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 JREF Forum What Happens to the Mass of Candle?

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 14th February 2009, 11:05 PM #1 Uncayimmy Banned   Join Date: Oct 2008 Posts: 7,485 What Happens to the Mass of Candle? 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.
 14th February 2009, 11:10 PM #2 LawnOven robot     Join Date: Sep 2001 Location: Illinois Posts: 1,306 Since some of the matter is being turned into heat energy I don't think it would weigh the same. __________________ Principia Discordia
 14th February 2009, 11:34 PM #3 Roma Master Poster     Join Date: Aug 2008 Location: Winnipeg, Canada Posts: 2,061 The mass of the candle is usually solidified on my nice coffee table. __________________ I only know what I want to know.
 14th February 2009, 11:34 PM #4 Evilgiraffe Scatterer of X-rays     Join Date: Aug 2007 Location: Oxfordshire Posts: 741 Originally Posted by LawnOven 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. 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.
 14th February 2009, 11:46 PM #5 Uncayimmy Banned   Join Date: Oct 2008 Posts: 7,485 I just remembered something I once read that might apply: Quote: http://www.straightdope.com/columns/...en-their-meals 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.
 14th February 2009, 11:59 PM #6 69dodge Illuminator   Join Date: Nov 2002 Posts: 3,607 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.
 15th February 2009, 12:00 AM #7 Evilgiraffe Scatterer of X-rays     Join Date: Aug 2007 Location: Oxfordshire Posts: 741 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. Last edited by Evilgiraffe; 15th February 2009 at 12:03 AM.
 15th February 2009, 05:49 AM #8 yairhol Graduate Poster     Join Date: Jan 2007 Posts: 1,403 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'.
 15th February 2009, 06:34 AM #9 sol invictus Philosopher     Join Date: Oct 2007 Location: Nova Roma Posts: 8,417 Originally Posted by yairhol 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.
 15th February 2009, 07:24 AM #10 Molinaro Illuminator     Join Date: Dec 2005 Location: Toronto Posts: 3,027 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. __________________ 100% Cannuck!
 15th February 2009, 07:36 AM #11 yairhol Graduate Poster     Join Date: Jan 2007 Posts: 1,403 Originally Posted by Molinaro 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.
 15th February 2009, 12:37 PM #12 Soapy Sam NLH   Join Date: Oct 2002 Posts: 25,885 http://www.gutenberg.org/etext/14474 Go here and download Michael Faraday's "Chemical History of a Candle". I believe he's some sort of English popular science chappie.
 15th February 2009, 12:58 PM #13 Ziggurat Penultimate Amazing     Join Date: Jun 2003 Posts: 26,183 Originally Posted by UncaYimmy What happens to the mass of a candle when it burns. For chemical reactions the mass attributable to potential energy is negligible, and you can safely treat all reactions as conserving mass directly without worrying about energy. Almost all of a candle's wax is made of carbon, hydrogen, and oxygen, and when the wax burns, this simply turns into carbon dioxide and water vapor. __________________ "As long as it is admitted that the law may be diverted from its true purpose -- that it may violate property instead of protecting it -- then everyone will want to participate in making the law, either to protect himself against plunder or to use it for plunder. Political questions will always be prejudicial, dominant, and all-absorbing. There will be fighting at the door of the Legislative Palace, and the struggle within will be no less furious." - Bastiat, The Law

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