Cloud in a bottle

Dr. Sarah Bearchell knows what it takes to excite a large crowd! Here's her tips for conducting the cloud in a bottle demo:

You will need:

  • 2 litre empty fizzy drink bottle (IMPORTANT: DO NOT use an ordinary plastic bottle because it might shatter when you pump it up.)
  • 4cm diameter ball of sticky tack
  • a tyre pump with ball-inflating pin
  • 200ml warm water. (More confident teachers could use 50ml vodka (or similar clear spirit) or 25ml isopropyl alcohol or 25ml nail varnish remover for a more reliable cloud). Remember, your cloud will be made of which ever liquid you choose. DO NOT sniff the cloud if it is made of anything other than water and take appropriate precautions regarding sparks near an alcohol cloud.
  • Black paper or black surface for contrast to show off the cloud.

Before Assembly/Demo:

  1. Complete your Risk Assessment.
    • Key hazards: precautions for safe storage of liquid (if not using water), use of bottle which held a carbonated drink so it can take the pressure, safe child-use of pump, non-inhalation of cloud, spark protection around flammable cloud (if not using water), safe disposal, paper towels to mop up any spillages quickly, double check the bottle is from a fizzy drink (see ‘You will need’).
  2. Warm up your sticky tack or modelling clay and wrap it around the ball-inflating pin of your pump. Make sure that no air can escape around your pin. This will be the bung. Attach it to your tyre pump.
  3. If you are using alcohol, add it to the bottle and seal the cap. Store appropriately. Give the bottle a shake to ensure the atmosphere inside it becomes saturated. OR if you are using hot water to make your cloud, hot (but not steaming) tap water is ideal. Allow it to stand in the bottle for about a minute so that the air above the water becomes water-saturated. You’ll need to think about the demo timing when using water because if the water goes cold, the demo does not work. You could use a thermos to keep the water warm.
  4. Have a practise of the whole technique. The components can all be re-used (unless your water goes cold). Remember to shake the bottle between attempts, this will help to make the air saturated with water/alcohol vapour. Get a feel for how hard the bung needs holding in and when is the right time to release it. Your confidence in doing this will make the demo all the more impressive.

Demo time!

  1. Tell the audience you are going to make a cloud. It’s likely you’ll get some scepticism. Amazing things are possible with the power of science at your fingertips!
  2. On the first run-though, do not explain the science. Just let the audience observe.
  3. Ask for a volunteer. They will be doing the hard work of pumping while you model the experiment. Pupils below Year 5 will struggle to do this quickly enough.
  4. Use your hand to hold the bung firmly in the neck of the bottle.
  5. Now get your volunteer to pump quite quickly as if they were inflating a ball. Get the audience to cheer them along.
  6. It will take 5-10 pump strokes to reach the point where it becomes difficult to hold the bung. Get the volunteer to stop whilst you are still able to hold it in place.
  7. Ask the audience if they are ready to see a cloud.
  8. Hold the bottle up in front of some black paper and quickly remove the bung. Bask in their appreciation of your prowess at cloud-making!
  9. Hopefully they are impressed by your powers. Ask if they would like to see it again and select a different volunteer to prove it wasn’t a one-off.
  10. This time, explain what is happening….
    • ‘In this bottle I have water/alcohol/nail varnish remover. You can see some of it in the bottom, but some of it has evaporated into the air inside the bottle. You can’t see it in the air because the particles are too small – but it is there.
    • When I put the bung in, I am closing the system, making an air-tight seal.
    • When my volunteer pumps, they are squeezing more air particles into the bottle. This creates high pressure inside the bottle – it’s why I had to use an old fizzy drinks bottle, a thin water bottle might explode under the pressure which we have in here.
    • As my volunteer keeps pumping, the lower air pressure outside the bottle means the bung is being pushed out and it becomes harder to hold the bung in place.
    • The tube of the pump nearest the bottle has become hot from the air particles bumping into each other as they get closer together.
    • Inside the bottle the high pressure air can hold a lot of water/alcohol/nail varnish remover without it condensing out. This is why the cloud is not there yet.
    • When I remove the bung, lots of air whooshes out. As it escapes there is a sudden drop in pressure (and a small drop in temperature) which causes the airborne water/alcohol/nail varnish to condense into visible droplets.’

Not finished yet? why not:

  • See what happens if you pump up the bottle when it has a cloud in it. Why does that happen? The audience will have all the information they need to work it out.
  • Just after making a cloud, hold the bottle in one hand and give it a sharp squeeze. Did you get a cloud-ring? You might need to practise but it will really impress the audience!