194 COMMON SUBSTANCES 



is because some of their elements are uniting with the 

 O of the air. It is for this reason that a draught of 

 air is necessary in stoves, lamps, and various fires. 



223. Hydrogen. Hydrogen also is a colorless and 

 odorless gas. It is an important element, forming a 

 part of all acids and of water. Animal and vegetable 

 substances contain large quantities of H, but it is not 

 common in its free state. The element may be sepa- 

 rated from acids by the action of metals upon them 

 ( 213). Hydrogen is the lightest substance known, 

 being 14 J- times lighter than air. Balloons are usually 

 filled with it, so as to make them rise in the air. 



Experiment 121. Arrange the apparatus the same as for mak- 

 ing O (Experiment 119), filling jars w.th water. Into the test tube 

 put 5 grams of zinc with 5 cubic centimeters (cc.) each of water 

 and HC1, but do not heat. After the gas has flowed a few seconds, 

 collect some in jars by the same method as in Experiment 119. 

 This gas is H. 2 HC1 + Zn = ZnCl 2 + 2 H. 



Experiment 122. Uncover one jar of H, at once holding a 

 lighted match near the opening. Always be careful with H, for it 

 burns in air and explodes if mixed with it. Using a small jar, 

 partly uncover it for an instant; then cover it again and shake it 

 once. Now apply a match to the opened jar, being careful not to 

 get the face too near. If the air and H are mixed, a slight explo- 

 sion may occur. Does H burn in the air? Does it kindle easily? 

 With what element does it combine ? 



Hydrogen combines with very easily if it be heated 

 to its kindling temperature; once lighted, it burns 

 readily. Pure H burning in pure O makes a very hot 

 flame. It is the H in many substances, such as kero- 

 sene, paraffin (candles), wood, paper, etc., that makes 

 them kindle easily. 



