The science of how candles burn

Science how candles burn

If you blow out a candle and then put a lighted match close to the wick (but not touching it), the wick will re-light. Most of us intuitively know this, but have you ever wondered why?

This week we did two simple experiments investigating the science of how candles burn. Both come from a free online Kitchen Chemistry course we’re enjoying.

Experiment 1 – Lighting gaseous wax

What you need

  • Candle
  • Match or lighter

What you do

Light the candle. Notice how you have to hold the match very close to the wick for a second or two before it ignites. Allow the candle to burn for a few minutes. As it burns, observe what happens to the wax.

Now blow out the candle and quickly hold a lighted match near the wick. This time the wick should easily ignite, even without the flame actually touching it.

Repeat the process a few times, experimenting with how far away from the wick you hold the match.

Science how candles burn

Lighting a candle by holding a flame near the wick

What’s happening?

When you light the candle, the solid wax melts and liquid wax is drawn up the wick. As the candle gets hotter, the liquid wax evaporates into a gas. This gaseous wax burns in the oxygen of the air.

The gaseous wax remains in the air after you blow out the candle. If you hold a lighted match near the hot wick, the wax ignites and the flame spreads to the wick. If you allow the candle to burn for long enough that it produces a visible white vapour when you blow it out, you can light the vapour from above.

Experiment 2 – Soot on a spoon

What you need

  • Lighted candle
  • Metal spoon

What you do

Briefly hold the spoon in the candle flame. Remove it and observe what you see on the spoon.

You should see black soot. You may also see a tiny bit of wax. (Don’t worry, it all washes off.)

science how candles burn

Holding a metal spoon in a candle flame

What’s happening?

As the candle burns, solid wax becomes liquid and then evaporates to become a gas. The gaseous wax burns in oxygen to produce water, carbon dioxide, heat and light.

The burning candle also produces carbon, in the form of the black soot we see on the spoon.  It is glowing soot that causes the candle give out light.

If there were enough oxygen to burn all the wax, only carbon dioxide and water would be produced and the flame would be blue, like in a gas burner.

The small amount of wax on the spoon is the unburnt gaseous wax which has condensed on the cold spoon and turned back into solid wax.

Golden gas and solid methane

These experiments are an interesting way to explore states of matter. My kids were surprised to discover that every element can exist as a solid, liquid and a gas.

C(10) wondered if gold can be a gas. The answer is yes – gold boils and evaporates at 2,800°C.  And solid gold becomes a liquid at just over 1000°C.

Meanwhile J(9) wanted to know if gaseous bodily emissions can take solid form (and if so could we google photos). He phrased it differently, as you can imagine. This led us to the fascinating topic of solid methane, a source of fuel which exists in very cold conditions at the bottom of the ocean and at the poles.

science how candles burn

Frozen methane bubbles: U.S. Geological Survey

Trick re-lighting candles

We also wondered how trick birthday candles work –  the ones that re-light by themselves after you blow them out.

The “magic” ingredient is usually magnesium in the candle’s wick. When the candle is burning, the magnesium is shielded by the liquid wax being drawn up the wick. But after the candle has been extinguished, the wick is no longer hot enough to draw up the liquid wax. The magnesium is exposed to the wick, which is hot enough to ignite the magnesium The burning magnesium in turn ignites the gaseous wax.  This article explains the process very clearly.

Apparently you can see tiny flecks of magnesium going off around the glowing ember of re-lighting candles.  I’ve ordered some so we can observe this for ourselves.

Further Resources

Scientist Michael Faraday gave a series of children’s lectures about the chemistry of candles at the Royal Science how candles burnInstitution, London in 1860. You can read an abridged version here.

How does a candle work?  (How Stuff Works)

The Mystery of the Periodic Table is a wonderful living book about the history of chemistry. This week we read about the discovery of oxygen, which was the perfect background to our experiments.

Kitchen Chemistry  Free online course from the University of East Anglia. (Runs until 26 May 2014.)

Main photo credit: Windliest

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