Hot Air Balloons: The Science Behind Rising

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

A. The volume of the balloon decreases.

B. The hot air inside the balloon becomes less dense than the air outside the balloon.

C. The hot air inside the balloon becomes denser than the air outside the balloon.

D. The mass of cold air displaced by the balloon is less than the mass of hot air inside the balloon.

Answer:

The answer is actually B. It becomes less dense.

Hot air balloons float due to the buoyant forces that act on them. When air is heated, the air molecules spread farther apart. This causes the density of the hot air to be less than the density of the cold air. Eventually, buoyancy causes the balloon to rise because the weight of the cold, outside air that is displaced by the balloon is greater than the weight of the hot air inside the balloon (plus the weight of passengers and balloon equipment).