Hot Air Balloons

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SWBAT use provided materials to construct a hot air balloon that lifts off the ground.

Big Idea

Students use the design process to create a hot air balloon that operates using candle power.


5 minutes

Hot air balloons always tend to make me lookup at the skies in wonder. I have always wanted to incorporate hot air balloons into my unit on the gas laws but I have never been satisfied with any of the hot air balloon kits that I have seen from science supply companies, either they were too expensive, too scripted, or too time consuming. 

When I came across a lesson that used garbage bags, birthday candles and other materials typically found in science classrooms, I knew I had finally come across an idea that I had to try (of course I was no longer teaching chemistry but engineering design projects transcend content, right?).

When students enter the class they are introduced to the challenge: choose from the materials provided to construct a hot air balloon that lifts off the table and be able to carry extra weight. Designs must incorporate a place to add extra mass (paper clips) once aloft so we can determine which design holds the most mass.

I provide students with some time to look at the materials table (described in the next section) and sketch out their initial design ideas. The following images show the type of sketches I require from the students.



150 minutes

I like to set up a table that has the materials students can choose from and allow them open access to these materials for building purposes. I like to use this strategy as it allows me to "hide" the materials students will ultimately need with similar materials that don't work as well/as often, for example, I have found one type of garbage bag works better than the others.  Rather than tell students, I allow them to experiment with both so they get to fully experience of the design process.  Plus, students often come up with ideas that I just don't think of; the "table of nonsense" (as we have come to call it) helps students think outside the box as they design their contraptions.

It is important to have a discussion with the students on respect and responsibility as I don't want them to be wasteful of these materials. By this point in the year, my relationship with the students is such that they really just need a reminder of the expectations, daily, to help them control their teenage impulses.

I include a few tips for design success in my reflection. These aren't tips I give my students but information that you might find useful.

The items I offer include, but are not limited to the following:

  • various size garbage bags, all with a thickness of less than 10 µm
  • straws
  • aluminum foil
  • birthday candles (I limit to between 2-4 candles, they can be cut in half to limit mass while increasing heat production)
  • thread
  • craft sticks
  • pipe cleaners
  • tape
  • hot glue/gun (I limit groups to 1-2 glue sticks to help them be conservative)
  • paper clips
  • wood and/or school glue

There are a few building requirements students must be aware of prior to building:

  1. Their candles/candle holding apparatus must be layered with foil to protect from dripping wax.
  2. Designs must consider location of the flame during testing. Balloons deemed unsafe must be corrected prior to being tested.
  3. Balloons must have a location for the extra mass (paper clips) to be added.
  4. Balloons must incorporate a place to secure to the tether.  Students can determine how to accomplish this task.

I keep the only lighter and students are not able to test their balloon until I am present. When students are ready for testing, they must secure their balloon to a tether to ensure they do not lose control of their balloon. I use a 100g mass from the triple beam balance that has a pre-measured string length with a slip knot students can secure to their balloon.

When testing, students need to hold their balloon away from the flame until the balloon fills with warm air. Also, keep a fire blanket handy during testing. On some occasions, some balloons can start melting or the strings can ignite. I have never had any major problems but better to be prepared. (I would not suggest using this activity if you do not have access to a fire blanket). 

The following images and video provides an idea on what students put together and how I test the balloons. Following each test I ask students some questions to get them thinking about their redesign, but honestly they develop their own ideas during the testing - I mostly just remind them to record on their original drawings what happened during the test and what are they going to try next and edit their drawing.




10 minutes

Post activity discussion questions:

  1. What factors affected if the balloon would fly?
  2. Which methods worked best for getting the balloon to lift?
  3. Did having more or less candles work better?
  4. Rate your balloon on a scale of 1-4, with 1 being the lowest and 4 being the highest rating.  Justify your rating with evidence from the testing.