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

Answer 1

The correct answer is:

C. Helium has a lower density than air.

A balloon filled with helium rises because helium is less dense than the surrounding air. According to Archimedes' principle, an object will float if it is less dense than the fluid (in this case, air) it is in. Since helium is lighter than air, the buoyant force acting on the helium balloon is greater than the weight of the balloon, causing it to rise. In contrast, a balloon filled with air has a similar density to the surrounding atmosphere, which is why it does not rise.