MadSci Network: Chemistry

Re: Why doesn't a frozen candle burn slower than a room temp candle?

Date: Tue Jan 23 09:20:09 2001
Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton College
Area of science: Chemistry
ID: 979706173.Ch

Why doesn't a frozen candle burn slower than a room temp candle?

My first question [was returned with a request to try the experiment. Olivia's mother puts candles in the freezer, saying that they last longer]. In experimenting, the room temp candles burn a tiny bit faster at first, but then both burn at same rate.

This doesn't work with birthday cake candles, so I tried with 8" tapered candles to see if there was a difference. I marked exactly 2" down from top of each, and made sure the wicks were the same length. The frozen paraffin must come to room temp quickly, because there isn't much time difference. The oil in the paraffin must freeze at a lower temp than water and then the molecules must come back together fast when they warm up and get close to the candle flame. Right?

So I think my mom is wasting the energy of the freezer by putting her candles in there, because there is hardly any difference in the time it takes to burn either candles.

Thank you.

I'm glad you tried the experiment, Olivia! Now you've learned something!

Fats and oils are chemically the same sort of thing ("triglycerides"). The difference is the melting point: fats are solid and oils are liquid at room temperature. You can fry with lard (a fat) or with canola oil (an oil) but when you're frying, the fat/oil is a liquid! And when you burn a fat or an oil, it's the liquefied fat or oil that is burned.

In fact, things burn best when vaporized. Substances that don't vaporize easily, like coal, are not as easy to burn as substances that can vaporize at the temperature of their own flames.

In order to help the vaporization process along, the liquid oil or melted fat is drawn up into a cloth wick (by capillary action) and vaporized by the heat of the flame; the burning takes place just above the wick. A thin layer of the liquid is present at the wick's surface. Because the liquid layer prevents oxygen from getting to the cloth, the wick doesn't burn up, but the heat of the flame does convert it to more-or-less pure carbon ("char").

In the same way, paraffins are hydrocarbons (like kerosene or gasoline) except that they are solids at room temperature. When you burn a paraffin or kerosene or gasoline, it's always the gas phase that burns--that's why natural gas is such an efficient fuel: it's already a gas! Part of the heat of the candle flame goes into vaporizing the paraffin so it can burn. In the engine in your car, a lot of ingenuity goes into reducing the liquid fuel to a fine mist of droplets that evaporate more easily in the cylinders; this increases the amount of heat that can be used to actually run your car.

What this means is that

  1. The wax in a candle has to be melted and evaporated before it can burn, and the difference between the temperature of your freezer and the temperature of your living room is much smaller than the additional rise in temperature required to melt and evaporate the candle wax.
  2. Candles just out of the freezer will have a harder time starting, partly because the wax at the top has to be heated that extra 50-60° F. (The main reason, though, is probably that when you take candles out of the freezer they acquire a film of frost from the moisture in the air, and that has to be evaporated before the candle can burn.)
  3. As you discovered, there is therefore no difference in the rate of burning of a frozen candle and one that's been stored at room temperature. About the only thing freezing will do for you is make the candle more brittle, so that it is more likely to break in storage but less likely to deform or bend.
Here's an article from Fantasy and Science Fiction that discusses candle flames.
Dan Berger
Bluffton College

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