The Principles of Chemistry, Volume I

by Dmitry Ivanovich Mendeleyev

In the case already considered, as in the case of formic acid in the researches of D. P. Konovaloff (note [47]), the constant boiling solution corresponds with a minimum tension—that is, with a boiling point higher than that of either of the component elements. But there is another case of constant boiling solutions similar to the case of the solution of propyl alcohol, C₃H₈O, when a solution, undecomposed by distillation, boils at a lower point than that of the more volatile liquid. However, in this case also, if there be solution, the possibility of the formation of a definite compound in the form C₃H₈O + H₂O cannot be denied, and the tension of the solution is not equal to the sum of tensions of the components. There are possible cases of constant boiling mixtures even when there is no solution nor any loss of tension, and consequently no chemical action, since the amount of liquids that are volatilised is determined by the product of the vapour densities into their vapour tensions (Wanklyn), in consequence of which liquids whose boiling point is above 100°—for instance, turpentine and ethereal oils in general—when distilled with aqueous vapour, pass over at a temperature below 100°. Consequently, it is not in the constancy of composition and boiling point (temperature of decomposition) that evidence of a distinct chemical action is to be found in the above-described solutions of acids, but in the great loss of tension, which completely resembles the loss of tension observed, for instance, in the perfectly-definite combinations of substances with water of crystallisation (see later, note [65]). Sulphuric acid, H₂SO₄, as we shall learn later, is also decomposed by distillation, like HCl + 6H₂O, and exhibits, moreover, all the signs of a definite chemical compound. The study of the variation of the specific gravities of solutions as dependent on their composition (see note [19]) shows that phenomena of a similar kind, although of different dimensions, take place in the formation of both H₂SO₄ from H₂O and SO₃, and of HCl + 6H₂O (or of aqueous solutions analogous to it) from HCl and H₂O.

[61] The essence of the matter may he thus represented. A gaseous or easily volatile substance A forms with a certain quantity of water, nH₂O, a definite complex compound AnH₂O, which is stable up to a temperature t° higher than 100°. At this temperature it is decomposed into two substances, A + H₂O. Both boil below t° at the ordinary pressure, and therefore at t° they distil over and re-combine in the receiver. But if a part of the substance AnH₂O is decomposed or volatilised, a portion of the undecomposed liquid still remains in the vessel, which can partially dissolve one of the products of decomposition, and that in quantity varying with the pressure and temperature, and therefore the solution at a constant boiling point will have a slightly different composition at different pressures.

[62] For solutions of hydrochloric acid in water there are still greater differences in reactions. For instance, strong solutions decompose antimony sulphide (forming hydrogen sulphide, H₂S), and precipitate common salt from its solutions, whilst weak solutions do not act thus.

[63] Supersaturated solutions give an excellent proof in this respect. Thus a solution of copper sulphate generally crystallises in penta-hydrated crystals, CuSO₄ + 5H₂O, and its saturated solution gives such crystals if it be brought into contact with the minutest possible crystal of the same kind. But, according to the observations of Lecoq de Boisbaudran, if a crystal of ferrous sulphate (an isomorphous salt, see note [55]), FeSO₄ + 7H₂O, be placed in a saturated solution of copper sulphate, then crystals of hepta-hydrated salt, CuSO₄ + 7H₂O, are obtained. It is evident that neither the penta- nor the hepta-hydrated salt is contained as such in the solution. The solution presents its own particular liquid form of equilibrium.

[64] Efflorescence, like every evaporation, proceeds from the surface. In the interior of crystals which have effloresced there is usually found a non-effloresced mass, so that the majority of effloresced crystals of washing soda show, in their fracture, a transparent nucleus coated by an effloresced, opaque, powdery mass. It is a remarkable circumstance in this respect that efflorescence proceeds in a completely regular and uniform manner, so that the angles and planes of similar crystallographic character effloresce simultaneously, and in this respect the crystalline form determines those parts of crystals where efflorescence starts, and the order in which it continues. In solutions evaporation also proceeds from the surface, and the first crystals which appear on its reaching the required degree of saturation are also formed at the surface. After falling to the bottom the crystals naturally continue to grow (see Chapter [X].).

[65] According to Lescœur (1883), at 100° a concentrated solution of barium hydroxide, BaH₂O₂, on first depositing crystals (with + H₂O) has a tension of about 630 mm. (instead of 760 mm., the tension of water), which decreases (because the solution evaporates) to 45 mm., when all the water is expelled from the crystals, BaH₂O₂ + H₂O, which are formed, but they also lose water (dissociate, effloresce at 100°), leaving the hydroxide, BaH₂O₂, which is perfectly undecomposable at 100°—that is, does not part with water. At 73° (the tension of water is then 265 mm.) a solution, containing 33H₂O, on crystallising has a tension of 230 mm.; the crystals, BaH₂O₂ + 8H₂O, which separate out, have a tension of 160 mm.; on losing water they give BaH₂O₂ + H₂O. This substance does not decompose at 73°, and therefore its tension = 0. In those crystallohydrates which effloresce at the ordinary temperature, the tension of dissociation nearly approximates to that of the aqueous vapour, as Lescœur (1891) showed. To this category of compounds belong B₂O₃(3 + x)H₂O, C₂O₄H₂(2 + x)H₂O, BaO(9 + x)H₂O, and SrO(9 + x)H₂O. And a still greater tension is possessed by Na₂SO₄10H₂O, Na₂CO₃10H₂O, and MgSO₄(7 + x)H₂O. Müller-Erzbach (1884) determines the tension (with reference to liquid water) by placing tubes of the same length with water and the substances experimented with in a desiccator, the rate of loss of water giving the relative tension. Thus, at the ordinary temperature, crystals of sodium phosphate, Na₂HPO₄ + 12H₂O, present a tension of 0·7 compared with water, until they lose 5H₂O, then 0·4 until they lose 5H₂O more, and on losing the last equivalent of water the tension falls to 0·04 compared with water. It is clear that the different molecules of water are held by an unequal force. Out of the five molecules of water in copper sulphate the two first are comparatively easily separated even at the ordinary temperature (but only after several days in a desiccator, according to Latchinoff); the next two are more difficultly separated, and the last equivalent is retained even at 100°. This is another indication of the capacity of CuSO₄ to form three hydrates, CuSO₄5H₂O, CuSO₄3H₂O, and CuSO₄H₂O. The researches of Andreae on the tension of dissociation of hydrated sulphate of copper showed (1891) the existence of three provinces, characterised at a given temperature by a constant tension: (1) between 3–5, (2) between 1–3, and lastly (3) between 0–1 molecule of water, which again confirms the existence of three hydrates of the above composition for this salt.

[65 bis] Sodium acetate (C₂H₃O₂Na,3H₂O) melts at 58°, but re-solidifies only on contact with a crystal, otherwise it may remain liquid even at 0°, and may be used for obtaining a constant temperature. According to Jeannel, the latent heat of fusion is about 28 calories, and according to Pickering the heat of solution 35 calories. When melted this salt boils at 123°—that is, the tension of the vapour given off at that temperature equals the atmospheric pressure.

[66] Such a phenomenon frequently presents itself in purely chemical action. For instance, let a liquid substance A give, with another liquid substance B, under the conditions of an experiment, a mere minute quantity of a solid or gaseous substance C. This small quantity will separate out (pass away from the sphere of action, as Berthollet expressed it), and the remaining masses of A and B will again give C; consequently, under these conditions action will go on to the end. Such, it seems to me, is the action in solutions when they yield ice or vapour indicating the presence of water.

[67] Certain substances are capable of forming together only one compound, others several, and these of the most varied degrees of stability. The compounds of water are instances of this kind. In solutions the existence of several different definite compounds must be acknowledged, but many of these have not yet been obtained in a free state, and it may be that they cannot be obtained in any other but a liquid form—that is, dissolved; just as there are many undoubted definite compounds which only exist in one physical state. Among the hydrates such instances occur. The compound CO₂ + 8H₂O (see note [31]), according to Wroblewski, only occurs in a solid form. Hydrates like H₂S + 12H₂O (De Forcrand and Villard), HBr + H₂O (Roozeboom), can only be accepted on the basis of a decrease of tension, but present themselves as very transient substances, incapable of existing in a stable free state. Even sulphuric acid, H₂SO₄, itself, which undoubtedly is a definite compound, fumes in a liquid form, giving off the anhydride, SO₃—that is, it exhibits a very unstable equilibrium. The crystallo-hydrates of chlorine, Cl₂ + 8H₂O, of hydrogen sulphide, H₂S + 12H₂O (it is formed at 0°, and is completely decomposed at +1°, as then 1 vol. of water only dissolves 4 vols. of hydrogen sulphide, while at 0·1° it dissolves about 100 vols.), and of many other gases, are instances of hydrates which are very unstable.

[68] Of such a kind are also other indefinite chemical compounds; for example, metallic alloys. These are solid substances or solidified solutions of metals. They also contain definite compounds, and may contain an excess of one of the metals. According to the experiments of Laurie (1888), the alloys of zinc with copper in respect to the electro-motive force in galvanic batteries behave just like zinc if the proportion of copper in the alloy does not exceed a certain percentage—that is, until a definite compound is attained—for in that case particles of free zinc are present; but if a copper surface be taken, and it be covered by only one-thousandth part of its area of zinc, then only the zinc will act in a galvanic battery.