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How can we prove the law of conservation of mass

  1. Dec 21, 2016 #1
    How can we prove the law of conservation of mass ? I said prove not the verification, how lavoiser decided that yes this law is followed by every chemical reaction, is there a proof
     
  2. jcsd
  3. Dec 22, 2016 #2
    Lavoisier was long dead before the idea of mass energy equivalence appeared.

    Weighing devices are not accurate to detect the very small mass changes assumed to take place in chemical reactions , but einstein came up with the famous equation and we are told nuclear reactors/bombs loose mass ... it does all make sense..

    But your right to doubt everything ... lots of dis-info out there , particularly in the field of astrophysics
     
  4. Dec 22, 2016 #3

    Andrew Mason

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    Science, generally, does not prove things. Proofs are left to mathematicians.

    To what level of certainty are you trying to "prove" the law of conservation of mass?

    AM
     
  5. Dec 22, 2016 #4

    Borek

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    I suggest you watch this classic:



    It doesn't speak about mass conservation, but about why we can't 'prove' things is physics.
     
  6. Dec 22, 2016 #5
    [QUOTEknorek, post: 5649643, member: 23711"]I suggest you watch this classic:



    It doesn't speak about mass conservation, but about why we can't 'prove' things is physics.[/QUOTE]

    Hey i know we can't prove things in physics or chemistry because physics and chemistry is all about satisfaction and i love this but the point is that , suppose i made a law that yes mass of reactant will always be equal to mass of product, i didn't applied it on every chemical reaction on earth, i just watch some of them, so there may be a fear that what if this law is not followed in that reaction, i simply wanted to know what must have made him satisfied that it holds for every chemical reaction(except for nuclear reaction due to Einstein's equation), so finally what made him to made this conclusion as a law, and thanks for this video i have already seen this.
     
  7. Dec 22, 2016 #6

    Borek

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    We have tested it zillions times, and we have failed to find a single case where it didn't hold - so we assume it does. We also did thousands of other experiments, not related to chemistry - and in all cases we have found mass to be conserved (actually sum of mass and energy, as E=mc2 and they can be interconverted). We have a very thoroughly tested theories that use law of mass conservation as one of the underlying principles - if the law was wrong conclusions would be wrong as well; they are not. That makes us quite sure this law holds always, but it will still take a single experiment where it doesn't to prove we are wrong.

    Don't hold your breath though.
     
  8. Dec 22, 2016 #7

    Andrew Mason

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    As Feynman says, the first step is to guess the correct law. Lavoisier did that based on some experimental data that had been accumulated. He did not need "proof" before promulgating his theory. It was up to scientists to then test the theory to see if the Law's predictions matched experiment.

    Some laws appear to be absolute laws (Law of Conservation of Momentum, the Second Law of Thermodynamics). Some are laws within certain limits or application (eg. Newton's Third Law, which does not apply to electro-magnetism and Newton's Law of Universal Gravitation which requires some very fine corrections provided by General Relativity). And some are just approximations (Hooke's Law). The Law of Conservation of Mass in chemical reactions would probably fit in the middle category. (i.e. chemical reactions release or absorb energy and, in doing so, they lose or gain mass. But the amount of mass lost or gained is so tiny that it is difficult to measure).

    AM
     
  9. Dec 23, 2016 #8
    Conservation laws apply to isolated systems and isolated systems cannot relase or absorb energy.
     
  10. Dec 23, 2016 #9
    Yes ...we sometimes forget that ...to clarify... if we put a mixture of H2 and O2 on a weighing device and ignited it , the weight would be the same ....only as the water cooled to room temperature would there be a mass change.

    Warming things up increases their mass! The atoms have greater velocity and so have an increase in mass. Make sure you buy your loose fruit and vegetables on colder days , you'll get more for your money.
     
    Last edited: Dec 23, 2016
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