Question

The hot air balloon rises because it has a gas that is
(A) Denser than air
(B) Less dense than air
(C) Equal dense as air
(D) The given statement is wrong

Answer 1

Hint: To find that property of the gas because of which hot air balloon rises in upward direction we will first find out that because of which force a gas moves up in the sky.
After finding that force we will understand the properties of that force on what condition it is applicable.
Then we will see the property of gas used to raise the balloon and find the correct option.

Complete step by step solution
We have to find the properties of the gas responsible for the rise of hot air balloons. So, for this we have to find the concept of the force which is applicable on the hot air balloon which makes it rise.
The force which makes a hot air balloon rise is the buoyant force. All liquids and gases in the presence of gravity applies an upward force called buoyancy on any object.
Hot-air balloons can float in the air due to buoyant force of buoyancy, an upward force that the air applies on them.
Since the balloon is heavier than air, it should be filled with a large volume of much lighter things—either hot air or a light gas, example: helium. Because the combined weight of the balloon and the gas is less than the weight of an equal volume of surrounding air, the balloon rises.
So, we notice that hot air balloons have hot air inside. Also, hot air is less dense than cooler air which surrounds the hot air balloon. This means hot air is lighter than cold air. Hence due to buoyant force the balloon rises.
So, by the above concept we see that the gas should be less dense than air.

Therefore, option B is correct.

Note: Remember the balloon must weigh less than the air it displaces (the air that would occupy the same space if the balloon wasn’t there).
The calculation of the buoyant force in gases is less difficult than compared to liquids, since a body is usually completely submerged in the gas.
Also remember that If the object is less dense than the liquid, buoyancy will make it float. For example, a cork floats in water because it is less dense than a cork-size volume of water.