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Hot Air Balloons

Public summary: 

Make your own hot air balloon with a plastic bag and a toaster!

Making hot air balloons with plastic bags and a toaster.
Useful information
Kit List: 
  • Bottle with balloon attached to the top (thinner balloons that expand more easily work better, check that there are no holes in the balloon near the neck of bottle).
  • Toaster with cardboard shield to fit around it.
  • The "balloons": Bin liners (preferably white) - Tesco drawstring swing bin liners fly really level. Otherwise, bits of gaffer tape should be attached to the open end to weight the balloons a little (a roll may be found in the box, else look in the gaffer box).
Packing Away: 

(NB: The box might not shut properly if the toaster is too big, so don't waste too much time trying to make this happen...)

Frequency of use: 

This is a neat experiment which kids often find quite exciting, particularly in a large room (preferably away from any outside doors on windy days, even small amounts of wind are a pain for this one).
You can draw nice links to with the vacuum demonstrations (which can involve similar ideas of the particle theory of gases and show that air is not nothing!)*.

Bottle with balloon

It's nice to start by asking the kids what is in the bottle (lots of kids will say nothing [more rarely they say water if the bottle hasn't been fully dried!], this experiment is a good way of seeing that air isn't nothing!), and what will happen if we heat it up (the balloon expands, some of them will wishfully say it explodes). Depending on how old they are and how much they know you can talk about the following while you do the heating:

  • Air is a gas (some children will be confused by this as they think it's a mixture of different gases which is not relevant here, but can be something to talk about if they're interested).
  • Gases usually expand when they get hot.
  • What this means in terms of particles, ie they gain energy, move faster so fill up more space. A reasonable analogy is that of a class of children all walking round past each other needing less space than the equivalent class running (you can extend this to sitting still in rows in assembly [solid] and walking in a line back to class [liquid]).

Heating is best done by running it under a hot tap, then you can cool under the cold tap, but you can use the toaster if you don't have a sink (for obvious reasons don't do a mixture of the two!). If using the toaster, you need to hold it a good distance away (at the top of the shield) to avoid melting the bottle.


You could ask the kids to think of something powered by hot air (many get it straight away, some need a remarkable amount of prompting and loads say aeroplane...).

To make a balloon fly:

  • Put the cardboard shield around the toaster, it's easiest if you turn it on first.
  • Put the bag over the shield, pull it all of the way down to the bottom (or, hold on to the top, and hold it about as high as possible - this way the top of the balloon is further from the heat source and less likely to melt and deform).
  • Let the air inside heat up and off it goes.

You can get sensible kids to do this themselves.


Things to talk about include:

  • What is at the bottom of a hot air balloon? Aside from the basket with people in, there is a burner that heats the air, and gives it more energy.
  • Pretend that the bag is air tight (actually not far off being true, very little air escapes out the bottom), thus it only has a finite amount of air in it, which initially weighs the same as all the surrounding air. When heated the gas expands, which can be seen as the balloon puffs up slightly (although the effect is subtle sometimes) and hence there is the same amount of weight spread over more volume, so each "bit" of air i.e. fixed volume element, weighs less than it did before, thus less than the surrounding air and the balloon experiences buoyancy (you could use an analogy about the difference between a sponge and a brick of similar sizes dropped in the bath and say that if you expand the brick it would become more like a sponge).
  • Now pretend like air is lost. Where does the air go when it expands? It fills up the bag then escapes out the bottom. This makes the bag lighter, so it floats up to the ceiling. N.b. either explanation works, the first is more thorough but the second may be more intuitive especially for younger children. They are both in part true, though the first is probably dominant. If they all say "it rises", you can explain that we're trying to understand why it rises, or hold the bag down and say that it can't be rising because there's a bag in the way!
  • Get them to make a prediction for what will happen - it's usually a balance of the bag will explode or the bag will rise up - you can then test their competing scientific hypotheses!
  • Why do we have the bits of tape on the bottom? They are weights helping the bag go up in a straight line, if it gets tipped over sideways a bit then the weight pull it back straight again (hold the bag over your arm and tip it to demonstrate this). Some kids will know that real hot air balloons carry sand bags as ballast, this is more to help them control their height than to stop tipping, the weight of the basket/passengers is enough for this. You can try launching a bag without any weights, you should find that it tips over and falls back down quite quickly, though you need a relatively high ceiling to see this. Even with the weights, the bag often tips over with the opening at the top - here is a good chance to prove that the hot air is what gives it buoyancy (not the bag itself) because the bag quickly deflates (as the air can now rise out of the hole) and sinks back down.
  • How can we make it go higher/faster? Heat up the air more at the beginning by holding down the bag. Be a bit careful doing this, you can melt the plastic bag quite easily and it all gets quite hot. Different sizes of bag would be an option as well - some definitely fly better than others!

Further discussion:

  • What difference does the temperature of the room make?
  • Will the balloon stop going up? Would this balloon work in space, why not? What would happen as it approached space? (tip: as it rises density of surrounding air drops until the weight of displaced air = weight of balloon and it stops rising)
  • How does this relate to buoyancy in water? Can lead into some great discussions about how fish and submarines change depth (by expanding and contracting their volume, one way or the other)

*(We used to have a vacuum chamber as well as the spheres and bazooka, this allowed you to show that air does have some mass, so the bag does really get lighter when you lose some air, this will surprise most children and their parents. With good groups this is best done after the balloons, though less attentive children may lose interest once they've seen the exciting bit. Vacuums can however be used to start off the theory and lead into the idea of gas particles.)

Risk Assessment
Date risk assesment last checked: 
Sun, 05/01/2020
Risk assesment checked by: 
Date risk assesment double checked: 
Sun, 05/01/2020
Risk assesment double-checked by: 
Andrew Sellek
Risk Assessment: 

Making hot air balloons out of bin liners fly in a room. The air is heated using a toaster at the bottom of a card tube over which a bin liner is placed.

Hazard Risk Affected Person(s) Likelihood Severity Overall Mitigation Likelihood Severity Overall
Heat Heat may cause burns from direct contact.
Tube or crumbs may catch fire.
Bin liners can be melted.
All 4 3 16 Make the tube out of card, which isn't very conductive and will not melt and stick to people.
Use a cool-wall toaster which will therefore not get too hot.
The tube should make it very difficult to put fingers in the hot parts of the toaster.
The tube is supported so it can't fall over easily.
Use a toaster that hasn't been used to toast bread so there are no crumbs.
The card is white so will go brown before it burns - if it goes brown turn it off.
If there are burning smells turn it off!
Do not hold bin liner over toaster for longer than a few seconds or use multiple bags.
In case of burns run under a cold tap for at least 10 minutes. In case of fire, turn off electricity then follow procedure in venue RA (raise alarm, evacuate...) Call first aider in case of injury.
2 3 6
Toaster cable/electrical Electrical hazard and cables present a trip hazard. All 3 3 9 Use a new or PAT tested toaster.
Ensure cable is either positioned where people will not be walking or taped down.
Call first aider in case of injury.
2 3 6
Bin liner Risk of suffocation if it lands on a child’s head. Public 2 5 10 Make sure spectators are not standing too close where the bag could land on their heads, and try to catch before it can land on them.
Suspend experiment if the children are getting silly and trying to catch it with their heads.
Call first aider in case of injury.
1 5 5
This experiment contains mains electrical parts, see separate risk assessment.
This experiment is sometimes run outside during CBS!, see separate risk assessment.
Publicity photo: 
Experiment photos: