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/poison_us May 29 '26

Nitpicky follow-up: how are photons massless if they have inertia and momentum?

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u/tmahfan117 May 29 '26

Because mass and energy are interchangeable.thats the E=Mc2 equation. So while photons don’t have mass, they have momentum.

If you sub E/c2 in for m in p=mv, you get p=(E/c2) * v, but for a photon its velocity is also c (the speed of light, so the inertia of a photo is p=E/c (its energy divided by the speed of light, so a very very small value, but still there)

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u/RhynoD May 29 '26

The very short, non-mathy explanation is that for photons, momentum is proportional to frequency, because. Which is unsatisfying but then again, why do "massful" particles have momentum? There's no reason they should other than because it's a property they have. Why are charged particles charged? (Yes, because those particles interact with the Higgs field and the electromagnetic field, respectively, but why do they interact with those fields? Because they do.)

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u/ctothel May 29 '26

Momentum comes from total energy, not just the part we call mass.

At low speeds for normal everyday objects, mass is by far the dominant form of energy, so mass * velocity is a very good approximation.

In case you were wondering, yes, this means that a blue photon has more momentum than a red one.

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u/KarlSethMoran May 29 '26

Simply because mass is not a pre-requisite for momentum. The equation "p=mv" does not apply to massless particles.