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Name: Eli
Status: student
Grade: 6-8
Country: Canada
Date: January 2009

Question:
I did a science experiment to see which burns faster, a frozen candle or a candle at room temperature. Everyone said that the frozen candle will burn slower, but when I did the experiment, the frozen candle burned faster! I did this experiment 3 times, all with the same results. Why does the frozen candle burn faster?



Replies:
Hi Eli,

First, nice job running your own experiment! Can you provide a little more information? How did you measure how much candle burned? Was it visual or weight or some other method? How and when did you measure how much burned, how/when did you end the experiment, and any observations you made along the way (such as condensation, differences in the wicks, etc.)? Did you burn it until it went out, or did you stop it part-way? How long (time) did it take?

As you can see, there are many variables that could impact this experiment, and some are related to temperature, but not all. Sometimes there are 'tricks' you can play to produce a counter-intuitive result, and sometimes there are important factors that were not controlled that cause the unexpected result. I am not aware of any tricks with candles, though, so I suspect there is some other factor not related to temperature that is causing this result. Some more info could perhaps shed light on what you observed.

Burr Zimmerman


I cannot be sure, Eli, but I have an idea that might fit.

Candle flames tend to be bigger if there is a longer length of wick exposed above the molten wax pool. Likewise, if there is only a short length exposed, the flame can get tiny and clearly will not consume wax very fast..


If your candle was relatively fat, i.e., wide compared to the flame height, then a warm candle would melt the wax for a larger radius around the wick, making the pool fill up with molten wax, shortening the exposed wick and slowing the burn rate.

If your candle was only medium-width, so that the flame consumes it top down instead of just digging a pit in the middle as some fat candles do, hmm, can this business still happen? Not sure it can. Maybe not a great idea.

Another idea:

Think, what trims the end of the wick as the candle burns down? The exposed wick curls and pokes out of the side of the flame. There it can be oxidized by air. All the part that sticks out of the flame is burned away. It happens at that little black charred tip, which maybe glows orange. The part of the wick that is inside the flame cannot be burned away. It stays pretty white, too. It is protected from heat by being wet with melted wax. Also there is not enough oxygen inside the flame, and any oxygen that does get inside prefers to burn (react) with the wax fumes that were evaporated off the wick by the heat of the flame.

This "burn-trimming" keeps happening as the candle burns down, otherwise the exposed part of the wick would grow longer and longer. It "regulates" the operating length of the wick.

A wick that curls fast will poke outside the flame only a short distance above the molten wax and be air-burned to a short exposed length. Then it will consume wax slowly. A wick that curls less will stay inside the flame for a longer distance and develop a longer exposed length and fast consumption.

If somehow, the wick from the frozen candle came up straighter... Maybe the warm candle has a deeper pool of molten wax. A candle that was cold underneath would have a shallower pool of melted wax. Suppose the wick-string has a built-in favorite curl diameter. Solid wax holds the wick straight, but wherever the wax is molten, the wick is released to start its curling at that place. A deeper pool of melted wax would have a wick that comes out of the pool more tilted. That wick would poke out of the flame sooner, be burned shorter by air, and then it would burn wax more slowly.

Jim Swenson


This is another of those "how fast" questions that are so difficult to pin down because so many variables are changing as a function of time, but I would guess the reason is that the frozen candle wax is first, not a very good conductor of heat so the pool of liquid wax at the base of the wick is deeper than the pool derived from a room temperature candle. This supplies more "fuel" to the wick. Check to see if the pool of molten wax is deeper for the frozen candle. Please let us know.

Vince Calder

Follow Up

Hi & Thanks for the reply,

What I did was I trimmed the wicks to the same height and froze one candle for 2 hours. The way I did my experiment was:

Got both candles and them on identical candle holders. burned them both at the exact same time and timed how long it took for them to burn out the frozen candle took: 25minutes 42 seconds the non-frozen candle took:33minutes 27 seconds Is that what you wanted? if not tell me because I really want to know the answer, because everyone I asked does not know! For some people I even did the experiment in front of them!

Thanks

Eli


Again, kudos for performing an experiment. That’s good. However, I would encourage you to think about *why* you are getting the results you are getting, and perhaps repeat experiments to see if your reasoning is correct. Your question highlights the challenge (and fun) of scientific experimentation. If you get results that are unexpected, sometimes you truly find something new, but often it is possible there is a problem with your experiment. In your case, I am guessing there is a problem with the assumption that “burning out sooner” means the same thing as “burning wax faster”. The question is if your experiment truly measures the rate of wax burning, or if something else is happening causing the cold candle to go out first.

Here is the gist. The candle wax has to melt, move up the wick, then vaporize and burn. A colder candle will take more energy to melt (there is a larger temperature change required to get to the melting point, and therefore more energy input is required). So one would expect a colder candle to take longer to burn as it takes more time for the flame to deliver the greater amount of heat. But that is not very quantitative… since the candle does not *stay* freezer-temperature once you take it out of the freezer, the difference likely would not last long (depending on the candle geometry). I would speculate that a long thin candle equilibrates with room temperature relatively quickly. So any temperature effect you might see I would guess would not last long.

But that does not explain your results. So I am left to speculate about possible scenarios – but without data/observations from you to back them up. For example, if you are only freezing the candle for a short time, the exterior of the candle might have reached freezer temperature, but the interior might not have chilled fully. In this instance, the candle might burn differently than the room-temp candle. Depending on candle geometry, the warmer interior might melt before the exterior, leaving the colder exterior unsupported (since the middle melted) causing it *not* to burn, but just to drop away once it melts. In this instance, I could imagine a “freezer-treated” candle burning out faster than a room temperature candle (although that is NOT to say that the wax burned faster – just that less wax burned on the frozen candle). This is just one example. Without good documentation of what you are seeing, I can only speculate.

Bottom line, there are many other factors that would need to be controlled or measured to really determine the cause of these results. A video of the experiment might answer many of these questions – but in any event the key is rigorously and thoroughly documenting your experiment. (for example, initial temperature, uniformity of temperature, uniformity of burning, mass of wax burned, humidity, etc.). I would recommend you repeat your experiment and test different possibilities. Measure the candle mass before and after burning, use a holder that makes sure all the wax is burned (does not drip away), freeze the candle for a very long time, try different candle shapes (long and thin versus short and wide), etc.

Report back and let me know what you have discovered!

Hope this helps,

Burr Zimmerman



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