Hot Air Balloon Science Experiment
Before we had helicopters, airplanes, or even blimps, there was only one way to travel the skies – in a hot air balloon! With the first successful flight occurring more than two hundred years ago in 1783, the humble hot air balloon will forever hold the record of being the first passenger flight device. The simple concept of using a sealed envelope (the balloon) to carry low density air was revolutionary at the time, and you can recreate the concept at home, or at school, using some basic materials.
Let’s find out how you can make your own small scale hot air balloon, and explore the science behind it.
- Aluminum foil.
- Plastic drinking straws.
- A thin plastic bag, such as a small trash bag.
- Scotch tape.
- Household scissors.
- Small birthday cake candles.
- A lighter or matches.
You will also need plenty of indoor space to conduct the experiment. You could use a classroom, a school gymnasium, a garage, or a living room. Because you will be working with an open flame, adult supervision is necessary.
This experiment will allow you to demonstrate the basic principles of an unguided hot air balloon, on a scale that is safe for indoor use. Before you begin, make sure you have all of the materials, and a clear table or desk to work on as you prepare the experiment.
1. Begin by using the aluminum foil to create a small basket. This will be where the candle will sit. Use the foil to shape a square basket with a flat bottom. The base of the basket should measure around 1.5 x 1.5 inches, with sides that are slightly raised to create a lip around the edge (roughly 1cm).
2. Take a birthday candle and light the wick. Carefully tilt the candle over your basket to line the bottom with melted wax. Extinguish the candle (simply blow on it) and quickly take another candle and attach it to the melted wax. Hold this in place until the wax cools, and the candle will be set in place. You will now have an aluminum basket with an upright candle.
3. Now it’s time to prepare the plastic bag for holding warm air. Using the drinking straws and scotch tape, create a rigid internal frame that follows the shape of the bag. The frame should be made up of four supporting sides that cross over as an ‘x’ shape at the top and at the bottom opening of the bag (think of two hula hoops that are connected to make a spherical frame). The frame should slightly protrude from the bottom of the bag, to hold the basket in place. Ask an adult for help if you’re having trouble with this step. There will be an element of trial and error, and you may need to cut some straws to achieve the best result.
4. Attach the foil basket to the bottom of the frame by using scotch tape. Because of the crossover ‘x’ shape, it should be easy to mount the basket.
Beginning the Experiment
Check that you have followed all of the steps up to this point. Make sure there are no holes in the plastic bag, and that the frame is holding the bag open. Support the framed bag so that it is upright, and have an adult light the candle underneath. Don’t push down on the bag, and only support it from the sides. As warm air fills the bag, your hot air balloon will begin to rise!
How Does it Work?
Why do you think the bag rises as warm air fills it?
As air warms, it starts to expand. Expansion means that the molecules are moving faster than before, and they’re further apart from each other. The result of this is that the warm air inside the balloon is less dense, and therefore lighter than the normal air in the atmosphere. Naturally, the balloon will float. As soon as the flame is removed, or the air inside cools too much, the balloon will begin to fall again, as the air inside begins to match the air outside.
With a commercial hot air balloon, it is possible to continuously regulate the temperature of the air inside the balloon, allowing pilots to raise and lower the balloon at controlled speeds.
What other gasses do you think could be used to make a balloon rise?
If your balloon doesn’t rise, then it is probably too heavy. Try a more efficient frame with less straw, or try to use more candles in your basket, with one on each corner of the basket.
Also, make sure the space between your balloon and your basket isn’t too far, and that there isn’t too much air moving around in the space where you are performing your experiment. (You want as much of the hot air from the candle raising straight up into your model balloon.)
As a final note, if you’re trying the experiment in a large room, attach a string to the frame so that you can easily pull it down in case anything goes wrong.
… and for those who would like to see a bit more speed and higher heights, these are fun ways to do that! ..