140 



and the sodium remains in the solution as sodium hydroxide 

 (NaOH). The process is best shown by a diagram: 



:2 Nai Cl -> C1 2 t (pos. wire) 

 (neg. wire) f H 2 *- 2H ; OH | 



The chlorine C1 2 and hydrogen H2 being liberated, leave behind 

 in the solution the constituents of 2NaOH: 



2NaCl + 2H 2 + Elect. - H 2 + C1 2 + 2NaOH. 



Preparation from Oxygen and Hydrogen Chloride. 



When hydrogen chloride and oxygen gases are heated, they inter- 

 act very slowly to give water and chlorine. The action is greatly 

 hastened by contact with copper chloride. Lumps of pumice, 



saturated with a solution of 



^Mm* ^^A^^^^^f^,^^^ m= this catalyst (see p. 32), are 



p IG 4g placed in a tube. When the 



mixture of gases is passed over 



the heated pumice (Fig. 48), steam, chlorine, and about 20 per 

 cent of unchanged oxygen and hydrogen chloride issue at the 

 other end: 



Skeleton equation: HC1 + 2 <= H 2 O + C1 2 . 

 Balanced equation: 4HC1 + O 2 = 2H 2 O + 2C1 2 . 



Longer heating does not alter the proportion of the materials 

 successfully transformed. This is Deacon's process. 



That 80 per cent is changed, and 20 per cent unchanged, is 

 due to the fact that the action is reversible. If we lead pure 

 chlorine and steam through the tube (read the equation back- 

 wards), 20 per cent of hydrogen chloride and oxygen are formed. 

 No more than 20 per cent is formed, because these products are 

 continually being used up again and reproduce steam and chlorine. 

 If one product could be separated (p. 128) from the other, to pre- 

 vent the backward action, the yield could be raised to 100 per 



