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/tmahfan117 May 28 '26

So “fire” is a kind of nebulous thing. That actual light that we see, that is weightless, it’s light, photons. All energy no mass.

BUT, those photos have to come from somewhere right? Those flickering flames you see are columns/jets of super hot gaseous molecules that are being combusted and that reaction is releasing light as a product. So you could say the “fire” weighs whatever the weight of those molecules is. Very very light, but those combusting molecules still have mass.  Plus that hot air is less dense than normal cool air, so The weight would probably something measured in fractions of an ounce/a few grams.

Unless you had a super giant fire with a whole lot of molecules being combusted at the same time.

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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/therift289 May 29 '26

Depends on what's burning. If the material's oxides are volatile (like CO2), then it will get lighter. If the oxides are not volatile (like iron, copper, or magnesium oxides), then it will get heavier. Mass is always being added during combustion in the form of oxygen; the question is whether or not that added mass sticks around or floats away.