Grade K-3: A fire is not all that different from a human. Humans need to eat food, or fuel, and to breathe air, or oxygen. Fires also need to burn fuel, and require oxygen to keep burning. That’s why you can put out a fire by dousing it in water. Just like you can’t breathe underwater, neither can a fire. In this demonstration, instead of water, the fire is covered in another thing it cannot breathe, called carbon dioxide, which is created by the reaction of baking soda and vinegar.
Grade 4-6: Fire requires oxygen and a fuel source to burn. If one of those two things is missing, the fire will go out. In this demonstration, carbon dioxide is used to starve the fire of oxygen. Carbon dioxide is heavier than air, and will sink in air, just like a rock will sink in water. The bubbles that form when baking soda and vinegar are mixed are made of pure carbon dioxide. The generated CO2 will remain in the bottom of the vessel, and slowly rise to the top, just like how water would fill the same vessel. When the level of CO2 reaches the level of the candle, the flame will no longer be able to get oxygen at that level, and it will go out, just like if the tank were to be filled with water.
Middle School: Combustion requires fuel and oxygen. If one of these components is removed, then the combustion will stop. It is obvious that if there’s no more fuel, the fire will go out, as with a candle burning out, a fire burning out, or even a car running out of gas. However, it is more difficult to comprehend a lack of oxygen putting out a fire. This is however, what happens when water is dumped on a fire. The water is void of an oxygen source for the fire to burn, just as its void of oxygen for a human to breathe. The gases evolved from the reaction of sodium bicarbonate and acetic acid are similarly void of burnable oxygen, and the fire is extinguished. As CO2 is lighter than air, it will remain in the bottom of the reaction vessel, and slowly rise up to meet the level of the flame.
College: Sodium bicarbonate, when mixed with an acid will evolve CO2 gas. As CO2 is heavier than the atmosphere, it will fill the vessel from the bottom up. As the CO2 fills the container, it will raise up to levels close to the lowest flame. The flame creates a convention current as heat from the flame rises, and new air is pulled up from below. For this reason, the flame will actually start to go out before the level of CO2 has actually fully reached the flame. It must also be remembered that the wick is not burning, but instead is a medium for evaporating paraffin from the candle into a flammable gas. As the CO2 reaches the wick height, the paraffin will continue to be evaporated, and the flame may move above the wick, to a point where there is still sufficient oxygen to allow for combustion, but the paraffin has not spread out so much that it is in a high enough concentration to still continue burning. Once either the wick has cooled below the paraffin evaporation point, or the flame level is too high to maintain an optimum paraffin:oxygen burn ratio, the fire will go out.