104 GENERAL SCIENCE 



boiling at about 108 C. and freezing at -21 C. The 

 density of a salt solution should be tested to determine 

 whether it is different from that of pure water. 



Experiment 29. Into a vessel containing some pure water at 

 about the temperature of the room, place a thermometer and note 

 the exact temperature. Next add some ammonium nitrate and 

 again read the temperature from the thermometer. 



This is the usual effect of dissolving solids in water. 



After salt has been dissolved in water, how may it be 

 recovered ? 



Water of Crystallization. When a substance crys- 

 tallizes, it takes up water which is known as water of 

 crystallization. If we place a few crystals of copper sul- 

 phate in an evaporating dish and heat them, they will 

 change to a grayish blue powder. This substance may 

 be recrystallized by dissolving it in water and allowing 

 the water to evaporate slowly. If sodium sulphate is 

 placed in a test tube and heated, it will be dissolved in its 

 own water of crystallization. 



Each substance has its own crystalline form. For 

 example, salt crystals are cubical in shape, while other 



Snow Crystals. 



crystals are shaped like those shown in Figure 97. Some 

 of the most beautiful crystals are those of snow, made 

 by the freezing of moisture. The different flakes have 

 varied patterns, but they all have six sides or points 

 (Figure 98). 



