Hot air is less dense than cool air, so hot air will rise above cool air. Increasing the air temperature inside a
hot air balloon makes it lighter than the surrounding air, so the balloon can lift up. The density of hot air is
about 0.25 kg/mº less than that of cool air, which means a cubic meter of hot air can lift about 0.25 kg.
What is the minimum volume of hot air needed to lift a hot air balloon carrying 500 kg?
Since one cubic meter of hot air can lift .25 kg/m and you want 500 kg, you take 500 and multiply it by 100/25 or 4. Which gives you 2000 cubic meters or hot air.