Why does the mass stay the same during combustion?

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When burning a substance, such as a fuse, the mass decreases if the gases produced are allowed to escape. This is because some of the solid matter is converted into gas, which leaves the system. However, if the combustion occurs in a closed environment where all gases are contained, the total mass remains unchanged. This is due to the principle of conservation of mass established by Antoine Lavoisier, which states that mass is conserved in a closed system, even though the composition of the matter changes during combustion. Understanding this concept clarifies why mass appears to decrease when gases escape but remains constant in a closed system.
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Noticed that when u burn something its mass decreased like if u place a fuse on a scale and burn it, its mass decreases. But Antoine Lavoiser determined that if all the particles in the gas collected, like if you burn the fuse inside a closed bottle the mass stays the same...i don't understand this concept n how it works. Please help me and thanks in advance :)
 
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Some fraction of the initial solid matter gets converted to gas*. If you let the gas escape, the remaining mass is lower than the initial mass. If you keep all gas in the system, you see that the overall mass stayed the same.
I don't understand the problem.

*it also changes its composition, that is the point of combustion, but it is not relevant here.
 
mfb said:
Some fraction of the initial solid matter gets converted to gas*. If you let the gas escape, the remaining mass is lower than the initial mass. If you keep all gas in the system, you see that the overall mass stayed the same.
I don't understand the problem.

*it also changes its composition, that is the point of combustion, but it is not relevant here.
My question was just why the mass stayed the same and you answered it...thanks
 
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