in this decomposition the molecules are resolved into atoms, thus,

H2OH+0+H

or whether elementary molecules are formed,

2H2O = 2H2+O2

has not as yet been satisfactorily determined.

On the other hand, it is known that the partial decomposition of hydrogen iodide into hydrogen and iodine takes place at 440° C., whilst the decomposition of the iodine molecules into atoms begins at 600°, and that of the hydrogen molecule, if at all, at much higher temperatures. So it is probable that the dissociation of hydrogen iodide at 400-500° C. takes place as follows::

2HI= HH+II.

The compound is, therefore, not resolved into atoms, and the decomposition is not a case of simple dissociation, but an instance of a chemical exchange.

§ 97. Dissociation of Liquids and of Solids.-Liquids, both homogeneous and mixed, undergo dissociation just as gases do; but in the case of liquids it is less frequent, and also much more difficult of demonstration. Still, the colourless liquid nitrogen peroxide, N2O1, is observed to assume a reddish colour when warmed; showing that even in the liquid state, as is the case with the gas, it is dissociated into the red compound, having half the molecular weight and the formula NO2. Liquids, and many solids also, are frequently dissociated when boiled. Concentrated sulphuric acid is not volatile as such, but at 325° is resolved into the anhydride and water, thus:

and these compounds on cooling reunite with each other.

This volatilisation is not true boiling, and therefore even under reduced pressure it takes place only at the same temperature as under the atmospheric pressure (Mendeléeff). Carbonic acid, H,CO2, and sulphurous acid, H2SO,, exhibit a similar decomposition, but at much lower temperatures.

DISSOCIATION OF LIQUIDS AND OF SOLIDS

173

Chloral hydrate when vaporised decomposes into chloral and water, thus:

CC1,CO,Hg = CC1,COH+H,O

and these recombine on cooling. The iodides, bromides, and chlorides of many tertiary alcohols behave in a similar manner; thus, amyl iodide yields amylene and hydriodic acid :

Inorganic chlorides, bromides, and iodides exhibit dissociation, e.g. phosphorus pentachloride:

Phosphorus pentafluoride is, however, volatile without decomposition.

The salts of ammonia and of substituted ammonias form a group of compounds which can only be volatilised by first undergoing dissociation. Thus ammonium chloride is decomposed into ammonia and hydrochloric acid:

NH,C1 = NH,+HCl

and tetra-ethyl ammonium iodide is resolved into tri-ethylamine and ethyl iodide :

N (C2H ̧),I= N(C2H5)3+C2H2I.

In the case of many liquids it has been observed that the density of their vapour is much greater at temperatures near their boiling points than at higher temperatures. Thus, according to Cahours, the density of acetic acid vapour at 250° C. is 2.08, air being the unit, which gives a molecular weight corresponding to the formula C,H,O,. At 125° C. the density is found to be 3.2 in comparison with air. This is generally explained by assuming that at the lower temperature the vapour consists in part of particles of greater molecular weight, e.g. CHÃO, which are dissociated by further heating, as also by reduction of the pressure on the vapour.

2°

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Sulphur, aluminium chloride, and many other substances behave in a similar manner.

§ 98. Dissociation in Solution.-Dissociation may be more frequently observed in mixed liquids, in solutions, than with homogeneous fluids. The occurrence may be evidenced by a change in colour, as, for instance, when a coloured hydrated salt loses or changes its colour in consequence of a loss of water (cf. § 78). Thus the red compound CoCl2+6H2O dissolves in water, and also in dilute spirit, forming a red solution; but on warming the solution becomes blue, either because an anhydrous compound or one containing less water is produced. On cooling the solution the red compound is again formed.

Crystallisation may in many cases be used to prove dissociation. At low temperatures from a solution of sodium sulphate, Glauber's salt, Na2SO4 + 10H2O separates out; whilst at 33° C. the anhydrous salt Na2SO, is deposited. Many other salts behave in a similar manner.

In the case of double salts and analogous compounds dissociation may be demonstrated, as has been done with gaseous compounds, by diffusion. For example, if an open vessel filled with a solution of alum, K2SO4, Al2, (SO4)3 + 24H2O, be placed in a larger vessel filled with water and allowed to remain, then, according to Graham's observations, in course of time the upper layers of water are found to contain more potassium sulphate and less aluminium sulphate than correspond to the composition of the alum. The two simple salts have, therefore, separated from each other in the solution; the double salt has been dissociated. This separation takes place because the potassium salt diffuses more rapidly than the aluminium sulphate, and therefore passes out before the other; the separation is consequently only recognised at first, as later on the inequality is compensated for. Almost all double salts behave in a similar manner, but their dissociation can only be demonstrated when the products have different rates of diffusion.

§ 99. Electrolysis.—Electricity offers a very powerful means of separating the dissociated products from one another. It has been known since the end of the last century that when an electric current is conducted through certain liquids the constituents are separated from one another at the points where the electricity enters and leaves the liquid. Faraday, to whom we are indebted for the investigation of the fundamental laws of

this phenomenon, styled this kind of decomposition 'electrolysis,' i.e. analysis by electricity. Those substances capable of undergoing this species of decomposition are styled 'electrolytes,' and described as 'conductors of the second class,' conducting electricity only when simultaneously decomposed, and are distinguished in this way from (1) the 'conductors of the first class,' or 'metallic conductors,' which allow the passage of electricity without decomposition, (2) from the 'non-conductors,' or insulators,' which do not conduct electricity at all. The conductor of the first class, which serves to bring into and carry away the electricity from the electrolyte, is styled the 'electrode' (from ódós, the way). The electrode situated upstream as regards the positive current is called the anode, whilst that situated down-stream is styled the 'cathode.' Finally, the constituent passing up-stream and deposited at the anode is called the 'anion' (Tò ȧviòv), whilst that going down to the cathode is the 'cation' (Tò KaTiòv). Both are spoken of as the 'ions.'

6

For a long time electrolysis was regarded as the result of the decomposition of the electrolyte by electrical attraction, until in 1857 Clausius adduced the proof that electricity is not the cause of the decomposition, but that it can only effect the separation of the constituents of compounds already decomposed by the action of other forces. For if electricity is needed to effect the decomposition of a compound in which the constituents are held together by the force of affinity, then electrical energy in the conductor cannot produce the decomposition so long as it remains weaker than the affinity, and must, therefore, give rise to a very violent decomposition so soon as its strength somewhat exceeds this. Experience, however, shows this not to be the case, for the smallest force produces a current the intensity of which increasing in proportion to the force is sufficient to cause the'ions' to collect together at the electrodes, or, as it is technically described, to produce the 'polarisation at the electrodes.' Since, therefore, the smallest electromotive force is sufficient to produce this effect, no expenditure of force can be needed for the decomposition of the electrolyte; this must have already taken place, the electrolyte must have been dissociated. This dissociation must have an origin similar to that spoken of in the

preceding sections, and have been wrought by the rapid motion of the particles communicated as heat to the substances.

The recent investigations of Arrhenius have drawn attention to the fact that electrolytes are exactly those substances which, as already shown in § 78, produce a greater depression in the freezing point of water than is consistent with the proportion in the solution of their molecules as represented by the generally accepted formulæ. Thus, whilst in the case of non-electrolytes, when their molecular weights in grams are dissolved in a litre of water, giving a so-called 'normal solution,' the freezing point of water is depressed by- 1.8° C., the haloid salts of the alkalis, for instance, give twice as great depression, for

NaCl -3.5°, KCl 3.3, etc. (v. § 78).

If it be assumed that these salts are entirely or in part dissociated, in the following manner,

then the depression of the freezing point would appear to be normal; for, as there are twice as many particles present, the depression of the freezing point must be twice as great as in the case of substances which are not dissociated.

At first sight it does appear not a little remarkable that the substances which are supposed to decompose so easily should be exactly those which are formed by bodies uniting with one another with considerable energy, and to which consequently strong mutual affinities are ascribed. A further consideration shows, however, that these very same substances take part easily in the most diverse kinds of chemical change, and therefore their constituents cannot be so firmly and indissolubly attached to one another. Clausius did not suppose that when, for instance, common salt is dissociated into sodium and chlorine, the individual atoms are permanently set at liberty, but rather was of opinion that reunion and decomposition recur continually, each atom combining not only with the one with which it was previously united, but with any others which it may meet in the throng of atoms. This conception would appear still to be permissible; making clear, as it does, why in a solution of common salt we find neither free chlorine nor free sodium, so