LEAD-COPPER-BISMUTH. 319 



mixture with undecomposed sulphide to a higher temperature lead is 

 formed, thus : 



PbS + 2PbO == 3Pb + S0 2 and PbS + PbSO 4 == 2Pb + 2SO 2 . 

 Lead owes its usefulness in the metallic state chiefly to its softness, 

 fusibility, and resistance to acids, which properties are of advantage 

 in using it for tubes or pipes, or in constructing vessels to hold or 

 manufacture sulphuric acid. 



Lead is exceedingly malleable and somewhat ductile, but not very tenacious. 

 It is a constituent of many alloys, as for instance of type metal, britannia metal, 

 shot, etc. Common solder is an alloy of equal weights of lead and tin. The 

 noble metals are rendered brittle and unworkable when alloyed with even a 

 small quantity of lead. 



Experiment 38. Dissolve 1 gramme of lead acetate or lead nitrate in about 

 200 c.c. of water, suspend in the centre of the solution a piece of metallic zinc, 

 and set aside. Notice that metallic lead is deposited slowly upon the zinc in a 

 crystalline condition, while zinc passes into solution, which may be verified by 

 analytical methods. The chemical change taking place is this : 



Pb(N0 3 ) 2 + Zn Zn(N0 3 ) 2 + Pb. 



Electrolytic solution tension. The precipitation of lead from solution 

 by zinc in the experiment above is represented by the ionic equation : 



Zn + Pb" + 2NO 3 ' = Zn" + 2NO 3 ' + Pb. 



The lead ions lose their charges to zinc which becomes ionic, while metallic 

 lead is precipitated. This action is pretty much like the liberation of hydrogen 

 from acids by some metals : 



Zn + 2H* + SO 4 " = Zn" + SO 4 " -f H 2 . 



The explanation of this type of chemical change is found in the theory of elec- 

 trolytic solution tension proposed by Nernst. According to this, a metal when 

 immersed in water or a solution sends some positive ions into the solution, and 

 itself assumes negative charges of electricity. This proceeds to a point where 

 the metal is sufficiently charged negatively that it attracts its positive ions at 

 the same rate at which they tend to be given off from the metal. An equilib- 

 rium between the two tendencies is reached. The tension or pressure that 

 drives the ions into the solution differs for different metals, the order of decrease 

 being the same as the order in the electrochemical series of the metals (page 

 198). Any metal in the series has a higher tension than those following, and 

 will displace them from solutions of their salts, but not vice versa. Zinc has 

 a much greater solution tension than lead. When it is placed in the lead solu- 

 tion, it acquires a greater negative charge of electricity than does a piece of 

 lead when it is placed in a solution. The result is that the lead ions are 

 attracted to the zinc and discharged, and metallic lead is deposited. This 

 process continues until all the lead has been deposited from the solution, which 

 then contains an equivalent amount of zinc salt. 



Lead oxide, Plumbi oxidum, PbO = 221.23 (Litharge). Obtained 

 by exposing melted lead to a current of air, when the metal is 



