Try This at Home:

Full of Hot Air

Make your own mini hot air balloon and experiment with physics

What you’ll need:

  • aluminum foil
  • birthday candles
  • scissors
  • ultra-thin plastic bag (ex: thin trash can liner, grocery store produce bag, or dry cleaner’s bag)
  • plastic straws
  • clear tape
  • matches or lighter 
  • string (optional)

Safety Note: Adult supervision required for lighting the candles and making your balloon fly. Be careful with fire and lit candles.

Here’s what to do:

  1. The plastic bag needs to be thin so that it is lightweight. If your bag has any holes or rips (aside from the opening on one end), use clear tape to patch. Use as little tape as you can because the tape adds weight. Note: If you are using a dry cleaner’s bag, tape closed the hole at the top where the hanger pokes through.
  2. Cut a 3 or 4 inch square of aluminum foil.
  3. Fold up the edges of the foil, about ¼” all around, to form a gondola.
  4. Measure the width of your bag’s opening and write it down.
  5. Gather a handful of plastic straws. If they have a bendable top, cut the flexible portion off and discard.
  6. Connect straight plastic straws together to form a rod the same length as the width of your balloon. These will make your frame. To connect the straws, cut a ¼” slit into one end of each straw. Push the uncut end of one straw into the slit end of another. Add a small piece of tape to secure. 
  7. Repeat Step 6 to make a second identical straw rod.
  8. Take your two straw rods and cross them in the center to form an X. Add a small piece of tape to hold the shape together.
  9. Place the X-frame into the opening of your bag and use as little tape as possible to attach it to the bag and hold the frame securely in place. 
  10. Ask an adult to do this step! Use a lighter to melt a bit of wax from the bottom of the candle to make a small pool of wax about ¼” away from one of the gondola’s corners. Quickly and carefully, set the candle in the melted wax and hold it upright until the wax hardens and holds the candle in place. Repeat for the other three candles.
  11. Carefully place your gondola of candles on top of the X-frame of your balloon with the candle wicks pointing up into the bag. Stagger the gondola so the candles rest between, not on top of, the straws below. Tape the gondola in place.
  12. If you will be flying your balloon outside or in a tall room where you may not be able to reach it, tie a long piece of string to the frame so you can reel your balloon back to you when you are done.
  13. Ask an adult for help with this step! While you hold the top of your balloon (the closed end of your balloon) up and away from the gondola, have your adult carefully light the four candles. Keep holding the top of the balloon until it fills with hot air and can stand upright on its own.
  14. After about a minute, your balloon should lift off the ground and begin to rise. If your balloon is full of air but not leaving the ground, try to remove some weight like tape or from the candles. Ask an adult to cut the candles in half and reattach them to your gondola, then try to lift off again.

Take it further:

  • How high does your balloon go? How can you measure this? Does it go higher than you are tall? Higher than your adult is tall? To the ceiling? Try to estimate how high your balloon goes. If you attached a piece of string, mark the place where you are holding it before you reel your balloon in, then measure the length of the string.
  • How much weight can your balloon carry? With the candles not lit, weigh your balloon, frame, and gondola on a kitchen scale and write it down. Then, tape some straw pieces (about 2 inches long) into your gondola to simulate passengers. Start with two straw people. Weigh your balloon again and write down. Try to fly your balloon again with the extra weight. Can your balloon carry two straw passengers? If yes, keep adding straw passengers until your balloon gets too heavy to leave the ground. Did you balloon carry more or less weight than you expected? How do you think real hot air balloons carry people?
  • What happens if you make your balloon from a different size of bag? Try trimming the open end of your bag to make a smaller balloon. Does it lift off the same way and fly as high as the larger balloon? If you used a trash can liner, try a much smaller produce bag. What do you notice? 

Gases, like air, expand (or get larger and take up more space) when they get hot and contract (or get smaller and take up less space) when they get cold. When air is heated, like by the birthday candles, each molecule begins to move faster and spread out further from the other molecules; this makes the hot air lighter than the cold air because the hot air is less dense — there are fewer molecules in the same amount of space. This expansion as a gas gets hot is why hot air balloons fly! The hot air in the balloon is lighter than the colder air outside the balloon. The air outside of the balloon, that is cooler and, more importantly, denser, allows the hot air balloon to float. Air, in general, is not very dense which is why it is important that the balloon is made of lightweight materials and doesn’t get too heavy. The combined weight of the balloon, frame, gondola, and the hot air inside has to be less than the weight of the cooler air that would fill the space if our balloon wasn’t there. This is an example of Archimedes principle! Read about how he supposedly discovered this principle here and why that is probably not the whole truth here.

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