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Public summary: 

The idea of a vaccum is very different to what we experience in everyday life, with some surprising consequences! Oh, and the experiment involves balloons too!

Various demonstrations using a vacuum pump.
Useful information
Kit List: 
  • Vacuum pump (belongs to Dave)
  • Green vacuum chamber
  • Magdeburg hemispheres.
  • Balloons.
  • Various other bits.
Packing Away: 

Packs into a single large red box, if possible wrap the vacuum chamber in a bag to avoid damage.

Frequency of use: 

There are lots of different bits to this experiment, and there is probably too much to do with any given group of people. Here are some of the things that can be done:

Particle Theory

A good way to start (I think, depends on the age group) is often good to ask what the surroundings are made of, solids liquids gases etc, and introduce the idea of particles (kids, even young ones may have heard of atoms or molecules). You can talk about how particles behave in each state of matter, and then start to focus on the air around you. Many kids will know the names of the gases that make up the atmosphere, so that’s a nice question to ask, then you can explain that you get lots of these gas particles zooming around, but that you can’t normally feel them. Example, wind blowing you backwards. You can also explain that many, many particles bump into you each second, but because you're used to it you don't notice (and your body has evolved to deal with it, if they're old enough to know about evolution). Maybe make a comparison, such as a bag of flour (1kg) on an area the size of a postage stamp (1cm^2).

Weighing Air

Does air weigh anything? Most kids (and adults!) will answer no to this question when asked it straight off, it links in quite nicely with the hot air balloon experiment where we argue that it floats because some air gets forced out of the bottom of the balloon making it lighter. Have a discussion about the difficulties in weighing air. A good analogy is a swimming pool; if you try to lift someone in the pool they feel much lighter than in the air. There may be a plastic pint glass on a piece of string in the box; so if you have a bucket of water you can test the weight of a glass when it is:

  1. Empty, hanging above the water.
  2. Full, hanging above the water.
  3. Full, hanging in the water.

Get a volunteer to say heavy/light in each case. This should get across the idea that when you weigh something the surroundings matter. Now you can measure the weight of air itself. Take the plastic Vacuum chamber and put it on the scales. You can tare them to zero if you want, but this leaves you with a confusing negative mass measurement. Attach on the tube and then get someone to read the mass off, make sure no-one is touching it/leaning on the table too hard! Turn on the pump and (hopefully) they should see the mass go down by around 10g. (be very clear about what the pump is doing...lots of kids will be confused and think it's pumping air in rather than out!) Talk about how heavy this is, try weighing a marker pen/bunch of keys for comparison. So air really does weigh something!

Expanding balloon

Put a partly inflated balloon into the chamber. Evacuate the air from the chamber and see the balloon expand. Open the tap and it will shrink again (and if you do it too fast, it will shred into lots of pieces to the amusement of all). Talk about why this happens. No air can get in or out of the balloon, there are always the same number of particles inside it. Can talk about particles bashing against the inside and the outside of the rubber, when you remove them from the outside the balloon can expand more easily. You can even use a child tapping the palm of your hand on either side to demonstrate the point that if the tapping on one side gets taken away your hand moves. For a cheap "urgh!"; what would happen if I put you into a Vacuum chamber?

How strong is the Vacuum?

The other two experiments will have got across the idea that a Vacuum sucks things. Can talk about how this links in to Vacuum cleaners, which use pressure differences to suck up dirt (this is probably the only place they will have heard the word Vacuum before). Don't let them try to pull the lid off the plastic chamber (though they can put fingers on the end of the tap to feel the sucking effect!), but we have a metal Vacuum chamber that they can pull at hard! Evacuate it and let everyone have a go at pulling it apart. Can talk about the fact that you're fighting to pull against the millions and millions of air particles all bashing against the outside pushing it together.

Vacuum Bazooka

This leads nicely into the Vacuum bazooka experiment, which has a separate explanation/risk assessment (and can't be done in confined spaces!).

Risk Assessment
Date risk assesment last checked: 
Sun, 03/02/2019
Risk assesment checked by: 
Conor Cafolla
Date risk assesment double checked: 
Tue, 05/02/2019
Risk assesment double-checked by: 
Polly Hooton
Risk Assessment: 

Pumping on vacuum chambers to see what happens when you evacuate various things.

Hazard Risk Likelihood Severity Overall Mitigation Likelihood Severity Overall
Chamber The chamber could implode, throwing things out, which could hit spectators. 3 4 12 Use a polycarbonate desiccator as the chamber, as polycarbonate doesn't shatter. Check chambers for damage before using them. Only put a rubber balloon inside the chamber.
Can also use a metal vacuum chamber as this will not implode
Call a first aider in the case of an injury.
1 2 2
Oil mist The pump will produce oil mist in the exhaust. Could cause problems for audience and long-term exposure for demonstrator. 4 3 12 The exhaust from the pump is fitted with an air filter.
If someone expresses discomfort (e.g. feels sick/dizzy) stop using the machine.
In the event of an accident/adverse reaction call a first aider.
2 3 6
Metal vacuum chamber Risk of falling over when two people trying to separate metal vacuum chamber by "tug of war" method (especially if unexpectedly successful). 4 2 8 Don't allow "tug of war" in a confined space/area with risk of bumping into sharp objects/other hazards, sometimes best to make both parties sit down first. If group is (showing signs of becoming) over excited best for demonstrator to hold one end of the chamber to maintain control
Call a first aider in the case of an injury.
2 2 4
This experiment contains mains electrical parts, see separate risk assessment.
Publicity photo: 
Experiment photos: