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If I have a soap bubble then it will move rather randomly upwards perhaps due to the initial force from me blowing it, and eventually take ground moving downwards. enter image description here

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A bubble, while it still exists, is balanced by three factors:

  1. Surface Tension of the soapy water.
  2. Internal Pressure applied by the air inside the bubble on the surface.
  3. Atmospheric Pressure.

Now those factors don't take into account the initial force for the blowing of the bubble and does that really explain why the bubbles rise for some time instead of directly just falling?

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The air you exhale is in most circumstance warmer than the ambient air temperature. This is the air that ends up in the bubble. Due to the density difference (hot air rises), the soap bubble will rise. In my experience, bubbles do not keep rising, indefinitely. Thermal equilibrium is quickly reached, and the driving buoyancy force disappears. The gravitational force on the water will pull it downwards, but most likely the bubble bursts before you can observe this.

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    $\begingroup$ Your breath contains more CO2, which makes the air inside heavier. $\endgroup$
    – kristjan
    Commented Jan 25, 2015 at 19:51
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    $\begingroup$ @kristjan Interesting thought. Any quick calculation that compares it to the temperature effect? $\endgroup$
    – Bernhard
    Commented Jan 25, 2015 at 19:56
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    $\begingroup$ @kristjan - yes and more water vapour (mass 18) less than other components (and less oxygen (mass 32)) - this is a very interesting point. $\endgroup$
    – tom
    Commented Jan 25, 2015 at 20:00
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    $\begingroup$ @tom - In exhaled air (according to some sources), there is ~4% CO2. However, it seems that water vapor pressure will pretty much be just saturated to the maximum possible value. However, if the gas inside cools, it might start condensing out, making the bubble even heavier. It is very hard to estimate the effect of H2O on the bubble. $\endgroup$
    – kristjan
    Commented Jan 25, 2015 at 20:12
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    $\begingroup$ So the greenhouse effect should make the bubble keep rising! :-P $\endgroup$
    – CJ Dennis
    Commented Jan 26, 2015 at 3:37

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