232 



OUR PHYSICAL WORLD 



battery or other source of electricity. As a result of the charge, 

 the plates connected with the cathode are coated with lead per- 

 oxide (Pb 2 s ). Now, when the battery is being used, the lead 

 plates that were connected with the anode become the cathode, 

 and those that were connected to the cathode become the anode. 

 The lead peroxide (Pb 2 O s ) readily breaks down, yielding oxygen 

 which unites with the hydrogen of the sulphuric acid, thus 

 liberating S0 4 , which goes to the anode and unites with the lead, 



forming lead sulphate, PbS0 4 . As 

 the lead peroxide, Pb 2 s , changes to 

 the oxide, PbO 2 (Pb 2 O s = 2 Pb0 2 +0), 

 the lead is becoming less positive by 

 taking on electrons drawn from the 

 anode plate, thus leaving it positive. 

 At the other plate where the lead 

 sulphate is forming and ionizing, 

 neutral molecular lead is changing to 

 positive lead ions by giving up elec- 

 trons to the cathode, which is there- 

 fore negative. When the battery is 

 charged, the reverse process takes 

 place. The electricity flowing into the 

 battery decomposes the water, the 

 hydrogen going to one pole, the oxygen to the other. The oxygen 

 now changes the PbO 2 to Pb 2 s , while the hydrogen displaces the 

 lead in the lead sulphate, thus forming sulphuric acid. The 

 lead so displaced deposits on the plate. During the discharge 

 of the current, the movements of the electrons are just the reverse 

 of those described above. 



WHEN CHARGING 



Anode or PbSo 4 H 2 O Cathode 



positive decomposes decomposes or 



plate <- Pb So 4 <- H a O -> unites with PbO 2 negative 



unite to to form Pb 2 O s -> plate 



form 



FIG. 98. Diagram of a stor- 

 age battery. 



