

210 



ELEMENTARY GENERAL SCIENCE 



CHAP. 



passes off as vapour without actually boiling, as the student may 

 readily see by a simple experiment, and we speak of this as 

 evaporation. 



Distillation. We also see that we can by the evaporation of 

 the sol vent, i.e. the liquid in which the solution takes place, 

 separate it from the dissolved substance, and this is very 

 frequently used not only for obtaining the dissolved substance 

 but also for the purification of liquid from dissolved solid 

 material, since if the steam which is formed by boiling water 

 containing any of these dissolved substances be condensed, the 

 water formed is quite pure. To obtain pure water from any 



FIG. 108. The Distillation of Water. 



kind of water, then, whether fresh or salt, all that has to be done 

 is to boil it and condense the steam which is given off. The 

 dissolved materials are all left behind in the vessel in which the 

 boiling takes place. An arrangement for condensing steam or 

 vapour is shown in Fig. 108. The steam that is driven off from 

 the water in the flask passes through a long tube kept cool by 

 being surrounded with water, and is thus condensed. 



EXPT. 201. Evaporate a little distilled water in a platinum 

 or porcelain crucible. Notice the absence of any residue. 

 Repeat the experiment with tap-water, and note the residue. 



