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Is gas visible?

  1. Dec 26, 2005 #1
    Someone said that gases are invisible. But if you confine "lots" of gas into a small encloser so that on average a gas molecule will bump into another gas molecule by travelling a distance that liquide molecules tend to bump into each other than would one be able to see this extremely dense patch of gas in this small container?
  2. jcsd
  3. Dec 26, 2005 #2


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    It depends on what gas. Some are colored, some are not. Also, "Raleigh scattering," and things like that can affect the color.


    That would be... a liquid. Some are colored, some not, etc...

    I think I understand your thinking now. About how sparsely-placed the gas atoms are, and that affecting color.

    I think the gas still has a color, its just too weak to be seen by the human eye.

    What color is hydrogen?
    Helium? Liquid helium is blue right?

    Can somebody show me pictures of differnet liquified gases (i've seen N2)
    Last edited: Dec 26, 2005
  4. Dec 26, 2005 #3
    Last edited by a moderator: Apr 21, 2017
  5. Dec 26, 2005 #4
    Color by Rayleigh scattering is due to particles in the air, isn't it? And the size of particles determines the color.

    But actually some gases have colors. For example, ozone is light blue, though oxygen is colorless in the gas form. see this one.
    Last edited: Dec 26, 2005
  6. Dec 26, 2005 #5

    Why is my example a liquid? The particles in my small enclosed area are gas because they are above the temperture that the particles exist as liquids. In the case of H2O, the container might have a temperture of 120 celcius so they cannot be liquid because it is above H2O's boiling temperture.

    So my question is "Is it possible to see gasous particles?"

    The atomsphere is only made up of particles in their gasous form and we are able to see that the atomsphere is blue so does that answer my question? I am doubtful because I am not sure if the atomsphere is 100% made up of gas particles and my question is more about seeing the colour of one particular gas rather than many different gases mixed together as that of the atomsphere.
  7. Dec 27, 2005 #6


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    Keep in mind, pivoxa, that the state of the substance depends upon its pressure. In order to restrict the molecular motion of a gas to that of a liquid, you would have to compress it to the point that it becomes a liquid regardless of its temperature.
  8. Dec 27, 2005 #7
    So you are basically saying that if I restrict a gas so that I am able to see it, it will not be a gas anymore but a liquid. Therefore, no matter what I do, I will never be able to see the colour of particles in true gas form?

    What about the atomsphere?
  9. Dec 27, 2005 #8


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    Some gasses are colored, some are not. If the gas is clear, and its liquid form is colored, if you squeeze the gas, it will be a liquid. Then it will be colored. If the gas is colorless, but the liquid is not, it has no color until its sqeezed into a liquid.
  10. Dec 27, 2005 #9


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    No, that's not what I meant. There are some gasses that you can see while they're still in gaseous form, although this might be a matter of definition in which I'm wrong. When they're electrically excited, you can certainly see neon, argon, xenon, etc..

    I'm once again uncertain as to how this is defined. All of the individual gaseous components of the atmosphere are colourless. They collectively scatter the blue section of the EM spectrum, though, so the sky appears blue. I don't know anything about optics, and therefore am uncertain as to what 'colour' really means in these two instances.
  11. Dec 27, 2005 #10
    Could you name some of those colored gases and provide a reason as to why they are colored (in other words why are those gases visible)?
  12. Dec 27, 2005 #11
    Many vapours are visible and have colour. Vapours are gasses (just defined as ones that will revert back to liquid readily), you can check out web definitions of the two.

    Check out:

    I think that for most things we consider gasses (that are only liquified under extreme pressure or cold) the molecules/atoms are just way to far apart to provide any 'vapour colour'
  13. Dec 27, 2005 #12


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    We can clearly see the gases comprising the atmospheres of the gas giants in our Solar System.
  14. Dec 27, 2005 #13


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    Right. Never thought of that. :redface:
  15. Dec 27, 2005 #14


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  16. Dec 27, 2005 #15
    Bromine vapour pic on left side of screen

    http://www.schoolscience.co.uk/content/3/chemistry/materials/match2pg1.html [Broken]
    Last edited by a moderator: May 2, 2017
  17. Dec 27, 2005 #16
    But aren't fog and vapor like matter from dry ice many small drops of water/ice like smoke? So they aren't real gas.
    Last edited: Dec 27, 2005
  18. Dec 27, 2005 #17
  19. Dec 28, 2005 #18


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    Well, you both seem to be thinking the same thing. Its kind of iffy I guess. Look at the table at http://en.wikipedia.org/wiki/Colloid What do you think? You could look at it like, its a liquid aerosol, but you could ask yourself if a bucket of paint is a liquid or not.
  20. Dec 28, 2005 #19
    Yeah, we are thinking of the same thing. Smoke is a dense group of particles nearly the size of wavelengths of light, so no wonder it is visible and has color. And coloid is similar, except that in coloid, the size of each particle is larger than the wavelength of light.

    And now I'm wondering what makes the color of gas or even liquid. Molecules (1/10 - some nano meters) are usually much smaller than wavelength of light (some hundred nano meters.) So molecule itself won't affect color.

    So what makes the color of gas or liquid? I guess some color should be related to transition of energy states of electron in the atom (for ex, solution of blue vitriol, because copper compound usually has blue or green color)..... Can the color of gas/liquid be explained only by transition of energy states of electron (absorbed or emitted)?
  21. Dec 28, 2005 #20
    When I first post this question, I was really asking whether any gases can be seen normally (not excited). I assume that this question has been answered from the example of we being able to see the atomsphere as blue. But could it be that the blueness only comes from the liquid vapour that exists in the atomsphere? Hence we are not truly able to see the pure gas in the sky?

    But apparently most if not all gases are visible if you excite them such as shown http://en.wikipedia.org/wiki/Neon_sign
  22. Dec 28, 2005 #21
    My understanding about blue sky, I think it's due to small particles (dusts or whatever) in the air, reflecting light of shorter wavelengths. (or it may be the color of the air itself..I am not sure.)

    Wave reflects on objects greater than the wavelength, whilest it goes though objects smaller than the wavelength.

    So, at the same time, red sunset glow is also due to the same small particles in the air, through which only the light of longer wavelengths comes to the ground. At sunset (or sunrise), the sun is low and its light comes through "thick" air, so the longer wavelength has advantage. I'm sure of this one.
  23. Dec 28, 2005 #22


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    But vapour is not gas. Vapour is liquid droplets suspended in air.
    Last edited by a moderator: May 2, 2017
  24. Dec 28, 2005 #23
    I'm sure gases are visible. They would be needed in large concentrations though. Isn't it all to do with which colours of light are absorbed and which aren't?
  25. Dec 28, 2005 #24
    That’s not true. From thermodynamics look at a temperature/enthalpy diagram.
    Water, for ex, can be liquid (technically subcooled). Add heat and you will move from a subcooled liquid to a saturated liquid (the point where any more heat addition is simply going to change phase instead of raise temp.) As you add more heat there is a region of mixed phase, part liquid, part vapour. Once enough heat is added you reach a ‘saturated vapour’ Any more heat it becomes a ‘superheated vapour’ as its temp rises again. When does water become a gas in this explanation? The point is that the vapour is a gas, and vice-versa. Its just that most things we call gas are SO far into the superheat region that you aren’t going to see it condense on your lawn some chilly night.

    I got wondering about the same thing, and found this on the net (dont know if it is right or not) the energy transition they talk about is not the same as exciting a gas like neon.

    Color occurs when there is an electronic
    transition in a molecule whose energy difference corresponds to
    wavelengths in the visible part of the electromagnetic spectrum (400-700
    nm). That has nothing to do with the physical state of the substance (gas,
    liquid, solid). In some cases the absorption is weak, e.g. O2 so the color
    is not readily seen.
  26. Dec 28, 2005 #25
    Dry Ice fog: Gas CO2 is clear and colourless. The fog in this case is indeed liquid water drops in the air (or even possibly frozen solid water). As far as I understand, the reason it happens is the same as a cloud forming in the sky. The air temp is lowered by the dry ice to the point that the air can no longer hold its water vapour, (excess vapour). The vapour needs to condense out, so any dust in the air will provide the surface (nuclei) for this to happen. This is what you see as a cloud in this case.

    Saying that, I guess the sublimation of CO2 from solid to gas would be totally invisible if it were taking place in an atmosphere of 0 humidity.
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