Question

give reason of hot air balloons rise in air

Answer 1

When the air expands, its density reduces. The air in the balloon thus becomes lighter and less dense than the air outside the balloon. This is the reason why hot air balloons rise in air.

hope this will help you

Answer 2

Hot air is less dense than cool air; the heated air causes the balloon to rise simply because it is lighter than an equal volume of cold air.