Science Questions

Why does the wick of a burning candle shorten after you've blown it out?

Sun, 18th Dec 2011

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Question

Martin Schaefer, Facebook asked:

Why does the wick of a burning candle only smoulder at its end but shortens after you've blown it out?Ē

Answer

Dave -   The way a candle works is as follows:  You've got a bath of molten wax.  This is pulled up the wick by surface tension and then that evaporates.  When a candle is burning normally, itís this vaporised wax which is combining with oxygen and burning away.  The wick will stay there pretty much on its own and sit there.  If you blow the candle out, you'll get a load of smoke and that's actually the wax vapour recondensing into little droplets of solid wax in the air.  But the end of the wick can still carry on burning.  Itís still quite warm, and that will continue burning with oxygen.  Itís not actually the wick itself burning but the string inside the wick, and that will smoulder away for a bit like any bit of wood might.  It smoulders for a while and eventually goes out, but itís not hot enough to re-ignite the wax.

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