# Gas Laws and Pressure

bluegirlbalance
As a hot air balloon rises to increasingly higher altitudes describe what happens to the volume of the balloon if at constant pressure?

My teacher had us do a PTV cards activity, where we took an index card and wrote the letters P(for pressure), T(for temperature), and V(for volume). This activity was supposed to help us learn about pressure. For example, if the pressure increases and the temperature remains constant, what happens to the volume? It increases.

For the question above, if the pressure remains constant as the hot air balloon rises, the volume increases, right?

## Answers and Replies

Mentor
Correct. PV = nRT.

qwedsa
as the hot air balloon rises, temperature gets lower, right? as temperature drops but all other variables stay the same, volume must drop

berkeman why do you say the volume increases? what am i missing?

twotaileddemon
PV = nRT
If R is a constant, and temperature remains the same. Moles aren't changing. Therefore, if the pressure goes up, the volume must go down. If the volume went up too, then that would completely change the answer! Do you see why?

qwedsa
But we are supposed to assume a constant pressure according to the question

if the pressure is constant, the only variable changing as the hot air balloon rises, would be it is getting colder, which means less volume

Mentor
The pressure inside the balloon is constant, but the pressure outside is dropping. So the net force on the balloon walls would increase toward the outside, and the balloon would grow. Of course, that assumes that the temperature is not going down substantially (the problem did not address this explicitly), and that the balloon walls are elastic.

Mentor
The pressure inside the balloon is constant, but the pressure outside is dropping. So the net force on the balloon walls would increase toward the outside, and the balloon would grow. Of course, that assumes that the temperature is not going down substantially (the problem did not address this explicitly), and that the balloon walls are elastic.

Actually, the more I think about it, I am not sure of the answer. I don't understand what the problem statement means by "what happens to the volume of the balloon if at constant pressure?". What's at a constant pressure? The absolute pressure of the gas inside the balloon?

Homework Helper
Gold Member
Hot air balloons are open on the bottom. Volume remains constant. Density of hot air at constant P is less so the balloon is buoyant and rises. At lower external pressures (higher altitude) hot air is forced out the bottom of the balloon and therefore, the number of moles is less. Pressure inside the balloon must be the same as the outside at equilibrium regardless of altitude. If this condition is not met, air will flow from high pressure to low pressure (out the bottom).

This is not the case for Helium balloons where the number of moles of gas remains constant.

Staff Emeritus
Gold Member
chemistree said:
Hot air balloons are open on the bottom. Volume remains constant.
I'd go a step further. The balloon is not a closed container, so "volume" is meaningless (undefined). This question is a bunch of hot air!

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qwedsa
^i don't think volume is meaningless, the balloon can expand or shrink even with that opening at the bottom, so volume is defined in terms of the balloon

Homework Helper
Gold Member
Perhaps the question belongs in the brainteaser section...

To answer the question in the OP, Bluegirl, you are essentially correct that the volume would increase IF we were discussing a helium balloon. Keep in mind that the pressure would be decreasing inside the balloon and outside the balloon as it gained altitude. At some point, the pressure inside the balloon would start to exceed the pressure outside the balloon because the fabric stopped stretching. The balloon would probably fail shortly thereafter...

Helium balloons are launched to high altitude in massive balloons that contain relatively little helium. They look like mostly empty bags with a little bubble of helium at one end. As the balloon rises this small bubble eventually expands to fill the entire balloon (which can be very large).

http://topweb.gsfc.nasa.gov/balloon/launch.html [Broken]
http://topweb.gsfc.nasa.gov/balloon/launch2.html [Broken]

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