. Introduction to inorganic chemistry . vent. Measurement of Freezing-Points, — The task consists in measuring exactlythe temperature at which a previouslyweighed quantity of the solvent freezes,and then, after dissolving in it a knownweight of some soluble substance, deter-mining the freezing-point once more. Theabsolute values of these two points are notrequired, it is simply the difference be-tween them that has to be known withexactness (c/. p. 163). By means of avery delicate thermometer (Fig. 75) havingonly six degrees on the whole scale, thetemperature of the freezing liquid ihaybe read
. Introduction to inorganic chemistry . vent. Measurement of Freezing-Points, — The task consists in measuring exactlythe temperature at which a previouslyweighed quantity of the solvent freezes,and then, after dissolving in it a knownweight of some soluble substance, deter-mining the freezing-point once more. Theabsolute values of these two points are notrequired, it is simply the difference be-tween them that has to be known withexactness (c/. p. 163). By means of avery delicate thermometer (Fig. 75) havingonly six degrees on the whole scale, thetemperature of the freezing liquid ihaybe read to one one-thousandth of a reservoir at the top enables us to add to, or subtract from, themercury contained in the bulb and column, and so the same instrumentmay be used with solvents having widely different water is being employed as the solvent, the outer jar must befilled with a freezing mixture of ice and water containing salt. Withsolutions in benzene, ice and water are used alone. To avoid super-. FlG. 75. DISSOCIATION IN SOLUTION 291 cooling, the solvent or solution must be vigorously stirred after ithas been cooled down to a point just below the freezing-point. Laws of Freezing-Point Depression. — The depression isdirectly proportional to the weight of dissolved substance in agiven amount of the solvent. The depression is inversely propor-tional to the amount of solvent. Thus, if we double the concen-tration of the solution, the depression in the freezing-point is , equal numbers of molecules of different solutes in the samequantity of solvent give equal depressions. Or, in other words, thedepression is proportional to the concentration of the moleciiles ofthe solute. Thus, solutions containing 342 g. of sugar (Ci^H^^O^), or46 g. of alcohol (CjHjO), or 74 g. of methyl acetate (CHaC^HsO^), in 1000g. of water, show a depression below the freezing-point of water ° in each case. This depression produced by a mole of
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