When you heat the air inside a hot air balloon, it rises. Why does this occur?

Question

A.
The mass of cold air displaced by the balloon is less than the mass of hot air inside the balloon.
B.
The hot air inside the balloon becomes denser than the air outside the balloon.
C.
The hot air inside the balloon becomes less dense than the air outside the balloon.
D.
The volume of the balloon decreases.

Answer 1

The correct answer is C. The hot air inside the balloon becomes less dense than the air outside the balloon.

When you heat the air inside a hot air balloon, the air expands, and because it is heated, it becomes less dense than the cooler air outside the balloon. Since less dense air is buoyant in denser (cooler) air, the hot air balloon rises.