7 k 2 PRINCIPLES OF CHEMISTRY 



100 parts of water by potassium chloride there is required at 0, 29*2 

 parts, at 20, 34*7, at 40, 40'2, at 60, 45-7 ; and so on, for every 10 

 the solubility increases by 2 -75 parts by weight of the salt. Therefore 

 the solubility of potassium chloride in water may be expressed by a 

 direct equation : a=29*2 + 0*2752, where a represents the solubility at t". 

 For other salts, more complicated equations are required. For exam pie, 

 for nitre: a=13*3 + 0*5742 + 0*017172 2 + O0000036* 3 , which shows 

 that when 2=0 a=13*3, when 2 = 10 a=20'8, and when 2 = 100 

 a=246*0. 



Curves of solubility give the means of judging with accuracy the 

 amount of a salt separated by the cooling to a known extent of a 

 solution saturated at a given temperature. For instance, if 200 parts 

 of a solution of potassium chloride in water saturated at a temperature 

 of 60 be taken, and it be asked how much of the salt will be separated 

 by cooling the solution to 0, if its solubility at 60 = 45'7 and at 

 0=29*2 ? The answer is obtained in the following manner : At 60 a 

 saturated solution contains 45*7 parts of potassium chloride per 100 

 parts by weight of water, consequently 145*7 parts by weight of the 

 solution contains 45*7 parts, or, by proportion, 200 parts by weight of 

 the solution contains 62*7 parts of the salt. The amount of salt 

 remaining in solution at is calculated as follows : In 200 grams 

 taken there will be 137*3 grams of water ; consequently, this amount of 

 water is capable of holding only 40*1 grams of the salt, and therefore 

 in lowering the temperature from 60 to there should separate from 

 the solution 62*7 40*1 = 22*6 grams of the dissolved salt. 



The difference in the solubility of salts, <fcc., with a rise or fall of 

 temperature is often taken advantage of, especially in technical 

 work, for the separation of salts in intermixture from each other. 

 Thus a mixture of potassium and sodium chlorides (this mixture is met 

 with in nature at Stassfiirt) is separated from a saturated solution by 

 subjecting it alternately to boiling (evaporation) and cooling. The 

 sodium chloride separates out in proportion to the amount of water 

 expelled from the solution by boiling, and is removed, whilst the 

 potassium chloride separates out on cooling, as the solubility of this 

 salt rapidly decreases with a lowering in temperature. Nitre, sugar, and 

 many other soluble substances are purified (refined) in a similar 

 manner. 



Although in the majority of cases the solubility of solids increases 

 with the temperature, yet just as there are substances whose volume 

 diminishes with a rise in temperature (for example, water from to 

 4), so there are not a few solid substances whose solubilities fall on 

 heating. Glauber's salt, or sodium sulphate, historically forms a particu- 



