Why do helium-filled balloons float in air?

What mass of natural gas (ch4) must you burn to emit 276 kj of heat?

When the chemicals iron sulfide (fes) and hydrochloric acid (hcl) are combined, bubbles appear from the mixture. 1. does the appearance of bubbles indicate a physical or chemical change? 2. why do the bubbles indicate this change? 3. what property is this?

Gravitational potential energy is related to