WATER AS A SOLVENT 



29 



Liquids and gases, like solids, vary in their solubility in water. 

 Compare the solubility of alcohol, chloroform, and paraffin oil 

 by shaking the liquids, added drop by drop, in large test-tubes 

 of water. Compare the solubility of carbonic acid gas (sup- 

 plied) and air by inverting test-tubes of the gases in recently 

 boiled and cooled water, and noting the height to which the 

 water rises in the tubes. Since heat tends to promote change 

 from the liquid to the gaseous state, it is to be expected that a 

 gas will be more soluble in cold than in hot water. Heat some 

 rain water in a test-tube, and note that, as the temperature 

 rises, bubbles of gas are given off. 



The foregoing experiments indicate a method of obtaining 

 perfectly pure water by distillation', for on boiling water, 

 the dissolved gases are first evolved, the dissolved solids are 

 left behind, and the steam is pure water vapour which can be 

 condensed to pure water. Distil some water in a retort, and 



lead the steam into a flask kept cold by a trough of cold water, 

 in order that the temperature may be reduced below the con- 

 densing point of steam. Reject the first part of the distillate, 

 and collect that which comes over afterwards. Note that the 

 dissolved solids are left as a residue in the retort. 



Rain, Spring, and Sea Water.— Water exercises an appreciable 

 vapour pressure at all temperatures, and this increases as the temperature 

 rises, until at the boiling point it is equal to the atmospheric pressure. It is 



