When you light a candle, wax melts and travels up the wick via capillary action. As it gets close enough to the heat source, the wax vaporizes and ignites, providing more heat, which melts more wax, which is wicked up into the flame in turn. The cycle continues until fuel runs out, oxygen is depleted, or the heat source is removed (i.e. the candle is blown out).
In general, solid and liquid fuels burn when heat exposure causes them to release flammable vapors. Byproducts of gaseous combustion float away as smoke.
Extinguish a candle, and some wax vapor is left over in the smoke trail. In the moment before it dissipates, this column of flammable gas can act as a fuse and carry a flame back down to the candle wick.
This experiment depends on Earth’s gravity to work. Here on our home planet, the behavior of fire is predictably familiar. Warm air around the flame rises. Cool air rushes in to fill the void at its base, carrying a fresh supply of oxygen. The combustion reaction continues, heating more air and propagating the convection cycle.
Image credit: NASA/FLEX-2
Without Earth’s gravity to pull cool, dense air downward, hot air doesn’t float. For this reason, flames in space burn spherically, and a smoke trail would never rise from an extinguished fire!
This isn’t your typical cloud, and not just because it’s trapped inside a bottle! We chose rubbing alcohol as the raw material for our homemade tabletop cloud because it vaporizes so easily:
Rubbing alcohol is also highly flammable. Let’s explore this property by creating some combustion reactions! Every fire (or combustion reaction) requires fuel, an oxidizing agent (like oxygen), and activation energy. Activation energy is the trigger that causes the combustion reaction to start.
In this experiment, we touch a flame to the top of a cloud of fuel (rubbing alcohol vapor). The vapor is suspended in air, so there’s plenty of oxygen present. Heat from the lighter causes the closest alcohol molecules to react with nearby oxygen molecules. As they react, their atoms recombine to form carbon dioxide and water vapor. These products tie up less energy than the original materials; the leftover energy is released in the form of visible light and heat. Newly generated heat provides activation energy for the next layer of combustion reactions, and the process continues until all of the fuel has been consumed.
Isopropyl alcohol naturally burns blue, but it’s easy to change the hue of a flame. Just add salt! What our eyes interpret as colors are really waves of light stretched out to varying degrees. We say that the most stretched-out waves have the longest wavelength. We call the longest wavelengths we can see “red” and the shortest “violet”. A substance’s chemistry determines the wavelength– and thus the color– of the light that will be released by its combustion. Copper-based salts burn green. Strontium turns flames bright red. Sodium salts, such as table salt, burn yellow. If you’ve ever seen a multicolored fireworks show, you’ve experienced this science firsthand!
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