ELECTROLYSIS OF WATER 



95 



99. Composition of Water. By the "composition," 

 or "make-up," of water we do not mean a list of the 

 materials present in a particular sample of water, such as 

 the carbon dioxide, limestone, salt, etc., that are dissolved 

 in a natural water (c/. 80). What we mean is that pure 

 water, which we obtain by purifying natural water, is 

 still made up of two different substances : hydrogen and 

 oxygen. We say that water is "composed of," or "is a 

 compound of," hydrogen and oxygen. 



Water is formed when hydrogen is burned (c/. 52 

 and 105). Can we reverse the action that takes place in 

 burning, and break up water into hydrogen and oxygen, 

 just as we "decomposed" mercury oxide into mercury 

 and oxygen (c/. 50) ? Water, like mercury oxide, can be 

 broken up by heat; but the temperature required is very 

 high, and the method is hard to carry out. The decom- 

 position of water is easy, if we use the electric current. 

 The operation is called the electro- 

 lysis of water. Electrolysis means 

 "loosing," or "breaking apart," by 

 the use of electricity. 



100. Electrolysis of Water. The 

 "breaking apart" of water by the 

 electric current may be carried out 

 as follows (Fig. 77) : 



FIG. 77. 



Electrolysis of Water. The 

 electric current breaks up 

 water, containing a little 

 of an acid, into hydrogen 

 and oxygen. 



Two wires from a battery or other source 

 of the current (c/. 150 and 160) pass into a 

 vessel. The vesst/1 contains water and a very 



little sulphuric acid. The liquid to be changed by the electric current 

 is thus a very dilute sulphuric acid. The wires inside the vessel are 



