Why do hot air balloons rise?
Show answer & explanation
Answer: Hot air is less dense
Hot air is less dense ✓ — Correct! Heating air makes molecules move faster, spreading apart—same mass occupies more volume = less dense. Hot air inside balloon (~100°C) is less dense than cool outside air (~20°C). Buoyancy: denser fluid pushes less dense object upward. Volume of displaced cool air weighs more than hot air inside—net upward force (buoyancy). Same principle as boats floating. Cool balloon—descends. Heat air—rises! Vents control altitude.
Heat creates upward force — Wrong. Heat does create effect, but mechanism is density difference—hot air is less dense, so buoyancy pushes balloon up.
Air pressure pushes them up — Wrong. Pressure gradients exist, but specific mechanism is buoyancy from density difference between hot and cool air.