why does a hot air balloon expand when it goes up?

Question

answer must be for 7th standard child.

Answer 1

Good question. Here is the scientific answer. The equation of state says PV = RT where p, is the pressure, v is the volume, R is a gas constant and T is the temperature.

So as the temperature goes up in the rising balloon and the outside pressure goes down, the volume in the balloon must go up (expand) to balance the equation. Ok?

Answer 2

Air pressure is 14.7 psi at sea level. As you ascend, the pressure decreases. It could be demonstrated by placing marshmallows in a jar and apply vacuum. The marshmallows will expand greatly.