Question

Hot air balloons rise up because.
A. Hot air is heavier than cold air.
B. Hot air is lighter than cold air.
C. Hot air contains water vapour which pushes the balloon up.
D. None of the above.

Answer 1

In order to solve this question, we are first going to discuss the composition of the ‘hot air’ in a hot air balloon and then, giving the reason for it rising up. We will discuss the basic physical phenomena that help the hot air balloon to rise up in the air and how that phenomena is used here.

Complete step by step solution:
A hot air balloon is filled up with Helium gas. It consists of a gas burner. The burner is used to heat the helium gas filled in the balloon. Now, as we know that the Hot air rises up because it is lighter than cold air. As the air inside the balloon is hotter than the air surrounding the balloon because of the heat caused by the burner, the balloon rises up.
Thus, in spite of the composition of the balloon and the structure that has been provided with, the main physical phenomena which is responsible for the rising up of a hot air balloon is the comparative masses of the air at the different temperatures.
Hence, option (B) i.e. hot air is lighter than cold air is the correct answer.

Note: It is important to note that it is very crucial to give the functioning of a hot air balloon and how the air is heated with the knowledge of the phenomenon of hot air being lighter than cold air and rising up due to the difference in the masses. Only after that the correct answer is discussed.