Why does a candle need a wick? Why doesn’t it burn?

CandleWe’ve been using candles since at least 3000 B.C. Until fairly recent history, they have been one of the principle ways to light a room after the sun had set. In today’s world, however, candles have been demoted to lesser purposes: to help us relax, for use in the event of a power cut or on a birthday cake (my favourite use!).

Look closely at that burning candle and it has a strange appearance – there is a flame, but the wick doesn’t burn away. It looks almost magical. Here’s why:

Most modern candles are made of paraffin wax – a chemical that comes from crude oil which contains only hydrogen and carbon atoms. It burns in a similar way to gasoline and other fuels that we get from oil do. When you light the wick on a candle, the heat of the flame melts some of the wax. The liquid wax is absorbed into the wick and can travel up towards the flame in a process known as capillary action. As the wax moves up the wick, it gets hotter and hotter until the wax turns into a gas. This paraffin wax gas can then mix with the oxygen in the air and burn in the flame at the top of the wick. The heat of the flame then causes more wax to melt, which is also drawn up the wick and the process repeats itself until the candle wax is all burnt away (and so time to buy a new candle!).

So why does a candle need a wick at all? If you try to light just a lump of candle wax, all that happens is the wax melts a little bit. Liquid wax will not burn; it has to be a gas before it can burn in the flame. Believe it or not, but this is true of even gasoline – it will not burn as a liquid and only the vapours do. A great example of this is shown in this video, showing how easily (or hard) it is to burn different related chemicals. Some of the chemicals in the video can burn very easily as they turn into a gas at much lower temperatures while some have a very hard time burning – even jet fuel(!) – and must be sprayed into the flame in little droplets to have any chance of burning.

If you blow out the candle, you will notice a trail of grey-white smoke leaving the wick (as well as that amazing smell!). This trail is actually the wax vapour that has condensed into a visible form in the air. The vapour trail can continue to form as long as the wick is hot enough to turn the wax from a liquid into a gas. If you touch a lit match to the vapour trail, the flame will actually travel down the trail and the wick, as shown in the Youtube video below.


 
So now, when it’s someone’s birthday, you can show them all this cool party trick and relight the candle from the vapour. So if they don’t make a wish first time round, they can get a second go!

 

Answer by Nathan Beal

 

Photo Credit: José Pestana via Compfight cc


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Article by Nathan Beal

December 3, 2014

Nathan is currently in the throes of a PhD in Computational Chemistry at the University of Manchester. When he’s not behind a computer, he’s outside enjoying the sights of the city. A fan of The Walking Dead, he has a strange apathy to all things football-related.


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