92 NOT ALTOGETHER ABOUT PLANTS 



Thus in the case of sugar in tea, the sugar changes from 

 a solid state to a liquid state and yet remains sugar. 

 But the molecules of sugar cannot be changed into gas 

 without ceasing to be sugar ; they become other substances. 



There are some substances, however, which can exist in 

 all three states of matter. Water is an example. Water 

 when it evaporates changes from a liquid to a gas, but its 

 molecules still remain molecules of water. 



Thus we see that forms of matter can exist in one or in 

 more than one state of matter. Water may exist in all 

 three states ; it may be ice, it may be liquid water, or it 

 may be water vapor, which is a gas. Sugar may exist in 

 two states ; it may be a solid or a part of a Hquid, but it 

 cannot be changed to a gas. Gunpowder is an example 

 of the substances which can exist in only one state ; it 

 is a solid which cannot be dissolved nor can it be changed 

 to a gas without changing its nature. When gunpowder 

 explodes it changes into gases which immediately expand 

 with much force, and it has ceased to be gunpowder. 



We found that water dissolves more substances than 

 anything else. We now find that nothing exceeds it in 

 ability to exist in different states without changing into 

 other substances. 



32. Elements and Compounds. — Men found long ago 

 that they could change some substances into other sub- 

 stances. They found that when wood or anything else 

 burns it changes into substances quite different from what 

 it was before. The burning of wood leaves a solid called 

 the ash; it also leaves certain gases which are diffused in 

 the air. (By diffusion is meant a spreading abroad. The 

 spreading of the molecules of a solute in a solvent is an- 



