. Introduction to inorganic chemistry . former sub-stance turns into the latter. At transition points like this the values of solubility,vapor pressure, and some other properties, are always the same for both forms (seeFreezing-points of solutions). Properties of Solutions Proportional to Concentration:Vapor Tension. — Besides osmotic pressure (p. 151), there are severalproperties of solutions which are proportional to the concentration ofthe solution. If, instead of water, we introduce aqueous solutions of the samesubstance successively into the barometric vacuum (Fig. 39, p. 116), wefind tha
. Introduction to inorganic chemistry . former sub-stance turns into the latter. At transition points like this the values of solubility,vapor pressure, and some other properties, are always the same for both forms (seeFreezing-points of solutions). Properties of Solutions Proportional to Concentration:Vapor Tension. — Besides osmotic pressure (p. 151), there are severalproperties of solutions which are proportional to the concentration ofthe solution. If, instead of water, we introduce aqueous solutions of the samesubstance successively into the barometric vacuum (Fig. 39, p. 116), wefind that the vapor pressures of the solutions are less than that ofwater at the same temperature. The diminution in the fall of theme^^cury column, which measures the lo-wering in vapor pressure, isproportional to the concentration of each solution. The limit isreached with the saturated solution, although, if this is rather con-centrated, the proportionality does not hold strictly down to thatpoint (see Chap. xvii). 162 INORGANIC CHEMISTRY. This lowering in the vapor pressure of water is often considerableiThus at 100° a per cent solution of potassivun chloride shows avapor pressure of only mm., while that of water is 760 mm. Thedifference is mm. Hence the solution hq,s to be raised to a highertemperature (°) before it boils. This is almost exactly .037°per 1 mm., which is the value for pure water (p. 118). This conclusion may alsobe reached graphically (). The ordinates representthe vapor tensions correspond-ing to the temperatures shownby the abscissae. They in-crease in length with rise intemperature. The horizontaldotted line shows the vaportension of 760 mm. at whichany liquid will boil. Theboiling-point of the solvent istherefore the temperature atwhich its curve intersects thisline. Since the vapor tensionsof the solution are all belowthose of the solvent, its curve lies below that for the solvent but ascends along withthe latter. Hence it also
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