28 CHEMISTRY FOR AGRICULTURAL STUDENTS 



VII. WATER AS A SOLVENT 



Shake some common salt, gypsum, and sand separately with 

 pure water, filter the liquids, from undissolved substance, 

 through a cone of porous paper fitted in a funnel as figured, 

 into porcelain basins, and evaporate each to dryness over 



Fig. 12. 



Fig. 13. 



beakers of boiling water (Fig. 13). Judging from the residue, 

 which substance would be described as insoluble, which as 

 slightly soluble, and which as very soluble in water? 



Determine the solubility of gypsum. Stir excess of pure 

 gypsum with pure water (at 15° C.) in a beaker until, after 

 prolonged stirring, a saturated solution is obtained. Then 

 filter and evaporate 25 grams of the filtrate in a weighed 

 porcelain basin to dryness on the water bath. Weigh the dish 

 and residue. Calculate the amount of gypsum dissolved by 

 I gram of water (the " co-efficient of solubility " of gypsum). 

 Repeat the experiment until concordant results are obtained. 



The solubility of a solid in water usually increases with rise in tempera- 

 ture, for heat tends to promote change from the solid to the liquid state. 

 It is therefore necessary to adopt a standard temperature in the determina- 

 tion of solubilities. The usual temperature of the laboratories is 15° C, 

 and this is therefore generally adopted. 



