Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: Hot air balloon rises up in sky, because hot air is less dense than cold air.
Reason R: Volume of a gas decreases on heating.
In the light of the above statements, choose the most appropriate answer from the options given below.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A is correct but R is not correct
Assertion A — True. When the air inside the balloon is heated, it expands and becomes less dense than the cooler air around it. The cooler, denser air gives an upward push (buoyant force) greater than the weight of the balloon, so the balloon rises.
Reason R — False. A gas expands when heated: at constant pressure its volume increases with temperature (Charles’s law). That increase in volume is why hot air is lighter for the same space.