SODIUM 103 



not fit exactly together, some of the brine is caught be- 

 tween them. When the salt crystals are heated, some of 

 the water is changed to steam, and bursts the crystal mass. 

 Hence small salt crystals "crackle" when heated; while 

 large ones "snap " vigorously, and fly out of the dish. 



Salt is not changed, even when heated to red heat. At 

 white heat it melts. Like water, it can be broken up by 

 the electric current. When the current is passed through 

 melted salt, its elements, sodium and chlorine, are formed, 

 one at each pole. The chemical name of salt is sodium 

 chloride. 



109. Sodium. Sodium is a metal, like silver, gold, 

 copper, etc.; but it is so soft that it can be cut with a 

 knife. When it is freshly cut, its surface is white as 

 silver; but it is soon tarnished by the moisture, the oxygen, 

 and the carbon dioxide of the air. Sodium is kept under 

 gasoline, or some other liquid containing no oxygen. 

 When heated in air or oxygen, sodium burns, forming 

 sodium oxide (cf. 48 and 51). 



Sodium acts vigorously with water (Fig. 

 85). The action produces so much heat that 

 the sodium melts, forming a round ball 

 which floats upon the water. A gas escapes 

 with a hissing noise, and the sodium finally 



disappears. The gas produced is hydrogen. F IG. 



When the water is rubbed between the fin- 



gers, it feels soapy, or slimy. It contains 



sodium hydroxide, also called caustic (i. e., burning) 



soda (cf. 218). 



Sodium + water give sodium hydroxide + hydrogen. 



