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!
Written By: Caela Barry