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Adding equal amounts (by mass) of sugar and water -- combined weights don't add up...

  1. Feb 15, 2017 #1
    Today I was leading my students in an experiment that would reveal "the rule for what floats." We had previously floated (or sank) various objects in regular, room temperature tap water. I then weighed out 100 grams each of sugar and water, mixed them together and weighed them again. To my surprise, instead of having 200 grams of sugar water solution, I had 125 grams.

    At first I thought we had not tared our scale properly, so I poured the mixture into a newly tared beaker. We still came up 75 grams short. We then weighed out another 100 grams each of sugar and water. This time I tared the beaker containing the water back to 0.0 grams and then poured the newly weighed 100 grams of sugar directly into the water as it was on the scale. It only increased the weight by 25 grams.

    Then I thought there was something wrong with how the scale tared the beaker and weighed the beaker separately. It weighed about 115 grams, so this could not account for the loss.

    Given that you cannot destroy matter, can someone help me understand why adding the water to the sugar in equal weight resulted in a mixture that weighed less than 200 grams?? I'm flummoxed and I can't seem to find an explanation on the internet. I promised my students I would try to figure out why this happened.
     
  2. jcsd
  3. Feb 15, 2017 #2

    Doc Al

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    Staff: Mentor

    It's got to be some sort of measurement error. Try this: put the sugar in a zip lock bag and dunk it in the water and see if the same issue arises. (That would eliminate issues with dissolving.)
     
  4. Feb 15, 2017 #3

    Merlin3189

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    Gold Member

    I've just tried myself and within the accuracy of my kitchen force meter (2g) the results add up.
    As Doc says, has to be faulty method or equipment.
    Perhaps you could record actual readings for us to look at? Like
    Empty sugar container = 20g
    S.Container & sugar = 119g
    Empty water container = 75g
    W.Container and water = 177g
    W.Container and water and sugar = 276g or whatever it came out to.

    Have you tried a different force meter or have you tried calibrating the one you are using?
     
  5. Feb 15, 2017 #4
    I FINALLY figured it out!!! The OHaus scale I was using had a 200 gram limit and when I tared the 115 gram beaker back down to 0 grams, I exceeded the limits of the scale! I used a 5000 gram scale, and it added up to 200 g. Phew! I was SO perplexed. I was even asking my science colleagues how this could possibly be. It was such a reproducible result. No one could answer, but I thought I'd try another scale, and when it worked, it finally dawned on me what the problem was. My students will have a good laugh over this! Great lesson! So much for having a Ph.D.!!
     
  6. Feb 15, 2017 #5
    By the way, thanks so much for your thoughts. Much appreciated!
     
  7. Feb 15, 2017 #6

    Merlin3189

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    Gold Member

    Finally! Exactly! You knew something was wrong and kept going until you sorted it out. 10/10
     
  8. Feb 15, 2017 #7

    Doc Al

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    Good work! :thumbup:
     
  9. Feb 15, 2017 #8
    I think your students learned something more important than the planned lesson! Now you just need to convince them that was the plan all along. ;)

    Hah, easy to say in hindsight, but always check your instruments over the full range you expect to measure.
     
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