Why does a hot air balloon rise when the air inside it is heated?

Question

BlackTom AI · Accepted Answer

Consider the scenario: a hot air balloon rises because the air inside the balloon is heated and becomes less dense than the surrounding cooler air.
Option 1: 'The number of gas molecules increases.' In a closed balloon, heating does not create more molecules; it increases their average kinetic energy. The count of molecules remains essentially the same, so this explanation misattributes the rise to quantity rather than density changes.
Option 2: 'The temperature increases, causing the air to expand and become less dense.' This is the key physics: heating raises the air's temperature, which causes the gas to expand. Because the balloon's envelope confines the gas, the expanded air has a larger volume but the same amount of mass, leading to lower density. The less-dense inside air provides a buoyant force that lifts the balloon.
Option 3: 'The pressure inside the balloon decreases.' While external pressure is lower at altitude, the pressure inside the balloon does not need to decrease to rise; in fact, heating can raise internal pressure unless the envelope expands. The rise is primarily about density differences, not a simple drop in internal pressure.
Option 4: 'The air inside turns into a liquid.' This is physically incorrect for typical hot-air balloons operated at temperatures well above ambient air; the air remains a gas, not a liquid, so this option misstates the state of matter under normal operating conditions.

2254 CHEM 0101 SEC2010 HW 9

Why does a hot air balloon rise when the air inside it is heated?

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