SOLUTION 113 



12 (in water) <= I 2 (in ether). The partition of the substance 

 takes place in proportion to its solubility in each solvent. It 

 is found that any amount of the solute, up to the maximum the 

 system can contain, provided this does not involve too high a 

 concentration in either solvent, is divided so that the ratio of the 

 concentration in the two solvents is always the same. In the case 

 of iodine divided between water and ether, this ratio is about 

 1 : 200. 



This principle is used in Parke's process for extracting silver 

 from molten lead, by means of melted zinc as the second solvent. 

 It is employed in separating interesting compounds from animal 

 secretions and vegetable extracts, and in purifying such compounds. 

 Nicotine from tobacco and cocaine from coca leaves are secured 

 in this way. 



Temperature and Solubility. The solubility of every 

 substance in any solvent varies more or less with the temperature. 

 The solubility of niter (potassium nitrate KN0 3 ) in water shows 

 great variation, namely 13 g. in 100 c.c. at 0, 26 g. at 20, 140 g. 

 at 70. On the other hand, the solubility of common salt (sodium 

 chloride NaCl) is nearly constant,'_35.5 g. at 0, 36.5 g. at 20, 38 g. 

 at 70, 40 g. at 100. 



Usually, as in these two cases, the solubility of solids in liquids 

 (and of liquids in liquids) increases with rise in temperature, but 

 in a few cases it diminishes. Thus, the solubility in water of 

 slaked lime (calcium hydroxide Ca(OH) 2 , used to make lime 

 water) is 0.175 g. at 20 and 0.079 g. at 100, and that of an- 

 hydrous sodium sulphate (Na^SC^) is 55 g. at 32.5 and 42 g. at 

 100. 



The solubility of gases in liquids diminishes with rising tem- 

 perature. This may be illustrated by heating cold tap-water in a 

 beaker. The dissolved gases, originally obtained from the air, 

 appear in bubbles on the bottom and sides as the temperature 

 rises. 



