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Name: Patricia P.
Status: educator
Age: 50s

Location: N/A
Country: N/A
Date: 6/19/2004


Question:
I feel so dumb asking this, but it is in regards to oxygen and flammability. I always thought of oxygen as a flammable gas, but the textbook says that "oxygen is not a flammable gas-it does not burn. But oxygen does support the burning of other substances. A glowing wooden splint placed in a test tube of oxygen will continue to burn until the oxygen is used up. This ability to support burning is another example of a chemical property. By using the chemical properties of flammability and supporting burning, you can distinguish between the two gases hydrogen and oxygen." Somehow, I always thought that the oxygen underwent a change in the burning process also, but I must have been wrong. Can you shed a little more light on the matter for me? Thanks. I want to be sure my understanding is totally complete so I do not pass on error to others in their thinking.


Replies:
Hi Patricia-

I think you're fishing for the keyword/phrase "reactive gas" or "energetic gas". Or some similar word we do not quite have.

Virtually all flames are the energetic reaction between a fuel and an oxidizer. You understand rightly that both fuel and oxidizer are consumed, joined to create something else.

But since we live in an oxygen background, oxidizer is usually naturally present. Fire then becomes a likelihood only when fuel is added to your situation. So we started saying "flammable gas" for a strong or hi-energy fuel gas.

Oxygen is a strong or hi-energy oxidizer, true, but it is an oxidizer, so it is not a fuel, so they do not call it flammable.

Personally, I like your intuition better. It is like a 1000v battery. It is all part of a dangerous thing. No point in saying the negative terminal is not "High Voltage" just because it is not the positive terminal.

Actually, oxygen is the positive terminal, and fuel is the negative terminal, in a fuel cell. A fuel cell runs the same reaction as fire, just better controlled.

The point is, "reaction" is a purely bipolar relationship. Fire requires chemical opposites to happen.

As our use of technology grows, it seems that man's activities may at any time present us with any hazardous situation imaginable within the laws of science, regardless of its unlikelihood in natural surroundings.

We now have situations where fire starts because powdered oxidizers or pure liquid oxygen is released into piles of fuel. Inverting the earlier natural situation of a fire.

So oxygen really is as distinctively important as fuel, in making fires.

Oxygen is almost as prominent as flammable gasses, on our safety signs at work. In 100% concentration and under pressure, it is a "Hazardous Oxidizer".

Jim Swenson


Combustion, as the term is commonly used, means the reaction of a fuel with oxygen. In the case of carbon this would be usually C + O2 -----> CO2. Oxygen does undergo a chemical change, from O2 to CO2 in the example given. The process of combustion refers to the reaction of a substance WITH oxygen.

Vince Calder


Patricia,

The question is one of definition. For combustion or burning to occur you need two things -- a fuel and an oxidizing agent. By definition, the fuel burns in the oxidizing agent. Both fuel and oxidizing agent are chemically changed but (by definition) only the fuel "burns". For example carbon (in the form of charcoal) is a fuel and will react with oxygen (oxidizing agent) to form a new compound, carbon dioxide.

Oxygen is the most common oxidizing agent (there is a reason for the similarity in the words oxygen and oxidizing...) and it is also one of the strongest. Because oxygen is such a strong oxidizing agent there is nothing that will oxidize oxygen, hence oxygen does not "burn". However, oxygen is (almost) always a required component of a flame.

While oxygen itself is not flammable it is a very dangerous chemical because it causes any fuel in can react with any fuel in its presence. When high pressure oxygen is used extreme care must be taken so that it does not come in contact with any fuels or it will cause an explosion.

Handle oxygen with extreme care -- especially pure and / or compressed oxygen!

Greg Bradburn P.S. Another way to say this is that burning and combustion are names for specific types of chemical reactions in which one of the reactants is oxygen. Since oxygen does not react with oxygen it cannot burn.



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