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Uncayimmy
14th February 2009, 11:05 PM
I enjoy reading the physics discussion here, so I figured I'd start another one and learn.

What happens to the mass of a candle when it burns. I'm assuming it is being turned into a gas and the heat is from bonds in the molecules being broken. Suppose I put a large enough container around my candle to let it burn all the way down. Would the weight of the contraption remain the same?

I shall now sit back and enjoy the explanations.

LawnOven
14th February 2009, 11:10 PM
Since some of the matter is being turned into heat energy I don't think it would weigh the same.

Roma
14th February 2009, 11:34 PM
The mass of the candle is usually solidified on my nice coffee table.

Evilgiraffe
14th February 2009, 11:34 PM
Since some of the matter is being turned into heat energy I don't think it would weigh the same.

There is no conversion of mass into heat energy in a combustion reaction. The energy comes from the difference in bond enthalpies between the reactants (fuel and oxygen) and the products (CO2 and H2O).

To proceed, a combustion reaction must break bonds in the reactants, this takes energy. However, when new bonds are formed in making the products, energy is released. Heat is given out because more energy is released in making new bonds than was required to break the reactant bonds.

In answer to UncaYimmy's question. If the container was completely sealed then all of the atoms present in the candle before burning would still be present after burning. The contraption would have the same mass. This is a result of one of the central tenets of thermodynamics, The law of mass conservation (http://en.wikipedia.org/wiki/Conservation_of_mass).

The only caveat to your thought experiment is that the container must be sufficiently large to contain enough oxygen to complete combustion of the candle.

Uncayimmy
14th February 2009, 11:46 PM
I just remembered something I once read that might apply:

Don't bandy words with me, you slime. Despite what many of the Teeming Millions apparently believe, E=mc^2 applies to all reactions, not just nuclear ones. Permit me to quote from Space and Time in Special Relativity by N. David Mermin, a book I read myself to sleep with every night: "A loss of mass occurs whenever internal energy (nuclear, electrical, chemical, etc.) is converted into energy of motion. Only in the nuclear case is the amount of energy so large that [it results] in an observable change in mass, but in principle E=mc^2 is as descriptive of a chemical explosive, a gasoline engine, or a flying bird [or, I might add, a flying human] as it is of a nuclear explosion." Case closed.

69dodge
14th February 2009, 11:59 PM
Yes, E = mc2 applies, as always. But the amount of weight lost is tiny, nowhere near the weight of the original candle.

If the container is well-insulated, so that the outside doesn't get hot though there's a burning candle inside, no energy escapes, so not even a tiny bit of weight is lost.

Evilgiraffe
15th February 2009, 12:00 AM
Ok, I admit that I may have oversimplified. There is an imperceptible change of mass. It is the change of mass due to the change of electron binding energy. This is incredibly small.

If it wasn't, every combustion reaction would be just as powerful as a nuclear blast.

ETA. What 69dodge said applies too. If the system is truly insulated. The energy is still in the system and therefore the mass is still present in the kinetic energy of hot gas molecules.

yairhol
15th February 2009, 05:49 AM
If the container is made of glass, although glass is not a good heat conductor it still gets heated some and conducts this heat to the outside world thus removing energy from the 'closed system'.

sol invictus
15th February 2009, 06:34 AM
If the container is made of glass, although glass is not a good heat conductor it still gets heated some and conducts this heat to the outside world thus removing energy from the 'closed system'.

And some as visible light as well.

If you want to see how much this is, first estimate the energy escaping a candle as light and heat. I'd guess 30 watts. So over the few hours the candle burns, you might get out 10^5 joules.

Now - how much energy is in the mass of the candle? E=mc^2, which for a 100g candle is about 10^16 joules.

So about 1 part in 100 billion of the candle's mass is emitted as light and heat.

Molinaro
15th February 2009, 07:24 AM
On a somewhat related note.. back in my university days I once placed a burning marijuana cigarette on a digital physics lab scale.

It counted down at a rate of aproximately 0.001 grams/second, as the smoke rose.

yairhol
15th February 2009, 07:36 AM
On a somewhat related note.. back in my university days I once placed a burning marijuana cigarette on a digital physics lab scale.

It counted down at a rate of aproximately 0.001 grams/second, as the smoke rose.

:D

Soapy Sam
15th February 2009, 12:37 PM
http://www.gutenberg.org/etext/14474