes full of air, act in the same way.

Prolongations of the Isotherm.

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