8 MATTER AND ENERGY 



In general, gases change to liquids and liquids to 

 solids upon being cooled. Air and other gases are now 

 changed to liquids, and some even to solids, by cooling 

 to a very low degree. Steam is so commonly changed to 

 liquid particles that we carelessly call the liquid " steam." 



Experiment 4. Boil some water in a test tube. Hold a cool 

 dish in the steam just as it escapes from the tube. (A glass dish, 

 if clean and dry, may best serve the purpose.) Explain what you 

 observe. 



Experiment 5. Melt some paraffin, wax, or sugar, and let it 

 cool rapidly. What change takes place ? Under what condition 

 does the change occur? 



Most forms of matter occur commonly in only one 

 state, because the temperatures at which they would 

 change are unusually high or low. Some substances, 

 however, are common in two states: ice, wax, sugar, 

 and vaseline easily change to liquids ; while such liquids 

 as alcohol, ether, naphtha, and chloroform readily change 

 to the gaseous state. Water commonly occurs in all three 

 states ; as ice (a solid), water (a liquid), and steam (a gas). 



10. Composition of Matter ; Molecules. All matter 

 is made up of tiny particles called molecules. A mole- 

 cule is the smallest particle of any substance which can 

 exist alone without changing its nature. From this 

 definition it is clear that a molecule is too small to 

 be seen; for the smallest bit of matter which could 

 be seen would be capable of division into other bits 

 much smaller. Still we know that there must finally be 

 particles so small that they can no longer be divided ; 

 and although no one has ever seen them, we can give 

 them a name molecules. 



