Carbon Dioxide and Fire
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The Big Ideas
Combustion (burning) is a chemical reaction in which a fuel reacts with an oxidant, usually oxygen, ordinarily producing both heat and light. In the absence of an oxidant, such as in pure carbon dioxide, combustion ceases.
Gaseous oxygen is released in the chemical reaction of bleach and hydrogen peroxide. (See safety issues below!) Gaseous carbon dioxide is released when an acid (such as acetic acid in vinegar) is mixed with sodium bicarbonate (baking soda). Carbon dioxide is more dense than air and thus can be "poured."
Details
When a piece of wood burns, the molecules of wood - mostly cellulose - are being broken apart and combined with oxygen in a chemical reaction that releases energy in the form of heat and light. The energy that is released began as light energy that fell on the plant as it grew; this energy was captured through the process of photosynthesis and is stored in the bonds between the atoms in the molecules of cellulose. To begin combustion, we often have to supply external heat, such as a match or the spark in a lighter, to get the reaction started. This heat begins to evaporate molecules of the fuel and facilitates their reaction with oxygen. The resulting heat keeps the process going so long as there is oxygen and fuel. The following is the basic equation for the rapid combustion of a hydrocarbon, such as wood, butane or gasoline:
Hydrocarbon Fuel + Oxygen => Heat + Light + Carbon Dioxide + Water
Since the reaction requires oxygen to proceed, it can be halted by placing a burning object in an environment without oxygen - or by replacing the oxygen in the environment with a gas that will not oxidize the fuel, such as carbon dioxide. This is how C02 fire extinguishers work. In the video, when the burning skewer is placed in the beaker with CO2, the flame is extinguished. Since it is taken out of the gas immediately, the process of combustion has not entirely stopped, and the tip of the skewer is still glowing when it is removed; in the presence of a high enough concentration of oxygen, the skewer bursts back into flame. In this case, the residual heat from the glowing skewer is enough to get the rapid combustion reaction going again if there is a sufficient concentration of oxygen around for it to react with. While the glowing skewer does not burst into flames in the air, where there is only 20% oxygen, the very high concentration of oxygen in the second beaker does the trick.
In this demonstration, we generate oxygen with the following reaction:
Bleach + Hydrogen Peroxide => Salt + Water + Oxygen
or, more precisely
NaOCl + H202 => NaCl + H20 + O2
We generate carbon dioxide with this reaction:
Baking Soda + Vinegar => Sodium Acetate + Water + Carbon Dioxide
or, more precisely
NaHCO3 + CH3COOH => CH3COONa + H2O + CO2
Carbon dioxide normally makes up much less than one percent of the atmosphere (only about .038 percent or about 380 parts per million on average in 2007). Carbon dioxide is more dense than the surrounding air (more mass in a given volume, or more "stuff" per "space"), so when we release carbon dioxide gas in this reaction, it does not rise up out of the beaker. Thus, the gas can be "poured" over the row of candles to put them out. As the invisible wave of CO2 rolls down the valley of cardboard, it is pushing all the oxygen out of the way, ceasing the combustion of wax in the candles and extinguishing them.
Materials and Set-up
Mix baking soda and vinegar in one beaker and bleach and hydrogen peroxide in the other. Practice to get the amounts right so that enough gas is made but the beakers don't bubble over. Light the skewer and put it in the gas that has been formed by the reaction of baking soda and vinegar; it will go out. Quickly move the skewer to the other beaker and hold it in the gas that has been made by the reaction of bleach and hydrogen peroxide; it will flame back up. This can be repeated several times.
To generate carbon dioxide and get it in a beaker that does not have any liquid on the bottom, use a set up such as the one in the video. Note that you can use a soda bottle instead of a glass flask. Put some baking soda in the bottom of the flask. Put the funnel and the tubing next to one another through the top of the bottle, and, using modeling clay, create a stopper to seal around the tube and the funnel. The tube should just barely go through the clay, not down into the region where the reaction is taking place. (Otherwise, you will get liquid bubbling into your beaker with the carbon dioxide, which is what you are making this apparatus to avoid.) Put the free end of the tube into your collection jar, and pour vinegar through the funnel into the bottle. The carbon dioxide produced in the reaction will be forced through the tube into the collection jar.
Birthday candles can be set in a row in a "valley" made out of poster board using modeling clay to affix them to the board. Have your assistant hold this valley at an angle and then light the candles. (Note, the demonstration works best if the candles are short, so burn them down to half length before beginning.) Pour the carbon dioxide you have generated down the valley and watch the candles go out one at a time as the dense CO2 rolls down and replaces the oxygen around each candle.
Safety Issues
Never mix bleach with ammonia! Depending on the amounts mixed, one or more of several different toxic and /or explosive compounds will be created.
Whenever you are working with fire, be sure to have a fire extinguisher nearby.