52 SMITH'S INTERMEDIATE CHEMISTRY 



(Fig. 25). The gas, being much lighter than air, is collected by 

 downward displacement of the air (Fig. 26b). Heavy gases are 



collected by upward displacement of air (Fig. 26a). 

 f3 Extremely pure zinc is almost inactive, but com- 



mercial zinc, on account of the contact action of 

 the slight impurities it contains, gives a steady, not 

 too violent, evolution of hydrogen. Sulphuric acid 

 and hydrochloric acid, in each case diluted with 

 water, are convenient acids. Iron shows about the 

 same activity, but, on account of the impurities 

 FIG. 25 usually present in iron filings or wire, the hydrogen 

 contains other gases which exhibit a distinct odor. 



FIG. 26a FIG. 26b 



Zinc (65.4) + Sulphuric acid (98) -* Hydrogen (2) + Zinc sulphate (161.4). 



** (gjjt a 



Iron (56) + Hydrochloric acid (72.9) -> Hydrogen (2) + Ferrou^chloride 



Hydrogen Iron 



Chlorine Chlorine 



It will be seen that the action is of the form we have called 

 displacement (p. 51). 



The Proportions by Weight. It may be well to remind 

 ourselves that the weights of the various materials (given in 

 brackets) are obtained by laboratory measurements. It is always 

 found that the total weight of the product is exactly equal to that 

 of the materials used (p. 20). Also, that a given weight of the 

 metal, say zinc, will always displace and liberate the same weight 



