82 GENERAL CONDITIONS OF EQUILIBRIUM, chap.
Other special cases of the law of distribution are the laws that a solid substance dissolves in a particular liquid until a certain degree of concentration is reached (until a saturated solution is formed), and that liquids and solid substances at a particular temperature give ofif vapour until a certain pressure is reached.
Kinetic Considerations. — The development made above is based on the mechanical theory of heat (thermodynamics), and is therefore strictly exact. However, it is usual also to i-egard these laws from a kinetic point of view, and as this has been of great service and is of assistance in visualising the matter, a short account may be given here.
Let us suppose that we have water at a certain tempera- ture in a vacuum. Part of the water vaporises, and so fills the space above with water vapour. This evaporation takes place until the number of molecules which pass into the water per second is exactly equal to the number which leave it. The equilibrium which obtains is mobile. It is clear that this equilibrium depends only on the conditions in the immediate neighbourhood of the surface of separation of liquid and vapour. If the vapour-space be increased, the new volume must become filled with vapour at the same pressure as that in the original space, otherwise there would not be an equilibrium between this latter and the new portion. At the surface of separation no change whatever occurs. A liquid, therefore, at a given temperature must possess a certain definite vapour pressure which is inde- pendent of the quantity of vapour and liquid present.
In the same way it can be imagined that the solution of a solid substance in a liquid takes place imtil in unit time there are as many molecules leaving the solid as there are molecules separating from the solution. The same considera- tion as that used for the evaporation of a liquid leads to the conclusion that a solid substance in contact with a liquid forms a saturated solution, the concentration of which depends on the temperature, but is entirely independent of the quantity of solid and liquid present.
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