ON SOLUBILITY. 445 



V. A. (ii). — Solubility in relation to Heat of Dissolution. 



Le Chatelier 112 published a mathematical application of the laws 

 •,oo~ °f chemical equilibrium to the case of the dissolution of salts in 

 water. He showed the relation between the coefficient of solu- 

 bility of salts, their heat of dissolution at point of saturation, and the 

 temperature could be approximately expressed by an equation. This 

 conclusion was extended, 112 and he formulated the general law that 

 the solubility of a substance increases with rise of temperature when 

 the heat of solution is negative, and vice versd. 



Chancel and Parmentier 127 took exception to this law, not 

 considering it to be of general applicability in view of their obser- 

 vations on the relation between the solubility of normal calcium 

 butyrate and calcium iso-butyrate at different temperatures and the 

 thermal behaviour of these substances on dissolution. In reply to this 

 criticism Le Chatelier 129 pointed out that the last-mentioned investi- 

 gators had not measured the heat of dissolution in a saturated solution 

 but in dilute solution ; by direct experiment he proved the heat 

 of solution of hydrated calcium iso-butyrate in saturated solution 

 to be positive, as is required by his law of equilibrium. This brought a 

 reply from Chancel and Parmentier. 127 



In a publication entitled ' Deductions from Van't Hoff's Theory ' 

 iq«q 1S ^° be found a discussion by Payliani 1 ^* of an expression for 



finding the heat of solution of a gas. 

 lftQO Riecker and Van Deventer 160 found the solubility of copper 



chloride increased with rise of temperature, although this salt 

 dissolves in much water with development of heat ; in more concen- 

 trated solutions, however, this process of dissolution is endothermic. 

 A comparison of solubility and heat of dissolution led Timo- 

 ' fejew 171 to conclude that the molecular solubility of such sub- 

 stances as oxalic, succinic, benzoic, cinnamic, and salicylic acids in 

 methylic, ethylic, and propylic alcohols varies inversely as the heat 

 of solution. 



This was also found 171 to be the case with cadmium iodide, mercuric 

 chloride, and naphthalene in the same three alcohols. The ratio of 

 the heat of dissolution of one and the same substance in methylic 

 alcohol to its heat of dissolution in ethylic alcohol was found to be 

 practically identical with this ratio when ethylic alcohol and propylic 

 alcohol were the solvents employed. 



The supposed general law enunciated by Le Chatelier, connecting 

 variation in solubility with heat of solution, was shown by Van 

 1892- Deventer and Van d. Stadt 187 to be of only limited applicability. 

 Some interesting experiments were carried out by Pickering 189 on 

 the relationship between solubility, heat of vaporisation, and heat of 

 dissolution. Liquid substances of known heat of vaporisation were 

 dissolved separately in water, acetic acid and benzene, and their heat 

 of dissolution calculated. 



This author published 222 determinations of the heat of solu- 

 tion of calcium chloride ; the results were plotted as percentage of 

 CaCl 2 against molecular heat of solution. 



1910. g g 



