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Change in mass of a soda can

  1. Aug 10, 2015 #1
    My son did an interesting experiment with a can of soda.

    • Weigh the can before opening.
    • Open the can and weigh it again.
    • Wait a while (for the soda to go flat) and weigh it again

    I don't understand the result he got but here's what he saw:
    1. Immediately after opening the soda, the weight of the can (and soda) went up. (0.2g)
    2. Once the soda went flat, the weight went had gone down (after one hour) (0.2g)
    I put the results down to experiemental error, so I repeated his experiment and get the same results.

    Any ideas what effect we're observing?

    I'm surprised that the CO2 lost from a can going flat would have sufficient mass to cause any effect, and I definitely don't know why it would go UP in mass.
     
  2. jcsd
  3. Aug 10, 2015 #2

    mfb

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    CO2 has a higher density than air, so even replacing it with air should not increase the mass.
    Two data points are not that significant, however.

    At room temperature and pressure, about 1.5 g of CO2 can dissolve in a kilogram of water, 0.2g in a can of soda are not unrealistic. There could have been some evaporation of water as well.

    Multiple measurements with controlled conditions (e. g. soda can stays closed - maybe some condensation outside that increases the mass over time?) could help to find the experimental uncertainty for measurements in general.
     
  4. Aug 10, 2015 #3
    The second observation makes sense. The first one is counter intuitive. Likely just error, even though repeated a second time. What kind of scale are you using?
     
  5. Aug 11, 2015 #4
    It may be the reaction force produced by all these bubbles leaving the can. Not actually an increase in mass but an increased force on the scale.
    A bubble moving up (accelerated for at least part of the path) through the liquid will exert a downward force on the liquid.
    Or just the reaction to the buoyant force on the bubbles (which did not exist before opening the can).
     
  6. Aug 11, 2015 #5

    mfb

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    0.2g are ~130 cm3 of CO2 gas. Unlikely to have that many bubbles, no matter what they do.
    Their buoyancy does not change the force of the soda can on the ground.
     
  7. Aug 12, 2015 #6

    256bits

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    Yes, but wouldn't the CO2 be ejected at around a root mean square velocity of 400m/s from the open area ( temperature of the soda is unknown, but assuming somewhere in the vicinity of room temp ). In which case a thrust of 0.2 g would necessitate a "bubble" formation of 1.3 cm 2/sec.

    Perhaps, if little bits of Menthos were dropped into the soda to initiate bubble formation,but not to cause a large whoosh ejecting all the liquid also, a larger thrust could be obtained and observed.
     
  8. Aug 12, 2015 #7

    mfb

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    Those bubbles don't reach the speed of sound.
     
  9. Aug 12, 2015 #8
    Have your son do this experiment many times (20?) to reduce random error. Also, make sure the precision of your scale is adequate (I imagine it probably is).
     
  10. Aug 12, 2015 #9

    256bits

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    There was no reference to the bubbles obtaining the speed of sound.
    The molecules themselves have a velocity that has to be in the directed vertical direction.

    If they did not move upward the vapour pressure of CO2 above the liquid would remain constant and bubble formation would cease.

    .
     
  11. Aug 12, 2015 #10

    mfb

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    They have ~400m/s of random motion in all directions, with a slight asymmetry of a few centimeters per second due to the overall motion of the gas.
     
  12. Aug 12, 2015 #11

    russ_watters

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    Someone should calculate the velocity of CO2 flow required to generate 0.2 g-f thrust out a 1 sq cm hole, then see if it comes close to the rate of offgassing...
     
  13. Aug 12, 2015 #12

    mfb

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    1.6 kg/m3 * v2 * 1cm2 = 2mN
    v=3.4m/s2 or offgassing of 340 cm3 per second (that's the volume of the can). Quite unrealistic.
     
  14. Aug 12, 2015 #13

    Bystander

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    ... and now as an expanding hemisphere of gas? Maybe not so bad.
     
  15. Aug 12, 2015 #14

    SammyS

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    I expect that the increase in apparent weight that occurs as the can is first opened is due to a decrease in buoyant force. the volume of the can will decrease due to a decrease in pressure inside the can. This effect apparently overwhelms the mass of the small amount of CO2 initially released.
     
  16. Aug 12, 2015 #15

    Bystander

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    The total buoyancy correction (assuming 12 oz. can) would be 300-350 mg from air to vacuum wts.; little tough to come up with 0.2 g.
     
  17. Aug 13, 2015 #16

    CWatters

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    Gets my vote. By the time soda has gone flat it's warmed up and the condensation evaporated again.

    Repeat experiment with a can that is at room temp?
     
  18. Aug 13, 2015 #17

    SammyS

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    After actually looking up the density of air, I have to agree with you.

    Approx 1.2 grams/liter .
     
  19. Aug 17, 2015 #18
    It was a US-Extreme US Balance scale. Would be nice to try the experiment again with something more accurate.

    But I will definitely retry with a can that's already at room temp., as I did notice condensation.
     
  20. Aug 17, 2015 #19

    sophiecentaur

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    You could eliminate the reaction force idea by using a deflector (coolie hat) on top of the can.
    Condensation gets my vote. Do the experiment in very low and high humidity conditions and see if the result is different.
     
  21. Aug 17, 2015 #20
    The only possibility here, if this experiment is truly reproduceable, is the upward "force" of the escaping CO2 gas with the reaction of downward force on the can. To wit, if one takes a CO2 cartridge and punches a hole in one end the cartridge will accelerate in the opposite direction. Such acceleration will produce a force if one tries to resist it. The amount of force would be described in Toricelli's Law (v^2 = 2gh) where v is the linear velocity of the gas. One has to convert using the density of CO2 in the cylinder.
     
  22. Aug 17, 2015 #21

    SammyS

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    In my opinion the thrust from CO2 escaping upward is not relevant here.

    The initial surge of CO2 happens immediately upon opening the can. Any little amount escaping after that is at a very slow rate.
     
  23. Aug 17, 2015 #22
    You are quite right. The author himself noted much condensation on the cans. Initially the can would be weighed before condensation. After the can was opened and while 0.2 Gm of weight gain occurred initially from the condensation. This would evaporate in the hour described and the weight would drop. 0.2 ml of water collection isn't a whole lot of water on a 12 oz can.

    That's why repeated trials would be important. But, of course, without being snippy, who would care?
     
  24. Aug 19, 2015 #23
    This is clearly a kid's science project. And the gain of mass is best used as an example of why the simple act of measuring things leads to questions, hypotheses, and further testing, with a possible final model. Most likely the kid will determine the variables are the dry, closed can weight, the can temperature and ambient humidity, the CO2 evolution, the evaporation of water at ambient temperature, scale accuracy, ... and the price of soda and the need for research grants.

    I've had to ride herd on a pair of kids doing science projects. They generally have no idea what it means to measure a process and try to understand it. Condensation provides an example for kids to understand that it you have to think about something as simple as measuring weight, when temperature and humidity generate weight changes.
     
  25. Aug 19, 2015 #24
    LOL! Not at you but at the kids. Devious minds!
     
  26. Aug 22, 2015 #25

    mfb

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    In the interest of science, I tested condensation at a 0.5 liter PET bottle - I didn't have a can available. The scale was limited to 250 grams, so I had to open the bottle and reduce its content until the weight was low enough. This also means the part that still got cooled probably had a lower surface area than a can. Wiping it dry reduced the mass by 0.3 g. Repeatability of the weighting was better than 0.01 g, so the difference clearly came from condensation.
     
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