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Elementary Science Education

2 Questions about burning wood & matter vs mass

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    • #8268


      When my two sons (ages 8 & 10) and I discussed wood being burned (BFSU Vol II, A-14, page 45), my 8 year old asked why can’t we re-make the wood by “un-burning” the ash, carbon dioxide, water vapor, etc. I told him that some chemical reactions are reversible (like the examples on pg. 48 – electrolysis for one) whereas others are not. Is that correct? And if so, he is curious why certain reactions cannot be reversed. Why can’t we “un-burn” a piece of wood?

      My 10 year old was also wondering about the wood-burning example. He understands the concept behind conservation of matter but is wondering if that necessarily means there is also a conservation of mass. So, when you burn a piece of wood and it is changed into ash, etc. would all those components, when added, have the same mass as the original log (we didn’t think that would be the case unless there are huge quantities of gas… seems like a log would be much heavier than ash + gasses)? In trying to talk it through with him I realized I really can’t adequately articulate the difference between mass and matter…. If mass is the amount of “stuff” in something (similar to weight but not exactly) and matter is the “stuff” itself….

      If there’s any light Dr. Nebel or anyone else can shed on these questions we’d appreciate it very much!

    • #8269

      Bernard Nebel

      Thank you for your questions. They show good thinking.

      Why can’t we “un-burn” a piece of wood?

      Reese is quite right in his thinking. The formula for any chemical reaction can be written backwards as well as forwards, making it theoretically possible. Specifically the chemical reaction for burning wood (mostly cellulose) is:
      C6H12O6 + 6O2 —–> 6H2O + 6CO2 + release of energy
      The reverse (unburning) is:
      6H2O + 6CO2 + input of energy —-> C6H12O6 + 6O2 and is theoretically possible. In fact, plants do this with the aid of light energy. It is called photosynthesis.
      However, distinguish theoretical possibility from practical feasibility. Plants accomplish the unburning of wood (photosynthesis) through a dozen or more individual chemical steps driven by energy from light. To have it happen on one big step as occurs in the burning is what is impossible.

      Its a little like smashing a dinner plate on the floor. You can see the smashing apart occurs with one bang. You can see that the pieces might go back together to “unbreak” the plate. Then you might do so by carefully picking up each piece and fitting and glueing them back together. What remains impossible is having them go back together in one “bang.” Thus we say that the breaking of a plate (and unburning wood) are irreversible.

      Is there conservation of mass as well as conservation of matter?

      Yes! (But don’t confuse mass and weight.) Since each particle (atom) of matter has a certain mass and atoms remain the same, mass will also remain the same. Seeing this in practical way is what is difficult. Note from the above equation for burning wood, the major products are carbon dioxide and water (vapor) which go off in the air. (They retain their mass although weighing them is impractical.) A very small portion of the log ends up as ash. To prove that mass (in terms of weight) remains constant, you would have to burn the wood plus sufficient oxygen in a totally sealed container so that the water vapor and carbon dioxide are held in. (Scientists have done this.) It is found that the total mass and weight
      does not change.

      Please question more as you wish.

    • #8270


      Thanks for the great explanations! The plate analogy was especially helpful!

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