HYDROSCOPIC AND EFFLORESCENT SALTS 173 



122. HYGROSCOPIC AND EFFLORESCENT SALTS 



When salts are crystallized out of a water solution, they 

 contain a certain amount of water, by virtue of which they 

 are able to take the crystalline shape. This is called the 

 water of crystallization, and is a fixed amount for a given sub- 

 stance. The water of crystallization may be driven out by 

 heat, sometimes quietly, and sometimes with tiny explosions 

 caused by the expansion of the water when it turns to steam. 

 The color of the crystal is lost and it becomes white, showing 

 that the color is due to the arrangement of the molecules, 

 rather than to any inherent characteristics. If a crystal loses 

 its water of crystallization, when exposed to air, it is said to be 

 efflorescent. 



On the other hand, some substances attract the moisture of 

 the air and form chemical compounds with it. These are 

 called hygroscopic salts, and they may or may not be dry after 

 the absorption of the water. If they become wet, they are 

 called deliquescent. Ordinary lime, unslaked, is an example 

 of a hygroscopic substance which remains dry, although ab- 

 sorbing a considerable quantity of water. Calcium chloride 

 and zinc chloride are examples of deliquescent substances. 

 See Experiment 58. 



Since the absorption of the water vapor of the air is the 

 opposite to evaporation, a little calcium chloride placed in 

 the water of fire pails will prevent the water from evapo- 

 rating. 



Salts, when deprived of their water of crystallization, have 

 no fixed shape and are called amorphous. Some amorphous 

 salts are very hygroscopic. Molasses is an example of an 

 organic compound which is quite hygroscopic. 



