OXYGEN 



25 



into the tube. As the gas burns, drops of water are formed, showing 

 that it is passing back to its original condition, that is, it is uniting 

 with oxygen to form water. This gas is hydrogen. Elements always 

 unite in definite proportions to 

 form compounds, as in water the 

 proportion by volume is always 

 two parts of hydrogen to one 

 part of oxygen. 



Oxygen, when carefully pre- 

 pared, is found to be colorless, 

 odorless, and tasteless. Com- 

 bined with other substances, it 

 forms a very large part of the 

 composition of water, rocks, 

 minerals, and the bodies of 

 plants and animals. 



Oxygen has the very important property of uniting with many 

 other substances. The chemical union of oxygen with another 

 substance is called oxidation. When a candle burns, the oxygen 

 in the air unites with the carbon in the candle and forms a gas, 

 called carbon dioxide, which puts out a flame. This gas may be 

 tested for as follows : 



Demonstration 3. Burn a candle in a closed jar. After the candle 

 goes out, remove it carefully (the gas in the jar is heavier than air). 

 Add a spoonful of limewater — screw down the top of the jar and shake 



carbon dioxiote, 

 Cafborx ccnd 

 ^^cter- vetpor- 

 in the srnoks 



lisht^ and. 



I b&b energy 



ccr-bon 



Why is the burning of a match an example of 

 oxidation ? 



so as to mix the gas in the jar 

 with the limewater. What hap- 

 pens to the limewater ? This test 

 with limewater shows that carbon 

 has been oxidized, forming carbon 

 dioxide. 



Practical Exercise 1. Burn a 

 number of different substances in 

 closed jars and test in each case for 

 carbon dioxide. How many of the 

 substances produce carbon dioxide 

 when burned ? 



Oxidation. Oxidation may 

 take place slowly, as in the 

 rusting of an iron nail, which 

 is caused by oxygen uniting 



