Final answer:
As a helium balloon rises, it expands due to the decreasing atmospheric pressure and principle of the ideal gas law.
Step-by-step explanation:
As a helium balloon rises from the ground up into the sky, it gradually expands. This has to do with the principle of the ideal gas law. The surrounding air pressure decreases as the balloon rises higher into the atmosphere, thus allowing the helium balloon to expand. The volume of the balloon changes inversely with the atmospheric pressure. In simpler terms, the higher the balloon goes, the lower the surrounding pressure, and the more room there is for the helium gas particles to spread out, causing the balloon to expand.
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