What happens to the pressure inside a tied-off balloon as the pressure outside the balloon decreases?

I'm almost 100% positive it would decrease, but my meteorology professor (who also teaches physics) says it will stay the same, because no air is escaping the balloon!

Does the rubber of the balloon interfere that much with the equalization of pressure? Is there something else I'm missing? Who is correct?

• Going the other way, it seems intuitive to me that if you increase the "outside pressure" (by taking your balloon down to the bottom of the Mariana Trench, say), the pressure inside the balloon will increase as the volume decreases. Isn't that why deep sea divers get the bends? Oct 3, 2016 at 20:48
• Then your meteorology professor should take the balloon with him in his next ascent to the stratosphere and observe how the balloon will expand and maybe explode. This, of course, happens only with a thin rubber balloon, not with a very thick one. Oct 3, 2016 at 20:56
• Your professor would be approximately correct if the fabric of the balloon can not stretch. A toy balloon made of stretchy latex will expand (if it is not already at the breaking point) when the external pressure drops. And if its volume expands, but the quantity of gas within remains the same, then the absolute pressure of the gas must decrease. On the other hand, a toy balloon made of mylar film will barely expand at all (assuming it was already filled to capacity). Nov 16, 2018 at 21:42
• Reminds me of my high school physics teacher who insisted that a steel sphere filled with helium would be lighter than the same sphere filled with vacuum. May 2, 2019 at 0:37

Pressure inside the balloon as it is brought up to desired interior pressure, let's say at sealevel , is restrained by the exterior press (@14psi). If the exterior pressure drops, the vessel (in this case a flexible balloon )will expand, the volume inside increases and the interior pressure will fall. So you are correct, and should always question any answers by anybody if you aren't satisfied with the answer, or especially the explanation for the answer.

It is well known that for stratospheric balloons, only a small filling at ground level is used because upon ascent the atmospheric pressure decreases and the gas filling will occupy a volume much larger than before. Therefore the pressure in the balloon is approximately equal to the outside pressure and lower pressure gives a larger volume of the balloon.

• Freecharly , atmospheric balloons are never filled intheway balloons are, so your analogy is mute with respect to the question as asked. Aug 2, 2018 at 3:34

If your balloon is made of very strong material than its volume would not change and the pressure would be the same as your prof states. But that is not practical. Also its would be mostly assumed that balloon material is thin and elastic, the pressure inside the balloon is always greater than outside because the elastic exherts a contraction force on the gas inside. Or an atmospheric balloon is also carrying weight therefore pressure is higher inside. Helium is very light but can exert a lot of pressure for its mass due to kinetic energy (temperature) hence He balloons float.

If your professor agrees that the gas inside the balloon follows the ideal gas law, then the following applies:

$$PV=nRT$$ (1)

$$nRT$$ is constant for the tied-off balloon if the temperature is constant, which it should be. This means that given the right hand side of equation 1 is constant, the left hand side of equation 1 must also be constant. Thus, as volume goes up, pressure must decrease to preserve this condition. Conclusion - if the balloon expands, the pressure must fall as a consequence.