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Thread: hot air

  1. #1 hot air 
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    Is there a gas that is either of equal weight or heavier than air below 25C but that expands to be lighter than air beyond that temperature?

    Any ideas would be appreciated,

    Thanks


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  3. #2  
    Reptile Dysfunction drowsy turtle's Avatar
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    No. According to Charles's Law, if a gas is less dense than air at one temperature, it will be at all temperatures.

    http://en.wikipedia.org/wiki/Charles%27s_Law


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  4. #3  
    Universal Mind John Galt's Avatar
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    Of course that applies an Ideal gas. Might there not be something in reality that behaves differently?

    And since we are dealing with air, wouldn't condensation of entrained water mess up expected ratios?
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  5. #4  
    Reptile Dysfunction drowsy turtle's Avatar
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    The observed difference between how a real gas behaves, and how a real gas behaves, is very small, I believe.

    Although, now you mention water evaporating, couldn't the OP simply use a liquid with a boiling point of around 25 celcius? What exactly is the intended application of this?
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  6. #5  
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    Quote Originally Posted by Ophiolite
    And since we are dealing with air, wouldn't condensation of entrained water mess up expected ratios?
    Yes, in fact there's one very good example used in meteorology: a rising parcel of air. A non-saturated parcel will rise and cool due to decreasing pressure according to ideal gas law and first law of thermodynamics. It is called the dry adiabatic lapse rate.

    If it's saturated the water vapor will condensate, release heat and hence the entire parcel will be warmer and less dense than the surrounding dry air that might be rising at the same rate. The saturated parcel will cool at what's referred to as the moist adiabatic lapse rate.

    These lapse rates differences between rising or descending parcels when different than their surrounding result in buoyancy of the parcels and explain many phenomena such as nightly valley down slope winds, and thunderstorm convection.
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