Question

Which of the following best describes why hot air balloons rise?

The molecular weight of the gases in the air increases when heated.
The number of molecules in air increase exponentially when air is heated.
Hot air is more dense than cold air, creating a force or upward lift.
Hot air is less dense than cold air, creating a force or upward lift.

Answer 1

d

Answer 2

when heated ,air expands and a portion of air of balloon flows out thereby density of air inside balloon decreases.
It becomes lighter than outside air and hence lifts upward.