Chemistry

How a Candle Burns



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"How a Candle Burns"
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Take a candle, hold it sideways, and place its midsection over a flame. The candle melts, but does not catch fire in your hands, unless perhaps you keep it there until the wax has melted away to expose the wick. This suggests that it is the wick which burns, not the wax. But then observe a candle which has been properly lit. Although the flame is located near the wick, the wick is only very slowly consumed as the candle burns down as a whole. Certainly that little piece of string isn't enough to provide that long-lasting a flame. What mystery describes the mechanics of the burning candle?

Since you are already observing the flame, look closely at its structure. It seems to hover around the wick, some distance above the top of the wax portion of the candle. Unless the wick is long enough to extend out into the outer portions of the flame (in which case you really should trim it so it stops making smoke as it smolders) the wick is not in contact with any visible portion of the flame. Instead, it is surrounded by a fairly dark region of the flame. As you progress outwards, the flame becomes a bright yellow-white, and then fades down to orange at its outer reaches. Around and above the flame you may even notice a wavy effect in the air.

I hate to spoil the mystery, but continue we must. Beginning with a cool, unlit candle, strike a match and bring it close to the candle top, but not near the wick yet. You see the wax atop the candle begin to melt, but nothing else as yet. If you can, snip a piece of the wick and light it separately from the candle. It should burn briefly, only to extinguish forever. (Try to relight it if you are unsure.) Clearly, neither component is sufficient on its own. Now light the candle normally, but do so as slowly as you can. With good eyes, you might observe that the wax melts not only at the top of the candle, but also a small amount which is in the wick, and that this happens before the candle ignites.

What you really don't see is that not only does the wax melt, but some of it even evaporates - it turns to a gas. The gaseous wax is actually what burns, giving off enough heat to meltmore wax. The melted wax flows upward through the wick (just like water will climb up a narrow glass tube, a process called capillary action) closer to the flame, where the heat intensifies and turns it to gas, continuing the process. So long as the flame produces enough heat, the process continues.

Knowing what exactly is burning, we can also take a look at the flame. The central dark area of the flame is primarily the gaseous wax, not yet on fire. You might recall that to burn, fuel requires an oxygen source as well. The oxygen source is the surrounding air, which continually supplies the flame from the outside. Combustion occurs at the outer edge of the gaseous wax, where it mixes with oxygen. The bright portion of the flame corresponds to the combustion zone and the space just outside of it. The heat given off by the burning of the wax supplies the surrounding gas molecules with energy, bumping them to an excited state. When they return to normal, they release the energy as light. Bright yellow-white marks the region where there is the most heat, whereas the outer reaches of the flame, which are more orange, are more distant from the actual combustion, so they receive less heat, less energy, and only produce the lower energy, lower intensity orange. Looking past the flame, especially above it, you will note that images seem distorted - sort of wavy and swirly, in an ever-changing pattern. This, too, is due to the heat produced by the candle. It heats the air above and around the flame. Hot air is less dense than cool air, and rises. It also refracts light slightly differently, and the heating is uneven. The net result is that you have hot and cool air, circulating in the space around the flame, behaving as an ever changing, irregular lens. Light passing through this region does not follow a straight path, so the images you see are distorted.

That, in a nutshell, is how a candle burns. It actually relates very well to other flames as well - wood, alcohol, gasoline...they all require the fuel to be evaporated (or released as a gas) before combustion can occur. If you're wondering, heated wood actually releases the gas hydrogen. (The black that remains behind is just carbon - which did not combust.)

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