Why do bubbles float upward?
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Answer: Air is less dense than water
Air is less dense than water ✓ — Correct! Buoyancy principle: objects less dense than surrounding fluid float upward. Bubble contains air/gas (~0.0012 g/cm³) surrounded by water (1.0 g/cm³)—air is ~800 times less dense! Water's weight pushes air upward (buoyant force). Same reason hot air balloons rise, oil floats on water, helium balloons ascend. Bubble size affects rise speed (larger = faster). Pop at surface—air rejoins atmosphere!
Water pressure forces them — Wrong. Pressure exists but doesn't force bubbles up. Buoyancy drives upward motion—displaced water weighs more than air inside bubble.
Heat makes air rise — Wrong. Heated air rises (convection), but bubbles rise in cold water too—buoyancy from density difference, not temperature.