I just saw a post that suggested that 1 gallon of gas (6.3 lb) burned through a combustion engine generates 20+ lbs of CO2. I am sure they just missed a decimal or did not remember correctly what they saw or heard but

  1. what would be some practical/military applications of being able to increase mass 3x by burning or performing some action on a substance? and

  2. Are there any current processes that can be done on a substance that results in an increase in mass as the result?

I put this in world building because I was looking for a concept to build a story/world around. As the comments have shown, I was just missing all the mass inputs. I would still like to know if hand waving gave me more mass from energy somehow, what practical applications could come from it AND is there any process currently done or beautiful theory that can generate mass from energy so the hand waving would become unnecessary or significantly less?

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    $\begingroup$ Those numbers are correct. Combustion requires atmospheric oxygen, which combines with carbon from the gasoline itself to create CO2. Aside from nuclear reactions, you can't create or destroy mass, you just move it around and recombine it. 6.3lbs of gas plus a few dozen pounds of air gets you ~20lbs of CO2 - the extra mass doesn't come from nowhere. $\endgroup$ Nov 14, 2019 at 18:04
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    $\begingroup$ @Nuclear Wang correction - both nuclear and chemical reactions can add or subtract mass. Only in case of chemical reactions mass defect is so tiny it can be safely ignored. $\endgroup$
    – Alexander
    Nov 14, 2019 at 19:00
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    $\begingroup$ @Alexander I knew I'd get nailed on that point as soon as my comment-editing window expired! Absolutely correct, a small amount of mass is lost/gained through the formation/breaking of chemical bonds. $\endgroup$ Nov 14, 2019 at 19:03
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    $\begingroup$ @Alexander In nuclear reactions occuring on Earth, the effect is also safe to ignore. It's not like spent nuclear fuel is way lighter than new fuel. $\endgroup$ Nov 15, 2019 at 13:17
  • $\begingroup$ Whilst the fact that you have taken the trouble to edit you question is appreciated, your edit doesn't make the question any more on-topic. Please take the time to review the help center. $\endgroup$ Nov 15, 2019 at 19:12

3 Answers 3


There's actually a (beyond minuscule) loss of mass when fuel is burned. Your figures aren't much off for gasoline -- the extra mass comes from oxygen in the air. To oversimplify, let's assume gasoline is pure octane, C8H18 (molecular weight 114). To fully combust, it'll need 25 oxygen atoms (two for each carbon, and one for each pair of hydrogen), resulting in 8 molecules of CO2 and 9 of H2O have a combined mass of 514 Daltons (44x8 + 18x9).

If you can measure well enough, however, you'll find that some tiny fraction of a Dalton was lost -- it's the portion that represents the energy released by the reaction.

So, any chemical reaction that combines two substances, one of which is ignored before, will result in a similar "increase of mass" -- but only because you're counting something afterward that you weren't before. This is why, for instance, jet engines are so much "more efficient" than rockets -- because you don't have to pump oxidizer into a separate tank from the fuel the way you would with a rocket; the jet gets that from the air that's also its working fluid as a heat engine.


Take assorted hydrocarbons from the gasoline. As the name suggests, they consist mostly of carbon and hydrogen, and by weight they are mostly carbon.

Burn it, and one $C$ (carbon) atom bonds with two $O$ (oxygen) atoms from the air, forming $CO_2$. Each $O$ atom is about 1.33 times the weight of a $C$ atom, which means each $CO_2$ is only 27% carbon by weight.

The military useful action is to produce heat and to take (breathable) oxygen away. Typical applications are flamethrowers and fuel-air explosives.


With the large amount of CO2 generated by the burning gas, there could be a multitude of uses.

1.A) Terraforming, specifically on planets with little/no atmosphere, could use the quick greenhouse gases(though not practical)

1.B) The military could use it to gas-out areas that are crucial to capture or to kill/capture the opposing forces.

  • $\begingroup$ As there is no mass increase, this makes no sense. $\endgroup$
    – Aganju
    Nov 15, 2019 at 14:50

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