244 



shown by the way in which most solubilities increase with rising 

 temperature. The solution of a solid in a liquid, indeed, may be 

 regarded in normal cases as equivalent to the process of fusion, 

 which, like vaporization, always absorbs heat. The precipita- 

 tion of crystals from a supersaturated solution, similarly, may be 

 considered as analogous to the solidification of a pure substance, 

 the freezing-point being depressed below the normal va'ue, how- 

 ever, by the presence of the solvent (see p. 119). Freezing-point 

 lowering and solubility phenomena are therefore identical. When 

 we determine the depression of the freezing-point of water on 

 addition. of sugar by noting at what temperature ice begins to 

 separate out, we measure, simultaneously, the solubility of ice 

 in the solution at that temperature. In the same way, when we 

 determine the solubility of sugar in water at 25, we establish 

 also how much water must be added to a given quantity of sugar 

 to depress the freezing-point of sugar to 25. It is therefore 

 just as correct to say that sugar melts in tea as it is to say that 

 ice melts in iced tea. 



Often, however, extensive chemical reactions occur in the pro- 

 cess of solution (see p. 123), and the heat effect of these reactions 

 may be considerable. Thus when water is added to concentrated 

 sulphuric acid, so much heat is evolved that the water boils. 

 In the case of a few salts, also, we find that solution in water takes 

 place with evolution of heat, and we draw the conclusion that the 

 heat of the chemical reactions involved more than counterbalances 

 the heat absorbed in the process of fusion. 



When we heat a saturated solution, with excess of solid present, 

 crystals will separate out as the temperature is raised, if precipita- 

 tion is attended by absorption of heat. This is the case with 

 anhydrous sodium sulphate Na2SC>4 and some calcium salts in 

 water (see p. 113). 



Le Chatelier^s Law. The above-mentioned law is really a 

 particular case of a more general one, the law of Le Chatelier. 



