102 ELEMENTS AND COMPOUNDS 



it with a hammer, changing of the starch of a cracker to sugar by means 

 of the saliva in the mouth. 



2. Compare the properties of hydrogen with those of oxygen. 

 Compare hydrogen with water. 



3. What properties of hydrogen and of oxygen can you show only 

 by changing these elements chemically? 



4. In what proportions by volume would you expect hydrogen and 

 oxygen to unite in forming water? 



5. Coal, wood, and kerosene contain carbon and hydrogen. What 

 products are formed when they burn? 



6. How would you prove that wood ashes are a mixture of soluble 

 and insoluble substances? 



7. If you were making the hydrogen for filling a balloon, which 

 would be the cheaper to use as the metal, zinc or iron ( 103)? For 

 what reason would illuminating gas be better than hydrogen? In 

 what way would hydrogen be better? 



8. How many calories of heat are given off when 1 g. of hydrogen 

 burns ( 105)? 



9. Carbon dioxide is 1.5 times as dense as air. If, in the porous cup 

 experiment ( 106), you were to surround the cup with a jar of carbon 

 dioxide, which would pass through the porous wall more rapidly, the 

 air or the carbon dioxide? Would air be forced out of the tube, or 

 would water be forced in? 



108. Salt. Salt is found in large amount in sea 

 water; it is also mined as rock salt. To purify rock salt 



we add water to it, so that the 

 salt dissolves, while the impuri- 

 ties do not. The water is then 

 boiled off, either at ordinary 

 FlG . 84 . pressure or in "vacuum pans" 



Hopper-shaped^Massof Salt (^ g 99). g a lt wells form an- 



other source of salt. 



Salt crystallizes in cubes ( 95); masses of the crystals 

 are "hopper shaped" (Fig. 84). Because the crystals do 



