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Why does heat go UP?

  1. Apr 26, 2007 #1
    So I understand why heated molecules move faster--they have more energy.

    But, for my example, I'll use a water molecule. A water molecule has a certain density, and that density is more than that of air (which is why water stays in a pot). But when the water is heated to 100 deg. Celsius, it becomes steam. This steam always travels in an upward direction. I'm assuming this is because those H2O molecules have more energy--but why do they always go UP? Why don't they just go in some arbitrary directions?

    And sorry, I'm not well-versed in Chemistry at all. Actually quite an idiot on the subject. :-(

    But thanks for any useful input!
  2. jcsd
  3. Apr 26, 2007 #2


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    Heat does not go up. Heated materials are often more voluminous/lighter than surrounding materials, and thus can rise, but that does not mean that "heat rises".
  4. Apr 27, 2007 #3
    Yeah, I meant "heated molecules" when I said "heat". Sorry for bad wording.

    So when an H2O molecule is heated, the actual molecule expands to make its density lower than that of air?
  5. Apr 27, 2007 #4


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    This question can be answered statistically:
    If you have molecules with a lot of energy together with the ones that habe lower energy, then it's easier for the hi-energy ones to move up because they have more KE to change into PE and rise. So after a while you'd expect to find that more of the "hot" ones have risen than the "cold" ones.
  6. Apr 27, 2007 #5
    I'm glad this went into statistics quickly.

    'Air packet' models are very useful for solving problems, but terrible for explaining what's going on. I feel like slapping everyone who presents an air parcel explanation with a fish.
  7. Apr 27, 2007 #6


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    In other words, heated molecules go up in comparison with colder molecules because they have more energy to overcome gravity which is always down. If there were no gravity, there would be no "up" or "down" so there would be no preferred direction.
  8. Apr 27, 2007 #7
    Heated molecules rise in air because they are less dense. Therefore a hot air baloon works as long as the air inside the baloon is heated. Now steam rises in air because steam (H2O in gasious state) has a molucular weight of 18 and air has an effective molucular weight of 29 (actually O2 @ 32 and N2 @ 28). What you see when boiling water is condensed steam or liquid water, steam is invisible.
  9. Apr 27, 2007 #8


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    Individual molecules are all the same density regardless of temperature. What is different is the density of the gas. [edit] Lemme reword that: The concept of density does not really apply to individual molecules. Density is a property only of the fluid, the collection of multiple molecules. [/edit]

    I prefer the bulk fluid transport (buoyancy) model of warm air rising, but many physicists appear to prefer the statistical method. There are many cases, however, where the statistical method is inappropriate, such as in a helium or hot air balloon. Engineers are taught this way in fluids classes (at least in undergrad) because it is the most applicable to real-life engineering problems.
    Last edited: Apr 27, 2007
  10. Apr 27, 2007 #9


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    Nope. The heated molecule actually has more kinetic energy and is moving at a greater velocity. The collisions it makes with all of the other air molecules (nitrogen, oxygen, carbon dioxide, argon, etc...) causes them to be spaced further apart. Now the same number of molecules of 'air' occupy a greater volume. This is what is meant by lower density... less mass in a given volume. The surrounding air is cooler and therefore denser. In a gravitational field, the less dense air rises.
  11. Apr 29, 2007 #10
    Do molecules of all liquids go up when heated?
    H20 has less molecular mass than average air, what happens with liquids of greater molecular mass?
  12. Apr 29, 2007 #11
    They go down. If you put a heavy but volatile liquid in the bottom of a flask, it will evaporate, but stay in the bottom. You can make the experience with nasty solvents as carbon tetrachloride or chloroform. It may even work with nail enamel solvent. If you drop a soap bubble in the flask, it will float over the solvent vapors at some centimeters of the bottom (I did it).
  13. Apr 29, 2007 #12
    Thnx for the information (I have learned something new:).
  14. Apr 30, 2007 #13
    In this particular case both liquid and its sorrounding air are at same temperature
    , so the kinetic energies of air molecules and evaporating molecules are same.
    But when you heat carbon tetrachloride at 76.8 °C (boiling point of CCl4), it will certainly go up.
  15. May 4, 2007 #14
    COLD IS HEAVIER! picture cold as a damn ice cube, it would fall. and u no how on really hot days, u see like blurry lines on the pavement and off of cars? it goes up! picture an ice cube melted from the soild(ice cube) to liquid(water) to gas(steam). steam floats.(*THIS IS THE WAY I FIGURE AND REMEMBER IT*) i donno...these tips help me suring like a test so....yea...
  16. May 5, 2007 #15
    but if you put an icecube in water it goes UP ;)

    it is to do with density rather than temperature.
  17. May 5, 2007 #16
    No, it won't.
    The molecular mass of carbon tetrachloride is 153.8. The mean molecular mass of air is about 28. Then the density of ClC4 is 5.49 times that of air. I you heat ClC4 it will become lighter, but not enough to go up. You must heat ClC4 about 1350 °C over the temperature of air for the two to have the same density. I don't know the chemical behavior of ClC4 at these temperatures I don't know if it decomposes
  18. May 5, 2007 #17
    No. The molecule does not expand. Just its speed increases and it hits harder the other molecules and the wall of the container: at constant volume the pressure that it exerts increases. If the gas is allowed to expand, the speed (and the temperature) diminishes.
  19. May 5, 2007 #18


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    If you're interested, you can find a nice set of articles on the basic mechanism of free convection at the Nonoscience blog.

    - http://www.nonoscience.info/2006/07/10/free-and-paid-convection/" [Broken]
    - http://www.nonoscience.info/2006/07/22/free-convection-for-dummies/" [Broken]
    - http://www.nonoscience.info/2006/08/03/free-convection-and-the-rayleigh-number/" [Broken]
    Last edited by a moderator: May 2, 2017
  20. May 5, 2007 #19
    Does the extra energy the heated molecules have contribute to their going up or simply because the heated molecules are less dense, they go up?
  21. May 5, 2007 #20


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    The molecules themselves are not less dense, density is a macroscopic property and is not defined for individual or small numbers of molecules. It is because the individual molecules have more energy [on average] that the density of the gas (collection of molecules) decreases. The point that halls is making, is that if there were no gravity, the heated gas would still be less dense, but would not rise.
  22. May 5, 2007 #21
    I didn't mean individual molecules are less dense though, still i think I got the answer.
    So heated gas is generally less dense, that's why it rises up. If we heat a particular gas that is denser than air, would not go up unless it's density is lesser than sorrounding air- right?
    Last edited: May 5, 2007
  23. May 5, 2007 #22
    Yes, now we agree.
  24. May 5, 2007 #23


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    Indeed, you are correct :approve:. As Russ said, 'hot air rising' can be understood most intuitively through buoyancy. daniel et al. were alluding to the statistical method, which I must say I prefer, but often seem less intuitive initially.
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