Chapter X: Sodium Chloride--Berthollet's Laws--Hydrochloric Acid (3)

The hydrochloric acid gas evolved is subjected to condensation by dissolving it in water.[32] If the apparatus in which the decomposition is accomplished were hermetically closed, and only presented one outlet, then the escape of the hydrochloric acid would only proceed through the escape pipe intended for this purpose. But as it is impossible to construct a perfectly hermetically closed furnace of this kind, it is necessary to increase the draught by artificial means, or to oblige the hydrochloric acid gas to pass through those arrangements in which it is to be condensed. This is done by connecting the ends of the tubes through which the hydrochloric acid gas escapes from the furnace with high chimneys, where a strong draught is set up from the combustion of the fuel. This causes a current of hydrochloric acid gas to pass through the absorbing apparatus in a definite direction. Here it encounters a current of water flowing in the opposite direction, by which it is absorbed. It is not customary to cause the acid to pass through the water, but only to bring it into contact with the surface of the water. The absorption apparatus consists of large earthenware vessels having four orifices, two above and two lateral ones in the wide central portion of each vessel. The upper orifices serve for connecting the vessels together, and the hydrochloric acid gas escaping from the furnace passes through these tubes. The water for absorbing the acid enters at the upper, and flows out from the lower, vessel, passing through the lateral orifices in the vessels. The water flows from the chimney towards the furnace and it is therefore evident that the outflowing water will be the most saturated with acid, of which it actually contains about 20 per cent. The absorption in these vessels is not complete. The ultimate absorption of the hydrochloric acid is carried on in the so-called _coke towers_, which usually consist of two adjacent chimneys. A lattice-work of bricks is laid on the bottom of these towers, on which coke is piled up to the top of the tower. Water, distributing itself over the coke, trickles down to the bottom of the tower, and in so doing absorbs the hydrochloric acid gas rising upwards.

[32] As in works which treat common salt in order to obtain sodium
sulphate, the hydrochloric acid is sometimes held to be of no
value, it might be allowed to escape with the waste furnace gases
into the atmosphere, which would greatly injure the air of the
neighbourhood and destroy all vegetation. In all countries,
therefore, there are laws forbidding the factories to proceed in
this manner, and requiring the absorption of the hydrochloric acid
by water at the works themselves, and not permitting the solution
to be run into rivers and streams, whose waters it would spoil. It
may be remarked that the absorption of hydrochloric acid presents
no particular difficulties (the absorption of sulphurous acid is
much more difficult) because hydrochloric acid has a great
affinity for water and gives a hydrate which boils above 100°.
Hence, even steam and hot water, as well as weaker solutions, can
be used for absorbing the acid. However, Warder (1888) showed that
weak solutions of composition H_{2}O + _n_HCl when boiled (the
residue will be almost HCl,8H_{2}O) evolve (not water but) a
solution of the composition H_{2}O + 445_n_^{4}HCl; for example,
on distilling HCl,10H_{2}O, HCl,23H_{2}O is first obtained in the
distillate. As the strength of the residue becomes greater, so
also does that of the distillate, and therefore in order to
completely absorb hydrochloric acid it is necessary in the end to
have recourse to water.

As in Russia the manufacture of sodium sulphate from sodium
chloride has not yet been sufficiently developed, and as
hydrochloric acid is required for many technical purposes (for
instance, for the preparation of zinc chloride, which is employed
for soaking railway sleepers), therefore salt is often treated
mainly for the manufacture of hydrochloric acid.

It will be readily understood that hydrochloric acid may be obtained from all other metallic chlorides.[33] It is frequently formed in other reactions, many of which we shall meet with in the further course of this work. It is, for instance, formed by the action of water on sulphur chloride, phosphorus chloride, antimony chloride, &c.

[33] Thus the metallic chlorides, which are decomposed to a greater or
less degree by water, correspond with feeble bases. Such are, for
example, MgCl_{2}, AlCl_{3}, SbCl_{3}, BiCl_{3}. The decomposition
of magnesium chloride (and also carnallite) by sulphuric acid
proceeds at the ordinary temperature; water decomposes MgCl_{2} to
the extent of 50 p.c. when aided by heat, and _may be employed_ as
a convenient _method for the production of hydrochloric acid_.
Hydrochloric acid is also produced by the ignition of certain
metallic chlorides in a stream of hydrogen, especially of those
metals which are easily reduced and difficultly oxidised--for
instance, silver chloride. Lead chloride, when heated to redness
in a current of steam, gives hydrochloric acid and lead oxide. The
multitude of the cases of formation of hydrochloric acid are
understood from the fact that it is a substance which is
comparatively very stable, resembling water in this respect, and
even most probably more stable than water, because, at a high
temperature and even under the action of light, chlorine
decomposes water, with the formation of hydrochloric acid. The
combination of chlorine and hydrogen also proceeds by their direct
action, as we shall afterwards describe.

_Hydrochloric acid_ is a colourless gas having a pungent suffocating odour and an acid taste. This gas fumes in air and attracts moisture, because it forms vapour containing a compound of hydrochloric acid and water. Hydrochloric acid is liquefied by cold, and under a pressure of 40 atmospheres, into a colourless liquid of sp. gr. 0·908 at 0°,[34] boiling point -35° and absolute boiling point +52°. We have already seen (Chapter I.) that hydrochloric acid combines very energetically _with water_, and in so doing evolves a considerable amount of heat. The solution saturated in the cold attains a density 1·23. On heating such a solution containing about 45 parts of acid per 100 parts, the hydrochloric acid gas is expelled with only a slight admixture of aqueous vapour. But it is impossible to entirely separate the whole of the hydrochloric acid from the water by this means, as could be done in the case of an ammoniacal solution. The temperature required for the evolution of the gas rises and reaches 110°-111°, and after this remains constant--that is, a solution having a constant boiling point is obtained (as with HNO_{3}), which, however, does not (Roscoe and Dittmar) present a constant composition under different pressures, because the hydrate is decomposed in distillation, as is seen from the determinations of its vapour density (Bineau). Judging from the facts (1) that with decrease of the pressure under which the distillation proceeds the solution of constant boiling point approaches to a composition of 25 p.c. of hydrochloric acid,[35] (2) that by passing a stream of dry air through a solution of hydrochloric acid there is obtained in the residue a solution which also approaches to 25 p.c. of acid, and more nearly as the temperature falls,[36] (3) that many of the properties of solutions of hydrochloric acid vary distinctly according as they contain more or less than 25 p.c. of hydrochloric acid (for instance, antimonious sulphide gives hydrogen sulphide with a stronger acid, but is not acted on by a weaker solution, also a stronger solution fumes in the air, &c.), and (4) that the composition HCl,6H_{2}O corresponds with 25·26 p.c. HCl--judging from all these data, and also from the loss of tension which occurs in the combination of hydrochloric acid with water, it may be said that they form a _definite hydrate_ of the composition HCl,6H_{2}O. Besides this hydrate there exists also a crystallo-hydrate, HCl,2H_{2}O,[37] which is formed by the absorption of hydrochloric acid by a saturated solution at a temperature of -23°. It crystallises and melts at -18°.[38]

[34] According to Ansdell (1880) the sp. gr. of liquid hydrochloric
acid at 0° = 0·908, at 11·67° = 0·854, at 22·7° = 0·808, at 33° =
0·748. Hence it is seen that the expansion of this liquid is
greater than that of gases (Chapter II., Note 34).

[35] According to Roscoe and Dittmar at a pressure of three atmospheres
the solution of constant boiling point contains 18 p.c. of
hydrogen chloride, and at a pressure of one-tenth atmosphere 23
p.c. The percentage is intermediate at medium pressures.

[36] At 0° 25 p.c., at 100° 20·7 p.c.; Roscoe and Dittmar.

[37] This crystallo-hydrate (obtained by Pierre and Puchot, and
investigated by Roozeboom) is analogous to NaCl,2H_{2}O. The
crystals HCl,2H_{2}O at -22° have a specific gravity 1·46; the
vapour tension (under dissociation) of the solution having a
composition HCl,2H_{2}O at -24° = 760, at -19° = 1,010, at -18° =
1,057, at -17° = 1,112 mm. of mercury. In a solid state the
crystallo-hydrate at -17·7° has the same tension, whilst at lower
temperatures it is much less: at -24° about 150, at -19° about 580
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).