Minor Constituents of Air 225 



Carbon + oxygen = carbon dioxide. 

 Hydrogen + oxygen = hydrogen oxide. 

 (" Hydrogen oxide " is the chemical name of water.) 



We cannot see that these bodies are formed because 

 the carbon dioxide is a colourless gas, while the water 

 formed at the high temperature of the flame exists as 

 steam and as such passes away from the neighbourhood 

 of the flame as a colourless vapour. If this vapour 

 meets a cold surface, such as the inside of the gas- jar 

 in Experiment 87, it is condensed, and forms small 

 drops of water. In this way the existence of the water 

 formed by the burning of the candle is rendered evident 

 to our sense of sight. 



In showing by means of the lime-water test that 

 carbon dioxide is also produced we depend on the fact 

 that the lime in the lime-water combines with carbon 

 dioxide. Thus : 

 Calcium oxide + carbon dioxide = calcium carbonate. 



(Lime) (Chalk) 



Calcium carbonate is the chemical name of chalk (the 

 chalk of our cliffs and chalk-pits, not the prepared 

 substance used for blackboard writing) and, as chalk 

 will not dissolve in water, it separates out as fast as it 

 is produced, forming the white substance which renders 

 the lime-water turbid and settles on the bottom of the 

 jar. When a solid substance separates out from a 

 clear liquid in this way, it is said to be "precipitated", 

 and our white sediment is called a " precipitate " of 

 chalk. 



It must be clearly understood that, as shown in 



Experiment 78, all the oxygen contained in the air in 



the jar does not combine with the constituents of the 



candle. A part only is thus used up, and a still smaller 



H. D. s. 15 



