What happens to a candle's wax?
November 23, 2008 12:52 PM   Subscribe

When a candle is burnt indoors, what happens to the wax? An amount of smoke equivalent to the mass of the candle is released as it's burnt (?)--does this manage to escape the house, or does most of it stay in the room and coat the walls, floors, ceiling, etc.?
posted by mstillwell to Science & Nature (24 answers total) 7 users marked this as a favorite
Except for the smoke, it's all turned into gases by the combustion. CO2 and H2O mostly I guess (I don't remember the chemical formula for wax.) Very little remains as wax; the majority of the wax that evaporates as it gets hot is burned along with the wick.
posted by anadem at 1:00 PM on November 23, 2008

Best answer: The wax is actually the fuel that is burned, usually paraffin. With good quality wax, the end products are all gaseous CO2 and H20.

Smoky soot is the byproduct of incomplete combustion, due to a poor flame structure (crappy wick) or due to low quality wax with lots of impurities.
posted by randomstriker at 1:01 PM on November 23, 2008

the mass of the candle does not become an equal mass of smoke. Some of that mass is being converted to light, light, and various gasses.
posted by munchingzombie at 1:02 PM on November 23, 2008

Wikipedia says "Paraffin waxes are hydrocarbons", so basically burning a candle is the same as burning gasoline: the carbon and hydrogen combine with oxygen to make CO, CO2 and H2O. Because a candle flame gets all the oxygen it wants, the CO is burned to CO2 (which is not the case with a car engine - some CO is emitted.)
posted by anadem at 1:04 PM on November 23, 2008

BTW for you pyromaniacs here's a neat (or potential dangerous) trick: get two tealights, the ones with foil cups. Suspend one above the other, one inch apart, so that the flame from the lower tealight heats the upper tealight. Best if you set them up before lighting them.

Eventually the wax in the upper tealight will boil and the wax vapor will ignite across the whole liquid surface, resulting in a huge flame. At this point, even if you remove the wick (e.g. with metal tweezers) the wax will carry on burning, demonstrating that it is the wax that is the fuel.

The way a candle works is:

1) independent flame (a lighter or lit match) melts and vaporizes wax on the wick.
2) wax vapor ignites.
3) more wax around the base of the wick melts and is drawn up the wick by capillary action
posted by randomstriker at 1:16 PM on November 23, 2008 [8 favorites]

Some of that mass is being converted to light, light
No, none of the mass is converted to light, unless it's a thermonuclear explosion. The light is the result of the intense heat produced in the chemical reaction; it's basically gas so hot it's glowing. The heat and light released are energy previously stored in the chemical bonds forming the paraffin molecules.
posted by beagle at 1:20 PM on November 23, 2008 [1 favorite]

> Some of that mass is being converted to light, light, and various gasses.

In an effort to prevent the spread of disinformation, none of the mass is being converted into light. The light created is going to come from the breaking of chemical bonds, not the conversion of mass to energy.

randomstriker's answer is correct.
posted by christonabike at 1:23 PM on November 23, 2008 [1 favorite]

I'm going to second chistonabike's comment. The candle closed system loses no mass to the light.

The reason candles burn is because the molecules of wax have a higher, less stable energy state than that of he combustion by-products (H2O, CO2, etc). Because of the applied heat, the bonds in the wax molecules break apart and react with oxygen to form lower energy bonds. When the electrons fell from their higher energy states to their new, lower energy states they release the energy in the form of heat and light.

Now, someone is thinking "But E=mc^2 means mass from the wax was converted into photon energy!" Not true. Energy from the wax electrons was released as photon energy. Energy to energy. You need a different type of reaction to convert mass to energy.
posted by sbutler at 1:37 PM on November 23, 2008

If you had God's own balance, there is some argument that you would find molecules with stronger bonds to weigh more than those with weaker bonds, even if they were stoichiometrically identical.

If you're balance is only getting you five or six decimal places (rather than 34 or so) the answer you're going to come up with is that the products weight the same as the reactants.
posted by Kid Charlemagne at 2:00 PM on November 23, 2008 [1 favorite]

This video may answer your question, and is worth the watch both for the awesomeness of seeing the candle be relit from its own "smoke" and the professor's sheer earnestness.
posted by ewingpatriarch at 2:05 PM on November 23, 2008 [5 favorites]

([BEGIN SCIENCE PEDANT MODE] Actually, even chemical bond energy shows up as mass, it's not just nuclear bond energy that does that. So yes, some absurdly tiny amount of the wax's mass leaves as light. [END SCIENCE PEDANT MODE])

But mostly it leaves as water vapor, CO2, and a bit of CO. Wax is mostly long hydrocarbons (a big chain of carbons with hydrogens hanging off the sides like legs on a centipede); when it burns cleanly, the whole thing dissociates and everything oxidizes. You can see some water vapor condense out if you hold a cold object a few inches over a flame. Or if you burn stuff in an enclosed area, you might see some condensation, but that's a bad idea because even a small amount of CO can be deadly.
posted by hattifattener at 2:34 PM on November 23, 2008

No, none of the mass is converted to light, unless it's a thermonuclear explosion.

I don't think this is true.

If photon absorption did not increase mass, classical black holes would violate conservation of energy every time they absorbed a photon, because the energy would be gone from the rest of the universe, and the black hole itself would be unchanged in its only (measurable) property, its mass.

Reversing time, we can see that photon emission must involve a loss of mass.

So I would say the emission of photons show that the candle/air system must be losing mass somewhere, though it might be hard to say just where.
posted by jamjam at 2:35 PM on November 23, 2008 [1 favorite]

Mod note: few comments removed - can we keep this pretty much to the science needed to answer the question? thank you.
posted by jessamyn (staff) at 4:02 PM on November 23, 2008

When a candle is burnt indoors, what happens to the wax?

Most of the wax (and most of the cellulose that forms the wick) reacts with oxygen from the air to make carbon dioxide and water vapor.

A very small percentage of it fails to burn completely, forming smoke that consists mostly of carbon and tar. If the room you're burning the candle in is well sealed, most of that will end up sticking to your walls, floor, ceiling, furniture and lungs.

The mass of carbon dioxide, water vapor and smoke produced is, to an exceedingly good approximation, equal to the mass of wax, wick and oxygen consumed.
posted by flabdablet at 5:01 PM on November 23, 2008

The paraffin wax of the candle is an hydrocarbon of formula CnH2n+2, with n between 20 and 40. For n = 30, assuming perfect combustion giving only CO2, water and heat, we have:

C30H62 + 61 O2 → 30 CO2 + 31 H2O + Heat.
posted by Monday, stony Monday at 6:41 PM on November 23, 2008

Since burning hydrocarbons makes yellow light, the binding energy of CO2 is apparently a few eV. The mass-energy of each CO2 + H2O is about 60×109 eV. So burning wax changes its mass by about one part ten billion. The soot is a bigger effect.
posted by fantabulous timewaster at 7:39 PM on November 23, 2008 [1 favorite]

Reversing time, we can see that photon emission must involve a loss of mass.

Nope, sorry, photons do not have mass.
posted by beagle at 8:03 PM on November 23, 2008

Sorry, messed up that link, let's try again:

Reversing time, we can see that photon emission must involve a loss of mass.

Nope, sorry, photons do not have mass.
posted by beagle at 8:05 PM on November 23, 2008

I've nothing to add to the physics here (though I'm glad to hear that the Universe is still sticking to its conservation of mass policy, at least on a day-to-day basis), but did think I'd point out that I've read in a variety of places that if you are trying to maintain a very clean or hypoallergenic environment with air filters, you should never burn candles or light matches.

Although a candle or lamp with a well-adjusted wick will convert most of its fuel mass into harmless (from a dust/filtration perspective — they are toxic) gases, they almost always produce some soot. I think matches are significantly worse than candles in this regard. But, supposedly, burning candles can lead to increased load on filtration equipment if you make a habit of it.

I'm pretty sure the first place I read this was in the manual for a HEPA room-air filter, but I can't find a source right now.
posted by Kadin2048 at 8:31 PM on November 23, 2008

I agree with everyone else that only a small fraction of the candle's mass becomes soot. However, I'm not a big fan of candles, and this supports my preconceived notions:

Soot emissions from scented candles were significantly higher than those from non-scented candles. Forty five percent of non-scented (n=11) candles produced detectable amounts of soot, while 63% of scented candles (n=80) produced detectable amounts of soot. Soot production from non-scented candles, that produced soot, ranged from 20-175 µg/min/wick (mean 83 µg/min/wick). Soot production from scented candles, that produced soot, ranged from 20-3100 µg/min/wick (mode 1 = 180, mode 2 = 1650 µg/min/wick). The fine particulate matter collected from candle emissions was similar to that of diesel engine exhaust in particle size, morphology, elemental carbon content, and adsorbed chemical constituents...

(from the US Scented Candles Study)
posted by lukemeister at 8:55 PM on November 23, 2008

beagle: That's not what jamjam claimed. (S)he said that the emission must involve a mass loss from a black hole. And that is absolutely right.
posted by edd at 4:35 AM on November 24, 2008

Beagle, I happened to read the page you linked before I posted; I thought it was saying that the rest mass of the photon was zero, but I wasn't certain of it.

The clearest and least equivocal statement I was able to find that energy produced by ordinary chemical reactions (of which photon emission is one form) such as combustion, involve a loss of mass of the same sort as nuclear reactions (mass defect) comes from (the apparently well-thought-of) Special Relativity by Ulrich E. Schröder. On page 119, Schröder says:

In exothermal chemical reactions, which of course are connected with processes in the atomic envelopes, only extremely small amounts of mass are liberated. For example, the mass defect related to the burning of 1 kg carbon is 4x10E-7 g, so that the relative mass defect AM/M becomes only 4x10E-10. The ultimate reason is that, because of the smallness of the fine structure constant and the electron- proton mass ratio, atomic binding energies are very small.

I brought up black holes to try to argue that any process involving emission and absorption of photons would produce a change in mass of the system in question. To expand the case slightly, if someone says they have such a process (no mass change with photon emission/absorption), I take it to a black hole, let it absorb a photon and quickly toss it into the black hole. The system embodying the process augments the energy of the black hole only by its mass, which is not increased by the absorption of the photon, and that violates the law of conservation of energy because the energy of that photon disappears from the universe altogether. Therefore, the assumption that the absorption of the photon did not increase the mass must have been wrong.
posted by jamjam at 11:13 AM on November 24, 2008

lukemeister, your link to the study you quote excerpts a 1999 masters' thesis which doesn't seem to live anywhere online. The excerpt includes the conclusions but not any detailed discussion of the experiment or the analysis, so I might be overlooking something the author considered elsewhere. But comparing five sooty candles out of eleven, or (45±14)% using counting statistics, to fifty sooty candles out of eighty, (63±7)% --- that really stretches the meaning of "statistically significant difference." I wouldn't put much stock on that study, especially since its author apparently never published it apart from his thesis submission, whether it supports a preconceived notion or not.
posted by fantabulous timewaster at 1:52 PM on November 24, 2008


Fair enough - I certainly agree that one shouldn't cherry-pick data to support one's preconceived notions. What struck me was the *far* higher emission in some of the scented candles, not the percentage of sooty scented vs. unscented candles. Nonetheless, sub-micron particles are small enough that can get in your lungs and cause problems:

AMBIENT PARTICLES IN THE DIAMETER range of 1 nm to 20 µm may be deposited in human lungs on inspiration ("respirable particles") and are therefore available for interactions with pulmonary surfaces (16). As a result, many diseases of the human respiratory tract can directly be linked to inhaled material, such as cigarette smoke and toxic occupational or environmental aerosols (32).

Since candle soot is chemically similar to diesel exhaust, it can't be good for you. I have no idea how many candles you'd need to burn to put yourself in danger, though.

We now return to our regularly scheduled program.
posted by lukemeister at 6:56 PM on November 24, 2008 [1 favorite]

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