A balloon expands when taken to a higher elevation if the temperature remains constant, because pressure is inversely proportional to volume.
Boyle law states that, if temperature of an ideal gas is kept constant, the pressure is inversely proportional to the volume of ideal gas.
pV =nRT
P α 1/V
Pressure decreases with elevation and so, the volume of balloon increases.
Thus, balloon expands as it goes to higher elevation.
Learn more about Boyle's Law.
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