Part 16 (2/2)
[Ill.u.s.tration: FIG. 55.]
[Ill.u.s.tration: FIG. 56.]
(_b_) _The freezing-point curve exhibits a eutectic point_ (Curve II., Fig.
54). {192}
In this case the freezing point of each of the components is lowered by the addition of the other, until at last a point is reached at which the liquid solution solidifies to a mixture or conglomerate of two mixed crystals.
Examples.--Curves belonging to this cla.s.s have been obtained in the case of pota.s.sium and thallium nitrates[275] and of naphthalene and monochloracetic acid.[276] The data for the latter are given in the following table and represented in Fig. 56:--
-------------------------------------------------------------------------
Liquid solution.
Solid solution.
------------------------------------------------------------ Temperature.
Per cent.
Per cent.
Per cent.
Per cent.
naphthalene.
acid.
naphthalene.
acid.
------------------------------------------------------------------------- 62
--
100
--
100 60
4.0
96.0
1.7
98.3 55
21.0
79.0
2.1
97.9 53.5
29.4
70.0
--
-- 55
31.3
68.7
59.6
40.4 60
42.4
57.6
80.3
19.7 65
53.3
46.7
89.2
10.8 70
69.7
2.3
95.4
4.6 75
84.4
15.6
96.6
3.4 79.9
100
--
100
-- -------------------------------------------------------------------------
At the eutectic point the liquid solution is in equilibrium with two different mixed crystals the composition of which is represented by D and E respectively. If, therefore, a fused mixture containing the two components A and B in the proportions represented by C is cooled down, it will, when the temperature has reached the point C, solidify completely to a _conglomerate_ of mixed crystals, D and E.
[Ill.u.s.tration: FIG. 57.]
[Ill.u.s.tration: FIG. 58.]
Changes in Mixed Crystals with the Temperature.--In the case of the different types of systems represented in Fig. 49, a h.o.m.ogeneous liquid solution of the two components will exist at temperatures above the freezing-point curve, a h.o.m.ogeneous mixed crystal at temperatures below the melting-point curve, while at any point between the freezing-point and melting-point {193} curves the mixture will separate into a solid phase and a liquid phase. In the case, however, of the two types shown in Fig. 54 the relations.h.i.+ps are somewhat more complicated. As before, the area above the freezing-point curve gives the conditions under which h.o.m.ogeneous liquid solutions can exist; but below the melting-point curve two different mixed crystals can coexist. This will be best understood from Figs. 57 and 58. D and E represent, as we have seen, the composition of two mixed crystals which are in equilibrium with the liquid solution at the temperature of the point C. These two mixed crystals represent, in the one case, a saturated solution of B in A (point D), and the other a saturated solution of A in B (point E). Just as we saw that the mutual solubility of two liquids varied with the temperature, so also in the case of two solids; as the temperature alters, the solubility of the two solid components in one another will change. This alteration is indicated diagrammatically in Figs. 57 and 58 by the dotted curve similar to the solubility curves for two mutually soluble liquids (p. 101).
Suppose, now, that a mixed crystal of the composition _x_ is cooled down, it will remain unchanged until, when the temperature has fallen to _t'_, the h.o.m.ogeneous mixed crystal breaks up into a conglomerate of two mixed crystals the composition of {194} which is represented by _x'_ and _x”_ respectively. From this, then, it can be seen that in the case of substances which form two solid solutions, the mixed crystals which are desposited from the liquid fused ma.s.s need not remain unchanged in the solid state, but may at some lower temperature lose their h.o.m.ogeneity. This fact is of considerable importance for the formation of alloys.[277]
A good example of this will soon be met with in the case of the iron and carbon alloys. The alloys of copper and tin also furnish examples of the great changes which may take place in the alloy between the temperature at which it separates out from the fused ma.s.s and the ordinary temperature.
Thus, for example, one of the alloys of copper and tin which separates out from the liquid as a solid solution breaks up, on cooling, into the compound Cu_{3}Sn and liquid:[278] a striking example of a solid substance partially liquefying on being cooled.
{195}
CHAPTER XI
EQUILIBRIUM BETWEEN DYNAMIC ISOMERIDES
It has long been known that certain substances, _e.g._ acetoacetic ester, are capable when in solution or in the fused state, of reacting as if they possessed two different const.i.tutions; and in order to explain this behaviour the view was advanced (by Laar) that in such cases a hydrogen atom oscillated between two positions in the molecule, being at one time attached to oxygen, at another time to carbon, as represented by the formula--
CH_{3}.C--CH.CO_{2}C_{2}H_{5} . ^ .
O<-h>
When the hydrogen is in one position, the substance will act as an hydroxy-compound; with hydrogen in the other position, as a ketone.
Substances possessing this double function are called _tautomeric_.
Doubt, however, arose as to the validity of the above explanation, and this doubt was confirmed by the isolation of the two isomerides in the solid state, and also by the fact that the velocity of change of the one isomeride into the other could in some cases be quant.i.tatively measured.
These and other observations then led to the view, in harmony with the laws of chemical dynamics, that tautomeric substances in the dissolved or fused state represent a _mixture_ of two isomeric forms, and that equilibrium is established not by _intra_- but by _inter_-molecular change, as expressed by the equation--
CH_{3}.CO.CH_{2}.CO_{2}C_{2}H_{5} <--> CH_{3}.C(OH):CH.CO_{2}C_{2}H_{5}
{196} In the solid state, the one or other of the isomerides represents the stable form; but in the liquid state (solution or fusion) the stable condition is an equilibrium between the two forms.
A similar behaviour is also found in the case of other isomeric substances where the isomerism is due to difference of structure, _i.e._ structure isomerism (_e.g._ in the case of the oximes
<script>