27% Domestic Science 



deposited. Allow evaporation to take place slowly at 

 the room temperature for a considerable time and the 

 small piece of alum will be observed to gradually in- 

 crease in size. After some days its shape should be 

 easily recognisable, and is usually somewhat like that 

 shown on the left hand in Fig. 88. Such regularly- 

 shaped lumps are termed "crystals" and the process 

 by which they are produced is termed " crystallisation". 

 188. When a liquid has dissolved as much of a 

 given solid as possible at a given temperature, the 

 solution is called a " saturated " solution. By raising 

 the temperature of the liquid it may in general be 

 caused to dissolve more solid, and unless more is thus 



. Fig. 88. 



dissolved, the solution becomes unsaturated as the 

 result of the rise in temperature. On the other hand, 

 cooling the liquid causes it to become supersaturated 

 and the quantity of solid in excess of that necessary 

 to produce a saturated solution at the new temperature 

 separates out. It occasionally happens that the begin- 

 ning of crystallisation is delayed in the case of clear 

 solutions in clean vessels, and it is thus feasible to 

 prepare a solution containing a greater quantity of 

 dissolved substance than would normally be the case. 



