Lowering of the Freezing Point.—Pure water freezes at 0° C. Raoult showed that the introduction of a non-ionizable substance, such as sugar or alcohol, lowers the freezing point of a solution in proportion to the molecular concentration of the solute. One gramme-molecule of the solute introduced into one litre of the solution lowers its temperature of congelation by 1.85° C. Thus a normal solution of any non-ionizable substance in water freezes at -1.85° C. The measurement of this lowering of the freezing point is called Cryoscopy, a method which is becoming of great utility in medicine.
Cryoscopy of Blood.—In order to determine the osmotic pressure of the blood at 37° C., i.e. 98.6° F., the normal temperature, we proceed as follows. On freezing the blood, we find that it congeals at -.56°. Its molecular concentration is therefore .56 / 1.85 = .30, or about one-third of a gramme-molecule per litre. Its osmotic pressure at 0° C. is therefore .3 × 22.35 = 6.7 atmospheres. The increase of pressure with temperature is the same as for a gas, viz. 1/273, or .00367 of its pressure at 0° for every degree rise of temperature. The increase of pressure at 37° is therefore .00367 × 37 × 6.7 = .9 atmospheres. The total osmotic pressure at 37° is therefore 6.7 + .9 = 7.6 atmospheres.
Rise of Boiling Point.—Water under atmospheric pressure boils at a temperature of 100° C. The addition of a solute whose solution does not conduct electricity, such as sugar, causes a rise in the boiling point proportional to the molecular concentration of that solute.
Lowering of the Vapour Tension.—The vapour tension of a liquid is lowered by the addition of a solute. A liquid boils at the temperature at which its vapour tension equals that of the atmosphere. Since an aqueous solution of sugar at atmospheric pressure does not begin to boil at 100° C., it is manifest that its vapour tension is then less than that of the
atmosphere. The addition of a solute such as sugar, whose solution is not ionizable, and therefore does not conduct electricity, lowers the vapour tension of the solution in proportion to the molecular concentration of the solute.
Corresponding Values.—We have thus found five properties of a solution which vary proportionally, so that from the measurement of any one of them we can determine the corresponding values of all the others. These are—
1. The Molecular Concentration.
2. The Osmotic Pressure.
3. The Diminution of Vapour Tension.
4. The Raising of the Boiling Point.