Unit E: The Nature of Matter

How does a hot-air balloon rise and stay in the air?

1. Get Set to Explore


  • density: The amount of matter in a unit of volume; we say that something which has a high density is dense.

Building Background

  • Review the definition of density with the class. Encourage students to show their understanding of the concept by using the words density and dense in context.
  • Ask students if they think air is dense. Guide them to realize that as a gas, air is less dense than liquids and solids. Then ask students how they think heating air would affect its density. Let students share and explain their ideas. Help students gain an understanding of the effects of heat on matter. Heating matter causes the particles that make up the matter to move faster and faster and to spread apart. In this way, heated matter is generally less dense than cooled matter.
  • Show students a picture of a hot-air balloon, or refer them to the picture on Student Edtion page E46. Ask students how they think heating the air inside the balloon affects the air. Elicit the idea that heating the air causes the density of the air to decrease. In other words, the air inside the balloon becomes lighter.
  • Read the Discover! question to the class: How does a hot-air balloon rise and stay in the air? Prompt students to put together the concepts you just discussed, related to density and to heat, to come up with answers. Write their answers on the board, and keep them posted while students do the simulation.

2. Guide the Exploration

  • Challenge students to launch the Discover! Simulation to learn more about how hot-air balloons fly. They should listen closely to the question and the directions.
  • If necessary, explain that moving the cursor over the balloon, instruments, and landing pad gives important information about each.
  • Point out that students will get a minute to fly the balloon and to try to land it on the landing pad. They click the buttons to turn on and off the burner and to open and close the vent. As they fly the balloon, the main thing they will need to watch is the fuel gauge, to prevent running out of fuel. Point out that if they run out of fuel in this game, they will not be able to reach the landing pad.

3. Review/Assess

  • After students complete the simulation, have them return to their ideas about what causes a hot-air balloon to rise and what causes it stays in the air. Let them make any corrections to their earlier ideas, posted on the board.
  • Then discuss Step 3's Wrap-Up text with the class. Encourage students to share observations and to ask any questions that they may have.
  • Let a volunteer read the Extension question to the class. Have students work in small groups to formulate an answer. Allow them to return to the simulation, as necessary. After three to five minutes, ask the groups to come back together to share answers with the class. Help students recognize the following relationships:
    • When the hot-air balloon is rising, the air inside the balloon is less dense than the air outside the balloon.
    • When the hot-air balloon remains level, the air inside the balloon has the same density as the air outside the balloon.
    • When the hot-air balloon is going down, the air inside the balloon is denser than the air outside the balloon.

If time permits, present children with the following questions:

  • Critical Thinking Evaluate What did you find most surprising about how the hot-air balloon worked? Why did you find this surprising? Answer: Encourage students to focus their thinking on scientific facts related to hot-air balloons and how they work. For example, students may be surprised that a hot-air balloon can be so heavy and yet can float in the air; accept other ideas that students relate to scientific concepts or principles.
  • Inquiry Skill Infer How do you think the wind would affect a hot-air balloon? Explain your answer. Answer: A hot-air balloon flying with the wind might need less fuel than a hot-air balloon flying into the wind; up-drafts might carry a hot-air balloon upward, but make it harder to land the balloon; down-drafts would do the opposite.

4. Reaching All Learners


Ask interested students to learn more about hot-air balloons, how they work, where they fly, what kinds of variations there are among them, and where the big hot-air balloon festivals are held. Students can prepare a poster report, an oral report, or both, and share what they learned with the class.