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Conservation of Mass?

  1. Jun 9, 2009 #1
    My chem teacher told me once that if you could gather all the atoms produced after burning a piece of wood, you would have the same amount of mass you started with.

    Is this true, or does some of it get converted to energy?
    I'm reading about "Pair Production and Annihilation", and the text says (sorry to quote a boring textbook) :
    So ...(please explain this to me like I'm a dumb highschool kid, because I am) that means a little of the mass is converted to photons, right? Or no?
     
  2. jcsd
  3. Jun 9, 2009 #2

    diazona

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    Yeah, a very tiny bit. Probably something like a millionth of a percent. You'd never actually be able to measure the difference.

    For comparison, nuclear fusion converts (if I remember properly) about 0.3% of the initial mass into photon energy.
     
  4. Jun 9, 2009 #3
    Hi there,

    Your teacher is saying the right thing. When you burn a piece of wood, you simply transform it into something else. There is energy freed from this process (touch the flame and you'll feel it), but it is not converted from the transformation of mass into energy. It is mainly energy that binds the molecules together, when burning will free some energy. This is a subject of classical chemistry.

    Here you are talking about something completely different, which is in the field of high energy physics. If a photon has enough energy, it can loose its energy to create a pair of electron-positron. The process is rather complex, so I'll spare you the details, but for your information you would need a photon with 512keV ([tex]8.202 \times 10^{-15}J[/tex]). To make the story complete, if the photon has more energy than needed to create the pair of particles, the excess of energy will be given to the particle as kinetic energy.

    The conservation of mass is a classical process, which does not include the creation of pair. Therefore, like mentioned in the post before, the creation of pair is a process that creates very, very, very, very, very little mass.

    Cheers
     
  5. Jun 9, 2009 #4

    sylas

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    I believe you are correct... a small amount of mass IS lost when you burn wood, even though you still have all the same atoms you started with. The amount of mass lost is tiny, but it is real because there is mass associated with the energy of chemical bonds. You could not detect the difference in mass directly.

    It may help to see just how tiny this amount of mass really is. Suppose you burn a kilogram of wood. Typically, that will release about 1.6*107 Joules of energy. The amount of mass converted to heat is about 1.8*10-10 kg, using (E = mc2, and c = 3*108 ms-1). You can't really hope to measure this mass loss, and so it's quite normal to say that chemical reactions conserve mass.

    That's an approximation; but it's a pretty good one! In principle, however, you could weigh the difference, if you were able to weigh things to about 12 significant figures of accuracy. If you burned all the wood inside a sealed container, so that no atoms could escape, and then let that container radiate away the heat generated as photons, you would find the container weighs a tiny bit less.

    Cheers -- sylas
     
  6. Jun 9, 2009 #5
    Thanks everyone for your extensive answers. XD
     
  7. Jun 9, 2009 #6
    I agree with Sylas. Let's consider a specific example. I looked at mass conservation in the reaction

    6H2O + 6CO2 <--> C6H12O6 + 6O2

    This is the well known photosynthesis reaction, in which about 29 eV of heat-of-combustion energy is stored in the sugar molecule. The question is whether that extra 29 eV of energy is stored as extra mass. It is a very small amount, compared to 180 + 192 = 372 AMU (atomic mass units) = about 346 billion eV stored in the sugar + oxygen, but the energy has to be stored somewhere, and there is nowhere else to store it.
     
  8. Jun 9, 2009 #7
    I don't know if you consider it valid but no mass is lost.
    As you stated a small amount is radiated away as photons and escape the container.
    But they are out there somewhere in the universe so they are conserved which is part of the Conservation of matter.
     
    Last edited: Jun 9, 2009
  9. Jun 9, 2009 #8

    sylas

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    You mean conservation of energy. Photons are not matter.

    The question is whether the mass of all the chemical reagents is changed by the chemical reaction. It is, technically, by an amount so tiny that it can't really be measured effectively. Conservation of energy means that the mass loss of the reagents is balanced by the energy radiated out into the rest of the universe.
     
  10. Jun 10, 2009 #9
    Possibly, the Conservation of Matter states "if I remember correctly" that matter can not be created or destroyed but can be changed into another form.
    Photons don't have mass but they do have energy so they are a changed form of mass.
    I might be confusing the conservation of energy with the conservation of mass.
     
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