Fire and Heat 139 



The hydrocarbons, such as kerosene, gasoline, and paraffin are 

 composed of hydrogen and carbon in different proportions. They 

 are important fuels. 



The carbohydrates, such as starch and sugar, are composed of 

 hydrogen, carbon, and oxygen. They are important foods. 



Carbon, hydrogen, and oxygen are elements. 



Starch, paraffin, and water are compounds. 



Changes in which new substances are produced are chemical 

 changes; other changes are physical. 



Immense amounts of carbon dioxid are produced by fires and liv- 

 ing animals, and decaying organic matter. 



Carbon dioxid for laboratory experiments is generally produced 

 from marble or limestone by the action of hydrochloric acid. 



Carbon dioxid neither burns nor supports combustion. 



Carbon dioxid is one of the gases which make up the mixture 

 called the air, and is dissolved in ocean- and underground waters. 

 It is combined with calcium in immense deposits of chalk, limestone, 

 and marble. 



The carbon dioxid of the atmosphere is one stage of the great cycle 

 of carbon in nature. 



Coal is an accumulation, mainly carbon, formed from plants which 

 obtained their carbon from the carbon dioxid of the atmosphere. 



REVIEW QUESTIONS 



1. What are the most common products of combustion? 2. How 

 can you demonstrate that water is a product of combustion ? 3. How 

 can you show that carbon dioxid is a product of combustion ?, 4. Men- 

 tion a substance which, when burned in air, produces only water ; a 

 substance which produces only carbon dioxid. 



5. What substance is always a constituent of all combustion prod- 

 ucts? What general name do chemists therefore give to all such 

 products? 6. What does the experiment of burning magnesium in 

 steam show as to the composition of water? 7. Describe a method 

 of separating water into its constituent elements. 8. In what ratio 

 of volume do the elements combine to form water? 9. Of the two 

 constituents of water which one burns? Which supports burning? 



10. What did Lavoisier contribute, about the time of the American 

 Revolution, to our knowledge of the air? 11. What evidence do you 

 find that hydrogen is lighter than air? 12. Mention two acids in 

 which zinc will replace the hydrogen. 13. Mention two metals which 

 will replace the hydrogen of water. 14. Describe the process of mak- 



