100 ANSWERS TO PRACTICAL QUESTIONS 



20. How much O would be required to oxidize the metallic 

 Cu which could be reduced from its oxide by passing over if, 

 when white-hot, 20 gr. ofH gas ? 



(See Key, page 60, Question 34.) 



21. How much O would be required to oxidize the metallic 

 Fe which could be reduced in the same manner by 10 grs. 

 of 'H gas? 



(See Key, page 61, Question 35.) 



22. Why are rose-balloons so buoyant ? 



They are filled with hydrogen or coal-gas, which is 

 lighter than common air. 



23. How much H must be burned to produce a ton of 

 water ? 



Find how much H is contained in 2000 Ibs. of water. 



(i). H a : H a O : : x : 2000 Ibs. 

 2 : 18 : : x : 2000 Ibs. 

 18 x = 4000 Ibs. 



X = 222| IbS. (H). 



zd Method. One-ninth of any weight of water is hydrogen ; hence, 

 2000 Ibs. -4- 9 = 222 1 Ibs. (H). 



94. i. Why does not blowing cold air on a fire with a 

 bellows extinguish it ? 



More heat is liberated by the O which combines with 

 the fuel than is withdrawn from the fire by the current of 

 cold air. Yet the temperature of the fire must be suffi- 

 cient to elevate that of the O to the point of union with 

 C and H, else the fire will be extinguished. 



3. Why is fire-damp more dangerous than choke-damp ? 

 Fire-damp, or marsh-gas, is inflammable, while choke- 

 damp, or " carbonic acid," is not. 



