dissolved substances proportional to their molecular weights have the same point of solidification.

*

Thus, centi-normal solutions of sodium chloride and potassium chloride (i.e. solutions containing 0.585 gramme NaCl and 0.746 gramme KCl respectively in one litre of water) will begin to freeze at the same fraction of a degree below o°. In other words, the depression of the freezing-point of the solvent is a function of the number of molecules of the dissolved substance, irrespective of the nature of the molecules. The extent to which the freezing-point of a liquid would be depressed by dissolving in 100 grammes of it one gramme-molecule of any substance is called the molecular depression of the freezing-point of that liquid, and it is found that in the case of all substances which are non-electrolytes, i.e. which do not undergo ionisation, this molecular depression for a given liquid is practically a constant. Thus in the case of water, when the substance dissolved is a non-electrolyte, the molecular depression is about 18.5°.

In the case of substances which dissociate into their ions in the solution, the molecular depression will be greater, depending upon the degree of ionisation. Thus in the case of strong acids, bases, and salts, that is, "electrolytes" which undergo dissociation to the highest degree, it is found that the molecular depression is practically double that given by non-electrolytes. The ions in the liquid acting as independent molecules, it will be obvious that if dissociation is complete there will be twice as many ions as there were molecules of the compound, and therefore the effect produced in respect of lowering the freezing-point should be twice as great. The relations thus established between the molecular weight of a compound and its influence in lowering the freezing-point of a solvent form the basis of a method for the determination of molecular weights (Raoult's method).

The process is carried out in a tube quite similar to tube A, Fig. 17 (the side tube in this case being merely closed with a cork). A weighed quantity of the solvent is introduced into this tube, which is then carefully cooled in a freezing-mixture, the liquid being gently stirred by means of a wire passing through a hole in the top cork. The temperature at which freezing begins to take place is noted. The tube is then withdrawn from the freezing

In actually determining depressions of freezing-point, solutions so strong as this cannot be used. The determination is made with dilute solutions, and the molecular depression obtained by calculation.

mixture and the solidified portion allowed to melt, when a weighed quantity of the substance whose molecular weight is to be determined is introduced, and the operation repeated. The molecular depression is calculated from the formula

where C=constant-the molecular depression of the freezing-point; g=grammes of substances in 100 grammes of the solvent ; /= the observed depression of the freezing-point.

and

CHAPTER XIV

SOLUTION

A SOLUTION may be defined as a homogeneous mixture of either a gas, a liquid, or a solid with a liquid, this liquid being termed the solvent.*

Substances that are capable of forming such homogeneous mixtures with a solvent are said to be soluble in that liquid. The solution of matter in its three states will be treated separately.

1. Solution of Gases in Liquids.-When a gas is dissolved by a liquid, the liquid is said to absorb the gas, and although it is held that most liquids are capable of absorbing most gases to a greater or less degree, most of the investigations in this direction have been made with the two liquids, water and alcohol, by Bunsen.

The quantity of a gas which a liquid is capable of absorbing depends upon four factors-(1) the specific nature of the liquid ; (2) the nature of the gas; (3) the temperature of the liquid; (4) the pressure.

(1.) The influence of the solvent may be seen by a comparison of the quantities of the same gas which equal volumes of water and of alcohol are capable of dissolving, thus

100 volumes of water at o° dissolve 179.6 volumes of carbon dioxide, while 100

alcohol

432.9

(2.) The various quantities of different gases which the same liquid will absorb are found to extend over a very wide range,

Mixtures of gases are sometimes regarded as solutions, one gas being said to be dissolved in the other. Gases also are sometimes spoken of as dissolving liquids and solids, when liquid and solid substances directly vaporise into them.

(3.) The volume of any gas which a liquid can absorb diminishes with a rise of temperature.* This will be seen from the following table, where the volumes of different gases are given which 100 volumes of water will absorb at various temperatures.

It was at one time believed that the solvent power of water for hydrogen was the same at all temperatures between o° and 25°. Recent experiments have shown, however, that there is no exception to the general law in this case; thus it has been found that 100 volumes of water

At o dissolve 2.15 volumes of hydrogen.

When a solution of a gas in water is heated, the gas being less soluble at the higher temperature is expelled, and in most cases the whole of the gas is driven off at the boiling temperature. This, however, is not invariably the case; for example, the solution of hydrochloric acid in water, when boiled, will distil, without further evolution of gas, when a solution of definite strength is reached (see Hydrochloric Acid).

(4.) The influence of pressure upon the volume of a given gas which a liquid can absorb was discovered by Henry (1803), and is known as Henry's law, namely, The volume of the gas absorbed by a liquid is directly proportional to the pressure of the gas. If the pressure be doubled, the same volume of liquid will dissolve twice the volume of the gas, the volume in each case being measured at o° and 760 mm. But since, according to Boyle's law, the volume of a gas is inversely as the pressure, this law may be thus stated: A given volume of a liquid will absorb the same volume of a gas at all pressures.

* Helium, between certain limits of temperature, is an exception.

Thus, if 100 volumes of water at o° dissolve 2.03 volumes of nitrogen, under the standard atmospheric pressure (the volume of the gas being measured at o° and 760 mm.), under twice this pressure, i.e. two atmospheres, the same volume will absorb twice the volume of nitrogen, viz., 4.06 volumes measured at 。° and 760 mm. But 4.06 volumes of gas measured at o° and 760 mm. occupy 2.03 volumes under a pressure of two atmospheres, therefore the liquid dissolves the same volume of compressed gas as of gas under ordinary pressure.

Henry's law is sometimes stated in a slightly altered form. If the quantity of gas present in a unit volume of both the liquid and the space above it be called the concentration of the gas, then the law may be expressed by saying that under all pressures, the ratio of the concentrations of the gas in the liquid, and in the space above it, remains constant. This ratio is termed the coefficient of solubility, or the "solubility" of the gas in the particular liquid.

The term coefficient of absorption, first introduced by Bunsen, is the volume of the gas measured at o° and 760 mm., which is absorbed by I cubic centimetre of a liquid at the same temperature and pressure; and it is therefore simply the volume representing the "solubility" of the gas, reduced to o°.

The solubility of gases in liquids is measured by agitating a known volume of liquid with a measured volume of the gas, under determinate conditions of temperature and pressure. The apparatus employed by Bunsen, and known as Bunsen's absorptiometer, is shown in Fig. 20. It consists of a graduated tube e, into which known volumes of the gas and liquid are introduced. The lower end of this tube is furnished with an iron screw, by means of which it can be securely screwed down upon an indiarubber pad, in order to completely close the tube (seen in the side figure). The tube containing the gas and liquid under examination is lowered into a tall cylinder g g, in the bottom of which is a quantity of mercury. The cylinder is then filled with water, and the cap screwed down. The thermometer k registers the temperature. The apparatus is then briskly shaken, in order that the liquid in the eudiometer may exert its full solvent action upon the gas, and on slightly unscrewing the tube from the caoutchouc pad, mercury enters to take the place of the dissolved gas. The tube is again closed and the shaking repeated, and these operations are continued until no further absorption results. Finally, the volume of gas is measured, the temperature noted, and the pressure