Which gas is lighter than air and commonly used to fill balloons?

• A. Helium
• B. Oxygen
• C. Nitrogen
• D. Carbon Dioxide

Answer: (A)

When a balloon rises, it’s because the gas inside is less dense than the surrounding air. Air has a density of about 1.2 g/L at sea level. Helium is much lighter, around 0.18 g/L, so a helium-filled balloon has a lower overall density than the air around it. That makes the balloon buoyant: it displaces air and experiences a net upward force that overcomes its weight, so it climbs.

Helium’s lighter-than-air property, combined with its inertness (it won’t react or catch fire), is what makes it the common choice for balloons. The other gases listed—oxygen, nitrogen, and carbon dioxide—are all denser than air, so balloons filled with them wouldn’t rise in air.