QUESTIONS AND ANSWERS 21 



Discuss the use of carbon as a reducing agent. 



Carbon is a very active reducing agent, because of its great 

 affinity for oxygen. By heating carbon in the presence of any 

 oxide, the latter is rapidly reduced, its oxygen forming CO or 

 CO2 with carbon. 



Mention and describe three allotropic forms of carbon. 



1. Piamond is the purest form of carbon, and is the hardest sub- 

 stance known. It occurs in crystals, octahedral iu shape. It is a 

 very brilliant gem, owing to its great refractive power. 



2. Graphite, also known as plumbago, or black lead, is a black, 

 greasy substance with a specific gravity of 2.15. It is a good con- 

 ductor of heat and electricity. Used as a lubricant for machinery, 

 and in the manufacture of lead pencils, stove polish, crucibles, etc. 



3. Amorphous carbon is always a black solid, but the different 

 kinds vary in hardness and specific gravity. It is seen as the prin- 

 cipal part of the various kinds of coal ; in the form of lamp-black 

 it is used in printer's ink ; and occurs in bone-black, which serves as 

 a decolorizing agent in the making of sugar, syrups and other liquids. 



Describe how each of the following may be prepared : (a) lamp-black, 

 (b) bone-black. Mention the important uses of each. 



(a) Lamp-black is made by burning tar, rosin, turpentine or 

 petroleum, with a deficient supply of air, and passing the smoke 

 into large chambers where the carbon is deposited. It is used in 

 making printer's ink. 



(b) Bone-black is made by carbonization of bones of animals 

 and is used as a decolorizing agent. 



Give the important physical and chemical properties of carbon dioxide. 



Carbon dioxide is a colorless, odorless gas which by cold and 

 pressure may be easily condensed to a liquid. Its specific gravity is 

 1.529, and its symbol, COj, being composed of one volume of carbon 

 and two of oxygen. It is not combustible and is not a good sup- 

 porter of combustion, in fact it has a decided tendency to extinguish 

 flames. It unites with water to form carbonic acid, H2CO3. 



Describe the method of preparing carbon dioxide from marble and 

 write the equation. 

 By heating marble (CaCOg) or by adding hydrochloric acid, 

 carbon dioxide is liberated. 



CaCOa -f heat = CaO + CO^. 



CaCoa + 2HC1 = CaCla -f H,0 + CO^, 



