WATER 



97 



few feet away. When the hydrogen in the test tube does not ex- 

 plode but burns slowly up the tube, the gas is pure enough to be 

 collected. 



Properties of Hydrogen. Hydrogen is the lightest 

 known substance, weighing only one sixteenth as much 

 as oxygen. A liter of hydrogen weighs .09 gram. Like 

 oxygen and nitrogen, hydrogen is a colorless, odorless, 

 and tasteless gas. In testing the gas for its purity, it 

 was noticed that the less the quantity of air mixed 

 with it the less violent the explosion ; and we learned 

 also that hydrogen would burn. However, if we desire 

 to burn hydrogen 

 in a jet, we must 

 exercise some care 

 in lighting it in 

 order to avoid the 

 possibility of a 

 dangerous explo- 

 sion. Arrange 

 the apparatus as 

 shown in the dia- FlG - 93 ' ~ Burning H y dro s en - 



gram (Figure 93). In lighting the gas it is well to 

 use a test tube in the following manner. Hold the 

 tube over the jet for a minute and .then carry it mouth 

 downward to a flame several feet away. When the 

 gas in the test tube burns slowly enough that we may 

 carry it back and light the jet with it, the hydrogen is 

 pure enough to be safe. 



Hydrogen burns with a very hot, colorless flame, with 

 the formation of water. To test this, hold a dry beaker 

 or evaporating dish over the flame and note the result. 

 The same result may have been noticed when the hydro- 

 gen was exploded in the test tube. 



