What kind of gas is generally used to achieve lift in a hot air balloon?

Hot air is generally used to achieve lift in a hot air balloon because the fundamental principle behind a hot air balloon's operation is based on buoyancy. When air inside the balloon is heated, it expands and becomes less dense than the cooler air outside the balloon. This difference in density creates an upward force, allowing the balloon to rise.

Heated air is circulated throughout the envelope of the balloon, typically using a burner that ignites propane stored in tanks. The continuous heating of the air helps maintain the necessary temperature differential that keeps the balloon aloft.

Other gases, such as helium and hydrogen, can lift lighter-than-air balloons but are not standard for traditional hot air balloons. Carbon dioxide is denser than air and would not provide lift. Therefore, the unique mechanism of using heated air makes it the correct choice for achieving lift in a hot air balloon.