Published: 28 January, 1999

Why is it that when you burn a contained candle the wax disappears? Does it evaporate? What is the chemical reaction?

First let's consider what it is that burns. When you light a candle, a cup of liquid candle wax soon forms near the wick. The heat of the flame first melts the wax in this cup, the wax is then drawn up the wick and finally the heat of the flame turns this liquid wax into a gas. It is the gas that burns. You can show this with a candle and a grill lighter. Light the candle and then gently blow it out without disturbing the air too much. You'll see a wisp of smoke and gas coming from the wick. Light the grill lighter and bring it close to the wisp of gas, but don't touch the wick with the flame. You'll find that the gas lights first and that the flame then moves down to the wick to relight the candle.

But it's not just the candle wax that disappears; something in the air disappears too. Try this. Put a candle to the bottom of a bowl, fill the bowl with water until it's about half-way up the candle, light the candle and then put a jar over the candle and into the water. The candle will go out and the water within the jar will rise to a level above that in the rest of the bowl. Antoine Lavoisier, the father of modern chemistry, used such an experiment to show that air was roughly 80% nitrogen (that didn't burn), and 20% oxygen (that combined chemically with the wax vapor). So, the candle reaction consumes oxygen (O₂) and wax. The wax is basically a long hydrocarbon (a molecule formed of carbon and hydrogen, generally with the formula C_nH_2n+2, with n, the number of carbon atoms, typically ranging from 22 to 27).

If you look carefully in the jar used to put the candle out, you'll find small droplets of water on the inside surface. So, water must be one of the products. The other product is carbon dioxide. A chemical equation for wax + oxygen --> water + carbon dioxide is then 2C_nH_2n+2 + (3n+1) O₂ --> (2n) CO₂ + (2n+2) H₂O.

Candles provide a wonderful opportunity to learn about science. The English scientist Michael Faraday, in his famous lectures at the Royal Institution [1], claimed, "There is no better, there is no more open door by which you can enter into the study of natural philosophy than by considering the physical phenomena of a candle." The experiments suggested here and several others can be found on the internet [2-4].

[1] Faraday Candle History

[2] Dynamic Experimentation

[3] Microgravity Combustion Science

[4] Chemical History of the Candle