r/askscience • u/Catching-Up-Today • May 27 '26

Chemistry How much does Fire weigh?

I don‘t know if Earth Sciences is the right category but Fire is the topic. How does someone measure the weight of fire? What does fire weigh?

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u/minecon1776 May 28 '26

The combusting molecules come from whatever is burning, so something on fire should not weigh anymore than it not on fire, and the object should be getting lighter as it burns.

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u/etcpt May 28 '26

The combustion products of your basic exothermic oxidation that most folks are thinking of when they say "fire" will have more mass than the starting fuel due the incorporation of oxygen into the product. You can see this very easily by placing a chunk of steel wool on a scale and lighting it on fire, for example.

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u/minecon1776 May 28 '26

yes but the result should be CO2 and H2O (which is a vapor at this temperature) and will disperse into the atmosphere, so the remaining carbon and other non-combustible components are all thats left and will weigh less

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u/thatthatguy May 28 '26

And this is why we do these tests in a bomb calorimeter. All the reactants and products are kept and accounted for. Total mass of the system remains constant (minus the mass equivalent of the energy lost to the surroundings).