Museum of Science and Industry

Fly a Hot-Air Balloon

See what happens when air is heated as you build and fly a huge hot-air balloon.

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Lesson at a Glance

When air is heated, the molecules in the air begin to spread out and the air becomes less dense than the surrounding air. The more dense air falls beneath the hot air, forcing it upward. See that principle at work as you build a huge hot-air balloon and watch it take flight.

Students Will Know:
Thermal energy - The movement of the molecules that make up an object
Convection - The movement of heat by a liquid (such as water) or a gas (such as air)
Convection current - Air or liquid moving from one location to another while carrying heat along with it

Students Will Be Able To:
Assemble and launch a hot-air balloon
Explain why the balloon rises when hot air is added

Illinois Learning Standards

Late Elementary
State Goal 12.C.2a; 12.C.3a-b; 12.C.4a; 12.C.5b

Background Information

The more thermal energy an object has, the faster its molecules move. These moving molecules bump into each other more often as the thermal energy increases. Imagine children standing in a sandbox. If they begin bumping into each other, they will spread out and require more space. This is what happens when molecules get more energy and start moving around – they spread out, thus decreasing the density of the substance.

Convection is one of the ways heat is transferred from place to place. Convection is the transfer of heat by the movement of warmed matter, especially liquids and gasses. As a liquid or gas moves from one location to another, it carries heat along with it in a convection current. Due to their lower density, heated fluids (such as water or air) rise and cooled fluids fall.

When air is heated, the molecules in the air begin to spread out and the air becomes less dense than the surrounding air. The more dense air falls beneath the hot air, forcing the warm air upward.