CHAPTER ELEVEN 



SOLUTION AND CHEMICAL ACTION 



SECTION 53. Chemical change helped by solution. 



Why does iron have to get wet to rust? 

 Is it good to drink water with your meals ? 



When iron rusts, it is really slowly burning (combin- 

 ing with oxygen). If your house is on fire, you throw 

 water on it to stop the burning. Yet if you throw water 

 on iron it rusts, or burns, better than if you leave it 

 dry. What do you suppose is the reason for this? 



The answer is not difficult. You know perfectly 

 well that iron does not burn easily ; we could not make 

 fire grates and stoves out of iron if it did. But when 

 iron is wet, a little of it dissolves in the water that wets 

 it. There is also a little oxygen dissolved in the water, 

 as we know from the fact that fish can breathe under 

 the water. This dissolved oxygen can easily combine 

 with the dissolved iron; the solution helps the chemical 

 change to take place. The chemical change that results 

 is oxidation, the iron combining with oxygen, 

 which is a slow kind of burning; and in iron this is 

 usually called rusting. 1 But when we pour water on 

 burning wood, the wood stops burning, for there is not 

 nearly enough oxygen dissolved in water to combine 

 rapidly with burning wood ; and the water shuts off the 

 outside air from burning wood and therefore^ puts the 

 fire out. 



Another chemical change, greatly helped by solution, 

 is the combining of the two things that baking powder 



1 The rusting of iron is not quite as simple as this, as it probably under- 

 goes two or three changes before finally combining with oxygen. But 

 the solution helps all these changes. 



349 



