(a) Reason: 1 Mark
(b) Working: 2 Marks
(a) Hot air rises up because, it has lesser density than the cooler air around it.
(b) A hot-air balloon works on the principle that hot air rises up.
It consists of a burner and a balloon envelope. When the burner is lit, it heats the air around it.
The hot air rises up and the cold air on the sides comes near the burner. The same process continues for a long time.
The hot air gets trapped in the balloon and the hot air balloon rises, depending on the amount of hot air in it..