1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Does temperature affect mass?

  1. Oct 31, 2004 #1
    We were told in chemistry class to weigh an evaporating dish after it cooled. Does it's temperature affect it's mass?
  2. jcsd
  3. Oct 31, 2004 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    In general, the temperature does not affect the mass of a closed system.

    In your class, you are probably allowing something to cool, so that there is no more evaporation happening. That, or they don't want you to burn your fingers touching a hot object. What's in the dish, anyway ?
  4. Oct 31, 2004 #3
    There was [itex]CuSO_4 \cdot 5H_2O[/itex] in the evaporting dish. We were evaporating the water out of this hydrate to get an anhydrous compound. The goal was to find the ratio of water molecules to copper sulfate molecules. One of the questioins she asked us was why we should take the mass of the evaporating dish after it cools.
  5. Oct 31, 2004 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I can only think this is a safety measure. Also, depending on what kind of balance you are using, weighing something that's hot may give a faulty reading or might even damage the balance. Do you use a metal balace which uses moments or a digical balance which uses springs ? And I'll repeat :Temperature does not affect mass directly.

    In your particular example, it would be more accurate if you weigh the dish while still little hot. When the copper sulfate cools, being slightly hygroscopic, it will start to re-absorb moisture from the atmosphere. If you wait too long for it to cool, it may have re-absorbed a considerable amount of moisture that your calculation will yield an underestimate.
  6. Oct 31, 2004 #5


    User Avatar
    Staff Emeritus
    Science Advisor

    Yes, temperature afects mass, but not enough so that you'd notice in a laboratory undergoing chemical reactions with a balance.

    You might be able to measure the differences if you were doing nuclear reactions rather than chemical ones, but it's hard to contain the resulting plasma in a typical set of laboratory beakers :-).
  7. Oct 31, 2004 #6
    The substance was left in a desiccator, which would probably keep it from re-absorbing moisture (another question we were asked :smile:). Maybe it's best to answer the question saying the temperature could affect the mass? I can't see any other possible answer... any ideas?
  8. Oct 31, 2004 #7


    User Avatar

    Staff: Mentor

    The warm dish can create convection (airflow), which affects the scale's reading.
  9. Oct 31, 2004 #8


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The only direct influence on mass will be too small to measure on any lab scales.

    The only way you can affect the mass in your experiment is by adding or removing something to the test substance. If you can find a way for the high temperature to be responsible for adding or removing material, then only would the answer be 'yes'.

    If I was conducting a high school chemistry lab with the question you've got, I'd give a full grade to anyone that said 'no', or if someone saying 'yes' can explain why.
  10. Oct 31, 2004 #9
    I can now answer this question, thanks!
  11. Nov 17, 2010 #10
    I think that temperaturte doesn't affect mass, but it do affect wieght of the body.
    becuase if we put a piece of iron above a scale, we can consider that its weigh represent the pressure excerted form it onto the scale, and when the tempreture gets raised, the movemnt of molecules of the lowest part of the iron mass increases, this would raise the pressure excerted from the iron onto the scale, and would make the scale raise the value of the weight of the body.
    here I think that it's the same as "increasing the temperature of gaz increses its pressure", its also increasing the temperature will increase the weight of any mass because of the incresed number of molecules of the body which hit the scale and cause it to raise the value of the weight of the body.
    but i don't think that it raises the mass...
  12. Nov 17, 2010 #11
    As Pervect has explained an increase in temperature results in an increase in mass though a balance scale would not be sensitive enough to show the slight increase.
    By increasing the temperature you are adding energy to your piece of iron so it would be slightly more massive than when it was cold.
    Much the same way as a charged battery is slightly more massive than an uncharged one.
    This amount is teeny weeny and too small to measure with a normall scale.
  13. Nov 17, 2010 #12
    you could calculate it , using Q=mct
    c= specific heat
    m= mass
    t= change in temperature
    and then use this increase in temperature and divide it by c^2 (speed of light) and get your mass increase, which would be very small.
  14. Nov 17, 2010 #13

    As temperature rises, the kinetic energy of the molecules rises, according to the Einstein's equation, their masses must rise.
  15. Nov 17, 2010 #14
    I guess this is the kinetic energy relative to the speed of light as well as the 'base energy' of the object before it was heated. In that case, for normal laboratory tests, the increased mass would be incredibly tiny. You'd have to heat the body high enough to emit gamma rays to notice any change.

    I think it might be possible to utilize this principle into a 'directed energy' space propulsion system. Or simply accelerate charged particles at very close to light speed and slow them magnetically at some point in the accelerator. Exhaust would be a high energy beam consisting of X-rays and/or gamma rays.
  16. Jul 3, 2012 #15
    what about subzero, the complete absence of heat?
  17. Jul 3, 2012 #16


    User Avatar
    Science Advisor

    The problem when weighting a hot dish is not that it's mass is changing but that:
    a) The mass of the hot air in the hot dish is lower than that of the cold air in the cold dish.
    b) The density of the hot dish is lower than that of the cold dish due to thermal expansion whence the hot dish has a higher buoyancy in the surrounding air.
  18. Jul 3, 2012 #17
    if your aim is to observe the gravity of electrons at different temperatures, you will need a very controlled environment, im not sure that a petry dish would be considered in such an experiment as you would break your petry dish when it reaches it's tolerance because you either would need a way to measure the gravity created by a large explosion or a way to succeed in total refrigeration, a process that, if successful, would create dark matter, destroying your entire lab by completely halting every electron and altering every molecule affected into a unified state of zero mass, so probably better off trying to measure the gravity of an atom bomb as it is more predictable and possible than halting electrons
  19. Jul 3, 2012 #18
    Weighing the empty dish before and after the experiment is just good lab practice. If the readings are different then the dish was not clean/dry before the experiment or something else got screwed up.

    The direct effects of temperature on the weight are fairly small and have been listed above.
  20. Jul 4, 2012 #19


    User Avatar
    Science Advisor

  21. Jul 4, 2012 #20


    User Avatar
    Science Advisor

    The reason for waiting for the dish to cool is to reduce air currents around the plate and balance that could affect the result.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook