. Introduction to inorganic chemistry . y fromthe neighborhood of the surface of the solid andmake room for others, and thus, if the systemremains undisturbed, the liquid will eventuallybecome a solution of uniform concentration. Ifa large enough amount of the solid has beenprovided, the ultimate condition will be that ofa saturated solution with excess of the solidbeneath. If we had proper means of measuringit, the tendency of the molecules to leave the solid in the presenceof a given liquid would give the effect of a kind of pressure. This isspoken of as solution pressure. Now the molecules,
. Introduction to inorganic chemistry . y fromthe neighborhood of the surface of the solid andmake room for others, and thus, if the systemremains undisturbed, the liquid will eventuallybecome a solution of uniform concentration. Ifa large enough amount of the solid has beenprovided, the ultimate condition will be that ofa saturated solution with excess of the solidbeneath. If we had proper means of measuringit, the tendency of the molecules to leave the solid in the presenceof a given liquid would give the effect of a kind of pressure. This isspoken of as solution pressure. Now the molecules, after having entered the liquid, move inevery direction, and consequently some of them will return to thesolid and attach themselves to it. The frequency with which thiswin occur will be greater as the crowding of particles in the liquidincreases, so that a stage will eventually be reached at which thenumber of molecules leaving the solid will be no greater than thatlanding upon it in a given time. If the whole of the liquid has mean-. FlG. 59. SOLUTION 153 while become equally charged with dissolTed molecules, there will be nochance that the field of liquid immediately round the solid will losethem by diffusion, so that a condition of balance or equilibrium willhave been established: NaCl (solid) ^ NaCl (dissd). The motion ofthe particles in the liquid produces what we have called osmotic press-ure ; and when the osmotic pressure, by the continual increase in thenumber of dissolved molecules, becomes equal to the solution press-ure, increase in concentration of the solution ceases. It is at thispoint that we speak of the solution as being saturated with respect tothe particular substance dissolving. • The analogy to vapor tensionand vapor pressure (p. 135) is evident. The necessity of distinguishing between the fictions used in thinking about anddescribing the phenomena of solution and the facts themselves must be empha-sized here as it was in the preceding chapter. The arr
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