SOLUTION 



115 



This is to melt the substance (without any solvent), and allow the 

 mass to cool slowly. When a part has solidified, the rest of the 

 liquid is rapidly poured off. Metals and many other fusible sub- 

 stances give good crystals in this way. Water itself, when it 

 freezes, deposits radiating, hexagonal crystals of ice. 



Supersaturation. When a hot, saturated solution is cooled, 

 there is quite commonly some delay before the crystals begin to 

 appear. The solution, pending the appearance of the crystals, is 

 then said to be supersaturated. In most cases the crystals soon 

 appear in due course, especially if the liquid is shaken or stirred. 

 But certain substances have a tendency to remain indefinitely in 

 the state of supersaturated solution. The hydrates of sodium 

 sulphate (Na 2 S0 4 ,10H 2 0) and of sodium thiosulphate (photog- 

 raphers' " hypo " Na 2 S 2 03,5H 2 0) give solutions in water of this 

 nature. The addition of a minute crystal of the 

 substance concerned (" inoculation "), however, 

 always starts the crystallization (Fig. 46). 



Many pure liquids, similarly, when cooled below 

 their freezing-point, do not always crystallize out 

 at once. Thus water can be taken down to 10 

 without the appearance of ice. In this condition 

 it is said to be supercooled. Shaking, or stirring, 

 or (better still) inoculating with a fragment of 

 ice, induces crystallization in this case also. It 

 may be noted that the opposite phenomenon has 

 never been observed; ice invariably melts sharply 

 at under atmospheric pressure. 



FIG. 46 



Heat of Solution. Most substances absorb heat when they 

 dissolve, making the solution cooler, and give out heat when they 

 crystallize. Thus, in the cases of the two sodium salts last 

 mentioned, when the crystallization is brought about in the cool, 

 supersaturated solutions, the rise in temperature is considerable. 



