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2 Continued on next page... Chemistry of the Flame When acetylene is burned in air, the end products are carbon dioxide (carbon plus oxygen) and water vapor (hydrogen plus oxygen). A chemical equation covering complete combustion reads like this: 2 C2H2 + 5 O2 = 4 CO2 + 2 H2O In everyday terms, this means that to burn two cubic feet of acetylene you must use five cubic feet of oxygen, or 2- 1/2 times as much oxygen as acetylene. Yet we have just said that the neutral flame is produced by burning a one- to-one mixture of oxygen and acetylene, and that the neutral flame does not contain an excess of either gas. This might seem to be a bit of a contradiction, but it is not, since the equation given above represents an over- simplification of the combustion process. Actually, combustion in the oxy-acetylene flame takes place in two distinct stages; for the first, the oxygen is supplied by the mixture leaving the torch; for the second, the oxygen is supplied by the air around the flame. In the first stage of combustion, the acetylene breaks down into carbon and hydrogen, and the carbon reacts with the oxygen to form carbon monoxide. In chemical terms: C2H2 + O2 = 2 CO + H2 In words, this means that you need one molecule of oxygen for each molecule of acetylene. A cubic foot of acetylene contains the same number of molecules as a cubic foot of oxygen. In the second stage of combustion, the carbon monoxide (CO) reacts with oxygen from the air to form carbon dioxide (CO2). The hydrogen reacts with oxygen from the air to form water (H2O). The chemical equations are these: 2 CO+O2= 2 CO2 2 H2 + O2 = 2 H2O (or, H2 + O = H2O)