Part 11 (1/2)
EAO Ice--salt.
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Deliquescence.--As is evident from Fig. 32, salt can exist in contact with water vapour at pressures under those represented by OAMF. If, however, the pressure of the vapour is increased until it reaches a value lying on this curve at temperatures above the cryohydric point, solution will be formed; for the curve AMF represents the equilibria between salt--solution--vapour.
From this, therefore, it is clear that if the pressure of the aqueous vapour in the atmosphere is greater than that of the saturated solution of a salt, that salt will, on being placed in the air, form a solution; it will _deliquesce_.
Separation of Salt on Evaporation.--With the help of Fig. 32 it is possible to state in a general manner whether or not salt will be deposited when a solution is evaporated under a constant pressure.[208]
The curve AMF (Fig. 32) is the vapour-pressure curve of the saturated solutions of the salt, _i.e._ it represents, as we have seen, the maximum vapour pressure at which salt can exist in contact with solution and vapour. The dotted line _aa_ represents atmospheric pressure. If, now, an unsaturated solution, the composition of which is represented by the point _x_, is heated in an open vessel, the temperature will rise, and the vapour pressure of the solution will increase. The system will, therefore, pa.s.s along a line represented diagrammatically by _xx'_. At the point _x'_ the vapour pressure of the system becomes equal to 1 atm.; and as the vessel is open to the air, the pressure cannot further rise; the solution boils. If the heating is continued, water pa.s.ses off, the concentration increases, and the boiling point rises. The system will therefore pa.s.s along the line _x'm_, until at the point _m_ solid salt separates out (provided supersaturation is excluded). The system is now univariant, and continued heating will no longer cause an alteration of the concentration; as water pa.s.ses off, solid salt will be deposited, and the solution will evaporate to dryness.
If, however, the atmospheric pressure is represented not by _aa_ but by _bb_, then, as Fig. 32 shows, the maximum vapour {131} pressure of the system salt--solution--vapour never reaches the pressure of 1 atm. Further, since the curve _bb_ lies in the area of the bivariant system solution--vapour there can at no point be a separation of the solid form; for the system solid--solution--vapour can exist only along the curve AMF.
On evaporating the solution of a salt in an open vessel, therefore, salt can be deposited only if at some temperature the pressure of the saturated solution is equal to the atmospheric pressure. This is found to be the case with most salts. In the case of aqueous solutions of sodium and pota.s.sium hydroxide, however, the vapour pressure of the saturated solution never reaches the value of 1 atm., and on evaporating these solutions, therefore, in an open vessel, there is no separation of the solid. Only a h.o.m.ogeneous fused ma.s.s is obtained. If, however, the evaporation be carried out under a pressure which is lower than the maximum pressure of the saturated solution, separation of the solid substance will be possible.
General Summary.--The systems which have been discussed in the present chapter contained water as one of their components, and an anhydrous salt as the other. It will, however, be clear that the relations.h.i.+ps which were found in the case of these will be found also in other cases where it is a question of the equilibria between two components, which crystallize out in the pure state, and only one of which possesses a measurable vapour pressure. A similar behaviour will, for example, be found in the case of many pairs of organic substances; and in all cases the equilibria will be represented by a diagram of the general appearance of Fig. 29 or Fig. 30.
That is to say: Starting from the fusion point of component I., the system will pa.s.s, by progressive addition of component II., to regions of lower temperature, until at last the cryohydric or eutectic point is reached. On further addition of component II., the system will pa.s.s to regions of higher temperature, the solid phase now being component II. If the fused components are miscible with one another in all proportions a continuous curve will be obtained leading up to the point of fusion of component II.
Slight changes of direction, it is true, due to changes in the crystalline form, may be found along this curve, {132} but throughout its whole course there will be but one liquid phase. If, on the other hand, the fused components are not miscible in all proportions, then the second curve will exhibit a marked discontinuity, and two liquid phases will make their appearance.
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CHAPTER VIII
SOLUTIONS OF SOLIDS IN LIQUIDS, ONLY ONE OF THE COMPONENTS BEING VOLATILE
B.--HYDRATED SALT AND WATER.
In the preceding chapter we discussed the behaviour of systems formed of two components, only one of which was volatile, in those cases where the two components separated from solution in the pure state. In the present chapter we shall consider those systems in which combination between the components can occur with the formation of definite compounds; such as are found in the case of crystalline salt hydrates. Since a not inconsiderable amount of study has been devoted to the systems formed by hydrated salts and water, systems which are of great chemical interest and importance, the behaviour of these will first call for discussion in some detail, and it will be found later that the relations.h.i.+ps which exist in such systems appear also in a large number of other two-component systems.
The systems belonging to this group may be divided into two cla.s.ses according as the compounds formed possess a definite melting point, _i.e._ form a liquid phase of the same composition, or do not do so. We shall consider the latter first.
1. _The Compounds formed do not have a Definite Melting Point._
Concentration-Temperature Diagram.--In the case of salts which can form crystalline hydrates, the temperature-concentration diagram, representing the equilibria of the {134} different possible systems, must necessarily be somewhat more complicated than where no such combination of the components occurs. For, as has already been pointed out, each substance has its own solubility curve; and there will therefore be as many solubility curves as there are solid phases possible, _the curve for each particular solid phase being continuous so long as it remains unchanged in contact with the solution_. As an example of the relations.h.i.+ps met with in such cases, we shall first of all consider the systems formed of sodium sulphate and water.
[Ill.u.s.tration: FIG. 33.]
Sodium Sulphate and Water.--At the ordinary temperatures, sodium sulphate crystallises from water with ten molecules of water of crystallisation, forming Glauber's salt. On determining the solubility of this salt in water, it is found that the solubility increases as the temperature rises, the values of the solubility, represented graphically by the curve AC (Fig.
33), being given in the following table.[209] The numbers denote grams of sodium sulphate, calculated as anhydrous salt, dissolved by 100 grams of water.
SOLUBILITY OF Na_{2}SO_{4},10H_{2}O.
-------------------------- Temperature.
Solubility.
-------------------------- 0
5.02 10
9.00 15
13.20 18
16.80 20
19.40 25
28.00 30
40.00 33
50.76 34
55.00 --------------------------
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On continuing the investigation at higher temperatures, it was found that the solubility no longer increased, but _decreased with rise of temperature_. At the same time, it was observed that the solid phase was now different from that in contact with the solution at temperatures below 33; for whereas in the latter case the solid phase was sodium sulphate decahydrate, at temperatures above 33 the solid phase was the anhydrous salt. The course of the solubility curve of anhydrous sodium sulphate is shown by BD, and the values of the solubility are given in the following table:--[210]
SOLUBILITY OF ANHYDROUS SODIUM SULPHATE.
-------------------------- Temperature.
Solubility.
-------------------------- 18