THE HEAT OF SOLUTION. 105 



HEAT. 



Heat is a very important factor in chemical action. In the heat factor 

 two points are involved: first, the general effect of heat; and, second, the 

 effect of change in temperature in consequence of the reactions. 



As to the first of these, in the lithosphere the higher the temperature in 

 general the more rapid the alteration. To this law there may be excep- 

 tions, but none are positively known to me. 



As to the second point, the chemical effect due to the change in tem- 

 perature in consequence of a reaction is much more complicated. 



In considering whether heat be liberated or absorbed as a result of a 

 chemical reaction it is necessary to take into account the heat changes in 

 solution, the heat changes in precipitation, the heat changes in mixing 

 solutions, and the heat effects of chemical reactions. 



"By the heat of solution is meant the quantity of heat produced by 

 the solution of 1 gram molecule of a substance in a large quantity of the 

 solvent.'" 1 It has already been seen that in general the volume of the 

 solvent and salt is greater than that of the solution, and that in this case 

 there is usually liberation of heat and consequently rise in temperature; 

 but in exceptional cases the volume of the salt and solvent is less than that 

 of the solution, and in this case there is generally absorption of heat, and 

 consequent fall in temperature. The total effect as to the liberation or 

 absorption of heat depends upon whether the total of the factors, change 

 in volume, change of the solid to its dispersed form in the solution, and 

 the heat factor of dissociation, provided this occurs, is plus or minus. 

 Decrease of volume tends to liberate heat; increase of volume tends to 

 absorb heat. The change from the solid to the dispersed state of solution 

 absorbs heat. The supposed dissociation of a substance into its ions is 

 regarded as attended with either a liberation or an absorption of heat, 

 though liberation is held to occur more frequently.'' 



In precipitation the heat effect is just the opposite from that of solution 

 and is equivalent to the heat effect of the solution of an equal amount of 

 the like salt. "In general, in comparing substances which are chemically 

 analogous and soluble with difficulty, the heat of precipitation (—the 

 negative value of the heat of solution) is the greater the more insoluble 



"Nernst, W., Theoretical chemistry, translated by C. S. Palmer, Macmillan & Co., London, 

 1895, p. 503. 



!>Nernst, cit, p. 562. 



