114 SMITH'S INTERMEDIATE CHEMISTRY 



Crystallization. If the solvent has been saturated while 

 warm, and the substance is one that is less soluble at lower tem- 

 peratures, then, when the temperature falls, the solute begins to 

 come out of solution. The amount appearing, of course, is only 

 the excess beyond what is needed to saturate the solvent at the 

 lower temperature. 



If the solute is liquid at the new temperature, it appears at 

 first as a cloud of drops, rendering the liquid milky. This may be 

 shown by cooling a solution of phenol (carbolic acid) in hot water. 



If the solute is solid, then the particles, as they appear, take 

 the form of crystals (p. 94). These grow by taking on more of 

 the separating solute. If the cooling goes on slowly, very large 

 crystals can finally be obtained. On the other hand, with rapid 

 cooling, new crystals are continually formed, and a fine crystal- 

 meal falls to the bottom of the solution. The crystals in this 

 meal, however, when viewed through a lens, are seen to be just 

 as perfect as the larger ones. 



When a more dilute solution is used, instead of a saturated one, 

 crystals may still be obtained. A part of the solvent must first 

 be removed, however. This may be done, either by boiling the 

 solution for some time, or by leaving it to evaporate in a wide 

 dish in which a large surface is exposed. 



When the dissolved substance can form a compound with the 

 solvent (e.g., a hydrate, see p. 67) which is stable at the tem- 

 perature of crystallization, the crystals are composed of this com- 

 pound. 



The whole of the solvent may be boiled off. But in this case, 

 good crystals of the solute are never obtained the residue is 

 usually a crust composed of imperfect crystals. 



When the substance is more soluble in cold than in hot water, 

 then crystallization is produced by raising the temperature. 



Crystallization from a Melted Mass. In this connection, 

 it should be noted that there is another way of obtaining crystals. 



