io GENERAL SCIENCE 



9. Molecules. Molecules are the smallest divisions 

 of a substance that have the same properties as the sub- 

 stance itself. The small, invisible particles that escape 

 when water is heated are called molecules of water. 

 The smallest parts of chalk, coal, or sugar that have the 

 properties of chalk, coal, or sugar are also molecules. 

 Many thousands of them could hang on the point of a pin. 



Why do these molecules escape when the water is 

 heated? If a dozen boys and girls were standing on a 

 platform just large enough to hold them, they could all 

 stay on as long as they did not move around too much. 

 But however quiet they might try to be, there would still 

 be some movement, though perhaps not enough to shove 

 them out of their position. When they become more 

 active and begin to move around on the platform, several 

 may have to jump off. The more active they get, push- 

 ing against one another, the smaller the number that can 

 stay on the platform, and in time only one may bs left. 



In a piece of ice (a solid) the molecules can move out 

 of their position, but not far not even far enough to 

 change, materially, the shape of the ice. When heat is 

 applied to the ice, the molecules become more active, and 

 finally they shove one another so far apart that the ice 

 loses its shape and becomes a liquid. In a liquid the mole- 

 cules move around so freely that the liquid will take the 

 shape of any vessel containing it. Since molecules of 

 water are so small and light that they can float around in 

 the air, some of them jump out of the liquid into the air 

 and fly away. 



The process by which molecules continually escape from 

 water into the air is called evaporation. The more 

 heat you add to the water, the faster the molecules 

 move, and hence they escape into the air very rapidly 



