How Do Candles Burn?

The scientist Michael Faraday wrote an entire lecture series in the 1860s on his observations of burning candles. Over one hundred years and many more candle experiments later, NASA sent candles into space to test the behavior of their flames under microgravity in the 1990s. In honor of the end of Hanukkah, the Jewish Festival of Lights, this week, let’s look at what makes candles burn and three interesting candle experiments you can do at home.

How Do Candles Work?

Candles burn through an efficient combustion process by combining wax, a poor conductor of heat that melts just above room temperature, with a wick, an absorbent material that will absorb liquid wax and transfer it toward the candle’s flame while it’s burning.

When a candle’s wick is lit, the heat from the candle’s flame both melts wax near the wick so that it can be absorbed and brought towards the flame by the wick and vaporizes the wax before burning it. The vaporized wax molecules (typically hydrogen and carbon atoms) react with oxygen in the air to create heat, light, carbon dioxide, and a little water vapor. That heat is enough to continue the process by further melting the remaining candle.

Three Candle Experiments

1. Burn a Candle Under Water.

Fire needs oxygen to survive. Dousing a flame with water will put it out. So can you ever burn a candle under water? In reality, the answer is no, but you can make it look like a candle is burning underwater.

First, stand a candle up in a bowl of similar height. You can try melting a bit of wax as a base and letting it set around your candle to hold the candle in place or positioning the candle using rocks, for example. Next fill the bowl with cold water to just below the top of the candle. Once you light the candle, the cold water will keep the outer surface of the candle at a cooler temperature than the inner portion which will then burn faster. The flame will appear to creep below the surface of the water as long as the outer walls of the candle stay intact. Eventually, the fire will burn through the candle’s edges, letting water in to put out the flame.

2. Suck Water into a Glass Using Only a Candle.

Candles also provide a useful demonstration of the role temperature plays in atmospheric pressure. Specifically, you can draw water into a glass using only a lit candle flame.

First, stand a candle on a plate and pour a thin layer of water around it like a moat. You can add food coloring to your water to make this effect easier to observe. Place a clear glass upside down on top of the candle. As the flame burns, it warms the air inside the glass which increases its pressure relative to the air outside of the glass. (You can think of this process as the air particles moving faster and pushing harder against the sides of the glass.) The higher pressure air will attempt to escape to join the lower pressure air outside of the glass. Bubbles around the edge of the glass offer evidence of this transfer.

Since the candle, and more specifically combustion, requires oxygen, the flame will eventually go out due to lack of oxygen under the glass. Once the flame goes out, the air inside the glass will cool and decrease in pressure. Now it is the air outside the glass which is at a higher pressure. As the higher pressure air attempts to restore balance by forcing its way into the glass, it pushes the water inside along with it. The water level will rise inside the glass until the air pressure inside and outside the glass are a match.

3. Relight a Candle From Its Own Smoke.

You can relight a candle by lighting its smoke trail connected to the wick. (Check the process out in slow motion here.) This trick works because the smoke contains tiny bits of wax that have not yet burned.

Still looking for more fun candle experiments? Try blowing out a candle using the wind from vibrations created by music.

Until next time, this is Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com.

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