CARBON AND SOME OF ITS COMPOUNDS 



169 



FIG. 116. Burning of mag- 

 nesium in carbon dioxide. 



distilled water. The filter paper containing the black powder should now 

 be placed in an oven and allowed to dry. 



The powder looks like carbon. Heat a deflagrating spoon till it is red 

 hot, place a portion of the powder on it and 

 quickly put it into a jar of oxygen. Identify 

 the gas formed. 



Preparation of quantities of carbon 

 dioxide. Though carbon dioxide is produced 

 naturally by burning and breathing, a.nd it 

 also escapes from the earth in some regions, 

 there are more convenient ways of obtaining 

 the gas. Experiments show that when an 

 acid is added to chalk, marble, or limestone, 

 a gas is given off which puts out flames, turns 

 clear lime-water milky, and possesses all the 

 properties of carbon dioxide it is, in fact, 

 carbon dioxide. The best way to prepare 

 bottles or jars of the gas is to place pieces of 

 chalk or marble about the size of peas into a 

 bottle fitted like that in Fig. 114. Dilute 

 hydrochloric acid is poured down the thistle 

 funnel, and when it comes into contact with 

 the marble, the gas is given off. Enough acid is poured in to cover the 

 lower end of the funnel, so the gas cannot escape iip the funnel ; 

 the carbon dioxide passes through the other tube in the cork. The 

 gas given off is heavier than air, and can therefore be collected as 

 shown in Fig. 114. As the gas accumulates in the jar, the air is 

 pushed out at the top. After several bottles or jars have been filled, 

 the properties of the gas can easily be examined. 



Properties of carbon dioxide. An examination of the gas shows 

 that it is colourless and has no odour. As it is heavier than air it can 

 be poured downwards just like a liquid (Fig. 116). 



Carbon dioxide is slightly soluble in water, and the solution which 

 is thus formed turns a blue litmus paper red, just as acids do. For this 

 reason the solution of carbon dioxide in water is often called carbonic 

 acid, and the carbon dioxide itself is sometimes spoken of as carbonic 

 acid gas. The gas has the property of extinguishing the flame of a 

 taper or match, and is consequently called a non-supporter of com- 

 bustion. 



Further evidence of the identity of the gas obtained from chalk 

 with that produced by burning carbon, is furnished by the burning 

 of magnesium in the gas. The magnesium burns with a crackling noise, 

 as if the gas was being torn asunder ; and it is. Magnesium oxide is 



