Chemical Change and Energy 339 



still turns black, it means that too much electricity is 

 flowing. In that case use fewer batteries. 



The electricity has started two chemical changes. It 

 has made part of the piece of nickel combine with part 

 of the solution of nickel salt to form more nickel salt, 

 and it has made some of the nickel salt around the copper 

 change into metallic nickel. Then the negative elec- 

 tricity in the copper has attracted the positive bits of 

 nickel metal made from the nickel salt, and made them 

 cling to the copper. If there is no dirt or grease on 

 the copper, the particles of nickel get so close to it that 

 they stick by adhesion, even after the electric attraction 

 has ceased. This leaves the copper nickel-plated, but 

 to make it shiny the nickel plating must be polished. 



Silver plating and gold plating are done substantially 

 in the way that you have done the nickel plating, only 

 gold salt or silver salt is used instead of nickel salt. 



Just as electricity helps chemical changes in plating, 

 it helps changes in a storage battery. But in the storage 

 battery the new compounds formed by " charging " the 

 battery change back again and generate electricity when 

 the poles of the battery are connected with each other 

 by a good conductor. 



Application 75. Explain how spoons can be silver plated; 

 how water can be changed into hydrogen and oxygen. 



Inference Exercise 

 Explain the following : 



471. Clothes dry best in the sun and wind. 



472. Proofs of photographs that have not been thoroughly " fixed " 



fade if left out of their envelope. 



473. Blowing a match puts it out, yet a good draft is necessary for 



a hot fire. 



