Cryohydric point

Modern inorganic chemistry is a quantitative science. Consequently, when performing experimental work, students must determine the yield of the substances obtained and certain constants such as the boiling points, solubility, and cryohydrate points, and also perform the required calculations with the use of the fundamentals of thermodynamics. 

How can the shape of the temperature-time curve be explained Define the eutectic point (cryohydrate point). 

TEMPERATURE OF COOLING MIXTURE CONSISTING OP ICE AND SELECTED SALTS (CRYOHYDRATE POINT)... 

Salt Formula Mass of salt per 100 g of ice, g Cryohydrate point t, C... 

Table 19. COMPOSITION AND CRYOHYDRATE POINT OF COOLING MIXTURES ...

The point C is known as the eutectic point or cryohydric point of the system. It gives the lowest temperature which can be attained in the system, i.e., -23°. At the cryohydric point the solution freezes at constant temperature without change of composition. The eutectic composition is 52% KI and 48% ice. 

The eutectic point in KI - H20 system is also known as cryohydric point. 

A study of the curves in fig. 5 is particularly interesting from the point of view of the Phase Rule. AB represents the various states of equilibrium between ice and ferric chloride solutions, a minimum temperature being reached at the cryohydric point B, which is —55° C. At this point ice, solution, and the dodecahydrate of ferric chloride are in equilibrium. The number of degrees of freedom is nil—in other words, the system is invariant, and if heat be subtracted the liquid phase will solidify without change of temperature until the whole has become a solid mass of ice and dodecahydrate. Further abstraction of heat merely lowers the temperature of the system as a whole. 

The lines correspoial to the following states a v salt, vapotir, anmaximum pressure of the saturated solution. 

B, cryohydric point of magnesium sulphate solution (— 6°). AG, freezing point of potassium sulphate solution. 

The solubility of the hydrates of hydrochloric acid existing at low temperatures may be drawn in the under part of Fig. 38. We may here also start from the point 0° for pure water, and going along the curve of freezing points of solutions of hydrochloric acid reach the cryohydric point for HCI.3H2O at b then follow the solution of that hydrate, b c d, which at c has the same composition as the solid hydrate and ends at d with separation of the hydrate HCl. 2H2O then the curve for saturation... 

Salt. Cryohydric point. Percentage of anhydrous salt in the cryohydrate. 

At the cryohydric point, therefore, we are not dealing with a single solid phase, but with two solid phases, ice and salt and, as we have already learned, the constancy of temperature and composition at the cryohydric or eutectic point is due to the fact that we are dealing with an invariant system. 

The cryohydric or eutectic point is thus clearly seen to be the point of intersection of the solubility curve of the salt and the freezing-point curve of water. At this point, also, the curves of the univariant systems ice—salt— vapour and ice—salt —solution intersect. The cryohydric point is therefore a quadruple point, and represents an invariant system. 

If in the neighbourhood of the cryohydric point solution should be accompanied by an evolution of heat, then as the solubility would in that case increase with fall of temperature salt would pass into solution. 

The relations which are found here will be best understood with the help of Fig. 72 In this figure, OB represents the sublimation curve of ice, and BC the vaporisation curve of water the curve for the solution must lie below this, and must cut the sublimation curve of ice at some temperature below the melting-point. The point of intersection A is the cryohydric point. If the solubility increases with rise of temperature, the increase of the vapour pressure due to the latter will be partially annulled. Since at first the effect of increase of temperature more than counteracts the depressing action of increase of concentration, the vapour pressure will increase on raising the temperature above the cryohydric point. If the elevation of temperature is continued, however, to the melting-point of the salt, the effect of increasing concentration makes itself more and more felt, so that the vapour-pressure curve of the solution falls more and more below that of the pure liquid, and the pressure will ultimately become equal to that of the pure salt that is to say, practically equal to zero. The curve will therefore be of the general form AMF shown in Fig. 72. If the solubility should diminish with rise of temperature, the two factors, temperature and concentration, will act in the same direction, and the vapour-pressure curve will rise relatively more rapidly than that of the pure liquid since, however, the pure salt is ultimately obtained, the vapour-pressure curve must in this case also finally approach the value zero.2... 

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