Why does a balloon filled with helium rise while a balloon filled with an equal volume of average atmospheric air does not?

Question

A.
Helium has a higher density than air.
B.
Helium increases the balloon's mass more than air.
C.
Helium has a lower density than air.
D.
Helium has no mass, but air does.

Answer 1

The correct answer is:

C. Helium has a lower density than air.

A helium-filled balloon rises because the density of helium is less than the density of the surrounding air. Since the balloon displaces a volume of air that is heavier than the helium inside, the overall buoyant force on the balloon allows it to rise. In contrast, a balloon filled with air (which has a similar density to the surrounding air) does not experience this buoyant effect and thus does not rise.