10 MATTER AND ENERGY 



3. How, in general, may the state of a substance be changed ? 

 Give any common examples of changes caused by heating ; by 

 cooling. 



4. Can steam be seen ? Why does steam always form a white 

 cloud when set free in the air ? Of what is that cloud composed ? 



5. Give examples of substances common in two states. Why 

 are not all substances common in more than one state ? 



6. What is a molecule ? What are pores ? 



7. State the molecular theory in your own words. What does 

 it attempt to explain ? Is it known to be true ? 



SECTION III 

 PROPERTIES OF MATTER 



12. Common Properties. The features which any 

 form of matter possesses are called its properties; such, 

 for example, as its color, density, hardness, and the like. 

 As there are many different substances, there are also 

 many different properties. Very few of these are pos- 

 sessed by all forms of matter, though several are com- 

 mon to many substances. We shall consider only a few 

 of the more common properties. 



13. Impenetrability. This long word names a prop- 

 erty which is common to all matter, that of completely 

 occupying the space in which it is. This fact is some- 

 times stated as follows: No two bodies can occupy 

 the same space at the same time. No body of matter 

 can enter a space already filled, without first driving 

 out the substance which fills it. Notice that we say the 

 same space; this does not mean that two things cannot 

 be in the same dish, for example, since they can both 

 be in a dish together without filling the same space. 



