Solution and Chemical Action 353 



ings when you made the toy balloon. The copper 

 drove the hydrogen out of the nitric acid and incidentally 

 broke down some of the nitric acid to make the brown 

 gas, and then the copper joined the rest of the nitric 

 acid to make a salt called copper nitrate. This salt is 

 green, and it dissolves in water. When you washed 

 the copper, the green salt was washed away and a 

 dent remained in the copper where the copper salt 

 had been. 



Here is a more practical experiment showing the 

 action of acid on metal : 



Experiment 106. Use two knives, one of bright steel 

 and the other a silver-plated one. If the steel knife is not 

 bright, scour it until it is. Drop a little lemon juice on each 

 knife and let it stand for a few minutes, while the teacher 

 does the next experiment. Then rinse both knives and 

 examine them. What has the lemon juice done to the silver 

 knife? to the steel one? 



The lemon juice acts in this way because it is acid. 

 Acids act on the taste nerves in the tongue and give the 

 taste of sourness; everything sour is an acid. The 

 black stuff formed on the steel knife by the lemon 

 juice is an iron salt. The iron in the knife drove 

 the hydrogen out of the lemon juice, but there was 

 not enough for you to see it coming off; then the iron 

 combined with the rest of the lemon juice to form an 

 iron salt. 



Whenever an acid acts on a metal, the metal drives 

 off the hydrogen and forms a salt. The salt is not 

 always good to eat ; for instance, the salt that tin forms 

 with acids is poisonous. 



