Brightly colored hot air balloons take flight. How do they work? Air is a mixture of many gases, primarily nitrogen and oxygen. When any gas is heated, its molecules gain kinetic energy and push harder against the inside walls of their container. The density of the gas also decreases because of the spreading out of the gas molecules. The hot air balloon expands and the heated air causes it to rise since the hot air is less dense than the cooler air outside the balloon. While in flight, the balloon operator keeps the air in the balloon hot by occasionally blasting the air with a large burner. In this chapter, you will learn all about the properties of gases and the mathematical relationships between the pressure, volume, temperature, and amount of a gas.
Garrett Heath. http://www.flickr.com/photos/garrettheath/10262331596/. CC BY 2.0.