mm. A mixture of fuming hydrochloric acid with snow reduces the

temperature to -38°. If another equivalent of water be added to the hydrate HCl,2H_{2}O at -18°, the temperature of solidification falls to -25°, and the hydrate HCl,3H_{2}O is formed (Pickering, 1893).

[38] According to Roscoe at 0° one _hundred_ grams of water at a pressure _p_ (in millimetres of mercury) dissolves--

_p_ = 100 200 300 500 700 1,000 Grams HCl 65·7 70·7 73·8 78·2 81·7 85·6

At a pressure of 760 millimetres and temperature _t_, one _hundred_ grams of water dissolves

_t_ = 0 8° 16° 24° 40° 60° Grams HCl 82·5 78·3 74·2 70·0 63·3 56·1

Roozeboom (1886) showed that at _t_° solutions containing _c_ grams of hydrogen chloride per 100 grams of water may (with the variation of the pressure _p_) be formed together with the crystallo-hydrate HCl,2H_{2}O:

_t_ = -28°·8 -21° -19° -18° -17°·7 _c_ = 84·2 86·8 92·6 98·4 101·4 _p_ = -- 334 580 900 1,073 mm.

The last combination answers to the melted crystallo-hydrate HCl,2H_{2}O, which splits up at temperatures above -17°·7, and at a constant atmospheric pressure when there are no crystals--

_t_ = -24° -21° -18° -10° 0° _c_ = 101·2 98·3 95·7 89·8 84·2

From these data it is seen that the hydrate HCl,2H_{2}O can exist in a liquid state, which is not the case for the hydrates of carbonic and sulphurous anhydrides, chlorine, &c.

According to Marignac, the specific heat _c_ of a solution HCl + _m_H_{2}O (at about 30°, taking the specific heat of water = 1) is given by the expression--

C(36·5 + _m_18) = 18_m_ - 28·39 + 140/_m_ - 268/_m_^2

if _m_ be not less than 6·25. For example, for HCl + 25H_{2}O, C = 0·877.

According to Thomsen's data, the amount of heat _Q_, expressed in thousands of calories, evolved in the solution of 36·5 grams of gaseous hydrochloric acid in _m_H_{2}O or 18_m_ grams of water is equal to--

_m_ = 2 4 10 50 400 _Q_ = 11·4 14·3 16·2 17·1 17·3

In these quantities the latent heat of liquefaction is included, which must be taken as 5-9 thousand calories per molecular quantity of hydrogen chloride.

The researches of Scheffer (1888) on the rate of diffusion (in water) of solutions of hydrochloric acid show that the coefficient of diffusion _k_ decreases with the amount of water _n_, if the composition of the solution is HCl,_n_H_{2}O at 0°:--

_n_ = 5 6·9 9·8 14 27·1 129·5 _k_ = 2·31 2·08 1·86 1·67 1·52 1·39

It also appears that strong solutions diffuse more rapidly into dilute solutions than into water.

The mean specific gravities at 15°, taking water at its maximum density (4°) as 10,000, for solutions containing _p_ per cent. of hydrogen chloride are--

_p_ _S_ _p_ _S_ 5 10,242 25 11,266 10 10,490 30 11,522 15 10,744 35 11,773 20 11,001 40 11,997

The formula _S_ = 9,991·6 + 49·43_p_ + 0·0571_p_^2, up to _p_ = 25·26, which answers to the hydrate HCl,6H_{2}O mentioned above, gives the specific gravity. Above this percentage _S_ = 9,785·1 + 65·10_p_-0·240_p_^2. The rise of specific gravity with an increase of percentage (or the differential _ds/dp_) reaches a maximum at about 25 p.c.[39] The intermediate solution, HCl,6H_{2}O, is further distinguished by the fact that the variation of the specific gravity with the variation of temperature is a constant quantity, so that the specific gravity of this solution is equal to 11,352·7(1-0·000447_t_), where 0·000447 is the coefficient of expansion of the solution.[40] In the case of more dilute solutions, as with water, the specific gravity per 1° (or the differential _ds_/_dt_) rises with a rise of temperature.[41]

_p_ = 0 5 10 15 20 _S__{0} - _S__{15} = 7·2 23 38 52 64 _S__{15} - _S__{30} = 34·1 42 50 59 67

Whilst for solutions which contain a greater proportion of hydrogen chloride than HCl,6H_{2}O, these coefficients _decrease_ with a rise of temperature; for instance, for 30 p.c. of hydrogen chloride _S__{0}-_S__{15} = 88 and _S__{15}-_S__{30} = 87 (according to Marignac's data). In the case of HCl,6H_{2}O these differences are constant, and equal 76.

[39] If it be admitted that the maximum of the differential corresponds with HCl,6H_{2}O, then it might be thought that the specific gravity is expressed by a parabola of the third order; but such an admission does not give expressions in accordance with fact. This is all more fully considered in my work mentioned in Chapter I., Note 19.

[40] As in water, the coefficient of expansion (or the quantity _k_ in the expression S_{_t_} = S_{_0_}-_k_S_{_0_}_t_, or V_t_ = 1/(1-_kt_)) attains a magnitude 0·000447 at about 48°, it might be thought that at 48° all solutions of hydrochloric acid would have the same coefficient of expansion, but in reality this is not the case. At low and at the ordinary temperatures the coefficient of expansion of aqueous solutions is greater than that of water, and increases with the amount of substance dissolved.

[41] The figures cited above may serve for the direct determination of that variation of the specific gravity of solutions of hydrochloric acid with the temperature. Thus, knowing that at 15° the specific gravity of a 10 p.c. solution of hydrochloric acid = 10,492, we find that at _t_° it = 10,530-_t_(2·13 + 0·027_t_). Whence also may be found the coefficient of expansion (Note 40).

Thus the formation of two definite hydrates, HCl,2H_{2}O and HCl,6H_{2}O, between hydrochloric acid and water may be accepted upon the basis of many facts. But both of them, if they occur in a liquid state, dissociate with great facility into hydrogen chloride and water, and are completely decomposed when distilled.

All solutions of hydrochloric acid present the properties of an energetic acid. They not only transform blue vegetable colouring matter into red, and disengage carbonic acid gas from carbonates, &c., but they also entirely saturate bases, even such energetic ones as potash, lime, &c. In a dry state, however, hydrochloric acid does not alter vegetable dyes, and does not effect many double decompositions which easily take place in the presence of water. This is explained by the fact that the gaso-elastic state of the hydrochloric acid prevents its entering into reaction. However, incandescent iron, zinc, sodium, &c., act on gaseous hydrochloric acid, displacing the hydrogen and leaving half a volume of hydrogen for each volume of hydrochloric acid gas; this reaction may serve for determining the composition of hydrochloric acid. Combined with water hydrochloric acid acts as an acid much resembling nitric acid[42] in its energy and in many of its reactions; however, the latter contains oxygen, which is disengaged with great ease, and so very frequently acts as an oxidiser, which hydrochloric acid is not capable of doing. The majority of metals (even those which do not displace the H from H_{2}SO_{4}, but which, like copper, decompose it to the limit of SO_{2}) displace the hydrogen from hydrochloric acid. Thus hydrogen is disengaged by the action of zinc, and even of copper and tin.[42 bis] Only a few metals withstand its action; for example, gold and platinum. Lead in compact masses is only acted on feebly, because the lead chloride formed is insoluble and prevents the further action of the acid on the metal. The same is to be remarked with respect to the feeble action of hydrochloric acid on mercury and silver, because the compounds of these metals, AgCl and HgCl, are insoluble in water. Metallic chlorides are not only formed by the action of hydrochloric acid on the metals, but also by many other methods; for instance, by the action of hydrochloric acid on the carbonates, oxides, and hydroxides, and also by the action of chlorine on metals and certain of their compounds. Metallic chlorides have a composition MCl; for example, NaCl, KCl, AgCl, HgCl, if the metal replaces hydrogen equivalent for equivalent, or, as it is said, if it be monatomic or univalent. In the case of bivalent metals, they have a composition MCl_{2}; for example, CaCl_{2}, CuCl_{2}, PbCl_{2}, HgCl_{2}, FeCl_{2}, MnCl_{2}. The composition of the haloid salts of other metals presents a further variation; for example, AlCl_{3}, PtCl_{4}, &c. Many metals, for instance Fe, give several degrees of combination with chlorine (FeCl_{2}, FeCl_{3}) as with hydrogen. In their composition the metallic chlorides differ from the corresponding oxides, in that the O is replaced by Cl_{2}, as should follow from the law of substitution, because oxygen gives OH_{2}, and is consequently bivalent, whilst chlorine forms HCl, and is therefore univalent. So, for instance, ferrous oxide, FeO, corresponds with ferrous chloride, FeCl_{2}, and the oxide Fe_{2}O_{3} with ferric chloride, which is also seen from the origin of these compounds, for FeCl_{2} is obtained by the action of hydrochloric acid on ferrous oxide or carbonate and FeCl_{3} by its action on ferric oxide. In a word, all the typical properties of acids are shown by hydrochloric acid, and all the typical properties of salts in the metallic chlorides derived from it. Acids and salts composed like HCl and M_{n}Cl_{2m} without any oxygen bear the name of haloid salts; for instance, HCl is a haloid acid, NaCl a haloid salt, chlorine a halogen. The capacity of hydrochloric acid to give, by its action on bases, MO, a metallic chloride, MCl_{2}, and water, is limited at high temperatures by the reverse reaction MCl_{2} + H_{2}O = MO + 2HCl, and the more pronounced are the basic properties of MO the feebler is the reverse action, while for feebler bases such as Al_{2}O_{3}, MgO, &c., this reverse reaction proceeds with ease. Metallic chlorides corresponding with the peroxides either do not exist, or are easily decomposed with the disengagement of chlorine. Thus there is no compound BaCl_{4} corresponding with the peroxide BaO_{2}. Metallic chlorides having the general aspect of salts, like their representative sodium chloride, are, as a rule, easily fusible, more so than the oxides (for instance, CaO is infusible at a furnace heat, whilst CaCl_{2} is easily fused) and many other salts. Under the action of heat many chlorides are more stable than the oxides, some can even be converted into vapour; thus corrosive sublimate, HgCl_{2}, is particularly volatile, whilst the oxide HgO decomposes at a red heat. Silver chloride, AgCl, is fusible and is decomposed with difficulty, whilst Ag_{2}O is easily decomposed. The majority of the metallic chlorides are soluble in water, but silver chloride, cuprous chloride, mercurous chloride, and lead chloride are sparingly soluble in water, and are therefore easily obtained as precipitates when a solution of the salts of these metals is mixed with a solution of any chloride or even with hydrochloric acid. The metal contained in a haloid salt may often be replaced by another metal, or even by hydrogen, just as is the case with a metal in an oxide. Thus copper displaces mercury from a solution of mercuric chloride, HgCl_{2} + Cu = CuCl_{2} + Hg, and hydrogen at a red heat displaces silver from silver chloride, 2AgCl + H_{2} = Ag_{2} + 2HCl. These, and a whole series of similar reactions, form the typical methods of double saline decompositions. The measure of decomposition and the conditions under which reactions of double saline decompositions proceed in one or in the other direction are determined by the properties of the compounds which take part in the reaction, and of those capable of formation at the temperature, &c., as was shown in the preceding portions of this chapter, and as will be frequently found hereafter.

[42] Thus, for instance, with feeble bases they evolve in dilute solutions (Chapter III., Note 53) almost equal amounts of heat; their relation to sulphuric acid is quite identical. They both form fuming solutions as well as hydrates; they both form solutions of constant boiling point.

[42 bis] Pybalkin (1891) found that copper begins to disengage hydrogen at 100°, and that chloride of copper begins to give up its chlorine to hydrogen gas at 230°; for silver these temperatures are 117° and 260°--that is, there is less difference between them.

If hydrochloric acid enters into double decomposition with basic oxides and their hydrates, this is only due to its acid properties; and for the same reason it rarely enters into double decomposition with acids and acid anhydrides. Sometimes, however, it combines with the latter, as, for instance, with the anhydride of sulphuric acid, forming the compound SO_{3}HCl; and in other cases it acts on acids, giving up its hydrogen to their oxygen and forming chlorine, as will be seen in the following chapter.

Hydrochloric acid, as may already be concluded from the composition of its molecule, belongs to the monobasic acids, and does not, therefore, give true acid salts (like HNaSO_{4} or HNaCO_{3}); nevertheless many metallic chlorides, formed from powerful bases, are capable of _combining with hydrochloric acid_, just as they combine with water, or with ammonia, or as they give double salts. Compounds have long been known of hydrochloric acid with auric, platinic, and antimonious chlorides, and other similar metallic chlorides corresponding with very feeble bases. But Berthelot, Engel, and others have shown that the capacity of HCl for combining with M_{_n_}Cl_{_m_} is much more frequently encountered than was previously supposed. Thus, for instance, dry hydrochloric acid when passed into a solution of zinc chloride (containing an excess of the salt) gives in the cold (0°) a compound HCl,ZnCl_{2},2H_{2}O, and at the ordinary temperature HCl,2ZnCl_{2},2H_{2}O, just as it is able at low temperatures to form the crystallo-hydrate ZnCl_{2},3H_{2}O (Engel, 1886). Similar compounds are obtained with CdCl_{2},CuCl_{2}, HgCl_{2},Fe_{2}Cl_{6}, &c. (Berthelot, Ditte, Cheltzoff, Lachinoff, and others). These compounds with hydrochloric acid are generally more soluble in water than the metallic chlorides themselves, so that whilst hydrochloric acid decreases the solubility of M_{_n_}Cl_{_m_}, corresponding with energetic bases (for instance, sodium or barium chlorides), it increases the solubility of the metallic chlorides corresponding with feeble bases (cadmium chloride, ferric chloride, &c.) Silver chloride, which is insoluble in water, is soluble in hydrochloric acid. Hydrochloric acid also combines with certain unsaturated hydrocarbons (for instance, with turpentine, C_{10}H_{16},2HCl) and their derivatives. _Sal-ammoniac_, or ammonia hydrochloride, NH_{4}Cl = NH_{3},HCl, also belongs to this class of compounds.[43] If hydrogen chloride gas be mixed with ammonia gas a solid compound consisting of equal volumes of each is immediately formed. The same compound is obtained on mixing solutions of the two gases. It is also produced by the action of hydrochloric acid on ammonium carbonate. Sal-ammoniac is usually prepared, in practice, by the last method.[44] The specific gravity of sal-ammoniac is 1·55. We have already seen (Chapter VI.) that sal-ammoniac, like all other ammonium salts, easily decomposes; for instance, by volatilisation with alkalis, and even partially when its solution is boiled. The other properties and reactions of sal-ammoniac, especially in solution, fully recall those already mentioned in speaking of sodium chloride. Thus, for instance, with silver nitrate it gives a precipitate of silver chloride; with sulphuric acid it gives hydrochloric acid and ammonium sulphate, and it forms double salts with certain metallic chlorides and other salts.[45]

[43] When an unsaturated hydrocarbon, or, in general, an unsaturated compound, assimilates to itself the molecules Cl_{2}, HCl, SO_{3}, H_{2}SO_{4}, &c., the cause of the reaction is most simple. As nitrogen, besides the type NX_{3} to which NH_{3}, belongs, gives compounds of the type NX_{5}--for example, NO_{2}(OH)--the formation of the salts of ammonium should be understood in this way. NH_{3} gives NH_{4}Cl because NX_{3} is capable of giving NX_{5}. But as saturated compounds--for instance, SO_{3},H_{2}O, NaCl, &c.--are also capable of combination even between themselves, it is impossible to deny the capacity of HCl also for combination. SO_{3} combines with H_{2}O, and also with HCl and the unsaturated hydrocarbons. It is impossible to recognise the distinction formerly sought to be established between atomic and molecular compounds, and regarding, for instance, PCl_{3} as an atomic compound and PCl_{5} as a molecular one, only because it easily splits up into molecules PCl_{3} and Cl_{2}.

[44] Sal-ammoniac is prepared from ammonium carbonate, obtained in the dry distillation of nitrogenous substances (Chapter VI.), by saturating the resultant solution with hydrochloric acid. A solution of sal-ammoniac is thus produced, which is evaporated, and in the residue a mass is obtained containing a mixture of various other, especially tarry, products of dry distillation. The sal-ammoniac is generally purified by sublimation. For this purpose iron vessels covered with hemispherical metallic covers are employed, or else simply clay crucibles covered by other crucibles. The upper portion, or head, of the apparatus of this kind will have a lower temperature than the lower portion, which is under the direct action of the flame. The sal-ammoniac volatilises when heated, and settles on the cooler portion of the apparatus. It is thus freed from many impurities, and is obtained as a crystalline crust, generally several centimetres thick, in which form it is commonly sold. The solubility of sal-ammoniac rises rapidly with the temperature: at 0°, 100 parts of water dissolve about 28 parts of NH_{4}Cl, at 50° about 50 parts, and at the ordinary temperature about 35 parts. This is sometimes taken advantage of for separating NH_{4}Cl from solutions of other salts.

[45] The solubility of sal-ammoniac in 100 parts of water (according to Alluard) is--

0° 10° 20° 30° 40° 60° 80° 100° 110° 28·40 32·48 37·28 41·72 46 55 64 73 77

A saturated solution boils at 115°·8. The specific gravity at 15°/4° of solutions of sal-ammoniac (water 4° = 10,000) = 9,991·6-31·26_p_-0·085_p_^2, where _p_ is the amount by weight of ammonium chloride in 100 parts of solution. With the majority of salts the differential _ds_/_dp_ increases, but here it decreases with the increase of _p_. For (unlike the sodium and potassium salts) a solution of the alkali _plus_ a solution of acid occupy a greater volume than that of the resultant ammonium salt. In the solution of _solid_ ammonium chloride a contraction, and not expansion, generally takes place. It may further be remarked that solutions of sal-ammoniac have an acid reaction even when prepared from the salt remaining after prolonged washing of the sublimed salt with water (A. Stcherbakoff).