Why Candles Burn

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Candle Light

Candles have been around for hundreds of years – longer, even.  They were one of the very first sources of man-made illumination, guiding people through the hours of darkness when the moon and the stars were not sufficient for their needs.  Matter-of-fact, we have grown so very familiar with candles, that we do not often stop to think about how amazing they really are.  We do not often stop to think about how a candle, and a wick, and some wax, and the substance we call smoke, and even candle light itself, are more complicated – and far more fascinating – than we realize.

A typical, short, tea-candle is essentially made up of Paraffin wax with a piece of cotton rope stuck in it as the wick.  Paraffin wax comes from petroleum – a mix of hydrocarbon chains – and is chemically written as; C25H52.  However, if one tries to light this wax – in liquid OR solid form – all on its own, it melts and turns to a fairly clear liquid, but otherwise does nothing of interest.  So, clearly, it is not the wax than makes a candle burn.  Does that make it the wick that is responsible for that tiny, dancing flame?  Well, if one were to light a piece of cotton rope all on its own, it would burst into flames quickly, before smoldering and dying.

So why is it that a candle can burn, but can also do so for hours?   It’s because we’re not using the wax and the cotton separately, but combining them.  Alone, they would not be able to complete this task.  Together, they are.  When the wick is first lit, the wax in the candle begins melting and forms a pool around the base of the wick.  Because the wax is now a liquid, and the cotton is very porous, the liquid wax moves upward through the wick because of a phenomenon called ‘capillary action’.  Since liquid wax is not flammable, the wick is kept from bursting into all-devouring flame.  However, wax, not flammable as a liquid or a solid, does become flammable as a gas.  When the liquid wax reaches the top of the wick and comes in contact with the flame, its molecules start moving so fast that it becomes a gas, which – as above mentioned – is flammable, and keeps the candle burning.  Amazing, right?

That’s why, if you blow out a candle and immediately hold a lit match above the wick, but not touching it, the candle will re-ignite itself.  There’s still enough gaseous wax floating above the wick to be able to light the candle.

A toothpick stuck into some solid wax and lit will stay lit for only a couple of seconds, before burning itself out.  Why should it do this when a cotton wick will burn for two plus hours in some cases?  It all goes back to the idea of Capillary Action.  With the cotton, the wax was able to move upward, keeping the wick from combusting while within it, and turning into flammable gas upon reaching the flame.  Because the wood is far less porous, the gas is unable to rise upwards towards the flame.

Now, C25H52 spits out a couple of bi-products when you combine it with oxygen.  Namely, Carbon Dioxide (CO2) and Water (H2O).  That sounds all cool and everything, but I can promise you that no matter how many times you sit on the ground and throw a wax block into the air, it’s not going to all of a sudden disappear into CO2 and H2O.  Why is this?  Well, there’s a theory out there – aptly named “the Collision Theory” – that says that particles must be moving REALLY FAST in order to break bonds so that new ones can form.  And how do we speed particles up?  We heat them up.  That’s where our flame comes in and does its work.

Once heated up with a handy-dandy little flame, one wax molecule (C25H52) turns into 26 H2O molecules, and 25 CO2 molecules (a simple matter of balancing equations).  Most people consider it common knowledge that fire will produce some CO2.  It’s not exactly visible to the naked eye, so we’re not surprised that we cannot see it.  On the other hand, many people are surprised upon finding out that burning wax also produces water molecules.  If one has any doubts about the fact, though, and the balancing of an equation does not appease them, they need simply to put a small beaker (open side down) over a lit candle.  The flame will quickly sputter and die (as there is no oxygen to feed off of), but there will clearly be some condensed water on the insides of the beaker.  That should be proof enough.

But Carbon Dioxide and Water aren’t the only things that lit wax can produce.  We’ve all seen “soot” around, haven’t we?  Well, when a flame is deprived of oxygen, it obviously can’t form CO2, and instead, forms another carbon-based formula, making soot.  If one feels the need to experiment this, it is easily done by holding the closed, bottom end of a beaker up to the candle flame, allowing almost no room for oxygen.  The bottom of the beaker will quickly turn brown and sooty.

A candle’s flame can also be handy for causing other chemical reactions when you bring new materials in.  For example, if you take a small, unbent piece of copper wire and stick it into the flame, the copper particles and oxygen particles are moving so fast that they are able to “collide” and make Cupric Oxide (CuO), which is the why the stretch of copper wire exposed to the flame will turn black.

Not that copper wire will always do this if exposed to flame.  For example, if one first twirls the wire around a stirring rod, or some other cylindrical shape, and then removes the coil of wire and holds it into the flame, the copper wire will go from red hot, to orange hot, to yellow hot, and then melt.  

The various different visible colors (red, orange, and yellow) that appear on the copper before the bottom melts off are the result of something called Incandescence.  Incandescence is when something is heated up, and begins giving off its own light.  If something is heated up enough, it will become “white hot”, which means that it’s giving off every color of the spectrum (red, orange, yellow, green, blue, indigo, and violet), and thus producing a white color – not unlike a light bulb.

To return to the copper wire – the melting temperature of copper is 1084 degrees Celsius.  Which means, that (since the wire was coiled and a greater surface area exposed to absorb the heat, rather than radiating it as it would if it were flat), our tiny, little candle flame was able to make something that hot! Isn’t that crazy?!

Or, perhaps it isn’t crazy at all.  Perhaps the fact is that we simply have under-valued and under-appreciated all of the amazing things that can be done with a tiny, standard-sized tea-candle.  Perhaps, we should all take a couple of seconds to think about it, the next time that we see some small, dancing, flickering, seemingly-feeble, candle light.

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