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by katscratcedme on March 1st, 2004

katscratcedme

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Why do helium balloons stop floating after a while?

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  • by Dan Whitacre on March 1st, 2004

    Dan Whitacre

    In brief, because the helium leaks out, they shrink, and become heavier than the volume of air they displace. This causes them to lose buoyancy and "sink" in the air.

    Sometimes you can catch a balloon right around the time it is neutrally buoyant, and applying heat (your hand, for instance) or cold (rub with ice cube) will change its volume just enough to make it rise or sink in the air.

    Rubber balloons lose helium quite quickly because of the nature of the material. Thin rubber is not a real good barrier for helium gas. Mylar does a much better job of keeping helium inside, and mylar balloons have much longer "lifetimes" than rubber ones.

    An interesting experiment would be to fill a balloon with helium, another identical one with normal air, and compare their sizes over time. I would imagine the helium balloon would lose size more rapidly, due to the high velocity of the helium gas atoms compared to that of normal air molecules. For a given temperature, the kinetic energy of the particles (atoms or molecules) of a gas are equal. Since helium is a nobel gas, it is monoatomic, and has an atomic weight of 4. Normal air, 80% N2 and ~20% 02 has particles that are diatomic, and thus around 7 times heavier. (N2 has a molecular weight of about 28, and O2 is around 32, but there's 4 times more N2 than O2.) Since the helium is 1/7th the size it is moving the square root of 7 times faster. (Kinetic energy is 1/2 mass times velocity squared, and mass is 1/7th, so velocity squared must be 7 times bigger for same KE, thus square root of 7.) The faster something moves, the more often it will find the hole to leak out of compared to a slower moving something. Another way to look at it is how far the particles travel. The distance the lighter particles cover as they collide about is longer than the heavier ones.

    If I had to make a prediction of the experiment's outcome, I'd say the volume change rate would be different by about the square root of 7, but that's just a guess.

    Comments

    • Thanks, very informative.

      katscratcedme

      by katscratcedme on March 1st, 2004

    • thanks for answering my question.. very informative in a simple manner

      abbygayle

      by abbygayle on February 27th, 2006

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