Find out What Happens to Candle Wax When a Candle Burns

When you burn a candle, the wax is oxidized.
an aruni photography/Getty Images

Have you ever noticed how you have less candle after burning than before? This is because the wax oxidizes (burns) in the flame to yield water and carbon dioxide, which dissipate in the air around the candle, in a reaction which also yields light and heat.

Combustion of Candle Wax

Candle wax (paraffin) is composed of chains of connected carbon atoms surrounded by hydrogen atoms. These hydrocarbon molecules can burn completely. When you light a candle, wax near the wick melts into a liquid. The heat of the flame vaporizes the wax molecules and then they react with the oxygen in the air. As wax is consumed, capillary action draws more liquid wax along the wick. As long as the wax doesn't melt away from the flame, the flame will consume it completely and leave no ash or wax residue.

Both light and heat are radiated in all directions from a candle flame. About one-quarter of the energy from combustion is emitted as heat. The heat maintains the reaction, vaporizing wax so it can burn, and melting it to maintain the supply of fuel. The reaction ends when there is either no more fuel (wax) or when there isn't enough heat to melt the wax.

Equation for Wax Combustion

The exact equation for wax combustion depends on the specific type of wax that is used, but all equations follow the same general form. Heat initiates the reaction between a hydrocarbon and oxygen to produce carbon dioxide, water, and energy (heat and light). For a paraffin candle, the balanced chemical equation is:

C25H52 + 38 O2 → 25 CO2 + 26 H2O

It's interesting to note that even though water is released, the air often feels dry when a candle or fire is burning. This is because the increase in temperature allows air to hold more water vapor.

When a Candle Burns, Do I Breathe Wax?

When a candle is burning steadily with a teardrop-shaped flame, combustion is extremely efficient. All that is released into the air is carbon dioxide and water. When you first light a candle or if the candle is burning under unstable conditions, you may see the flame flicker. A flickering flame may cause the heat required for combustion to fluctuate. If you see a wisp of smoke, that's soot (carbon) from incomplete combustion. Vaporized wax does exist right around the flame, but doesn't travel very far or last very long once the candle is extinguished.

One interesting project to try is to extinguish a candle and re-light it from a distance with another flame. If you hold a lit candle, match, or lighter close to a freshly extinguished candle, you can watch the flame travel along the wax vapor trail to re-light the candle.