3.5

## University of Exeter

Skip to 0 minutes and 5 secondsClouds, we see them everyday in the skies over our heads but did you know you could make your own, in your own home, in a bottle. You just need three things - some water, a bottle and some matches. First things first you need to put the water in the bottle . You can pretty much use any kind of plastic bottle but with one this size we found it work best if it is around about 3/4 full. Put the lid on and give it a little bit of a shake. This allows the air at the top to become saturated with water vapour.

Skip to 0 minutes and 32 secondsNow give it a squeeze, you're raising the pressure at the top of the bottle and the temperature, let it go and you're reducing the pressure and the temperature but nothing is happening. Well this is why we need the matches strike a match and pop it in the water. It soon goes out, you can't see any smoke particles in there, but now put the lid back on and give it a squeeze, as soon as you release, you'll see your cloud forming in the top of the bottle. So what's going on here?

Skip to 1 minute and 9 secondsWell initially not very much, not until we added the match because the match adds tiny smoke particles and you need those tiny particles - condensation nuclei - for the water droplets to form. You can't see the water vapour in the air initially, but give the bottle of squeeze and it raises the pressure and the temperature. Let it go, the pressure drops, that allows the water to cool and condense and it will form those tiny droplets, but only if you've got those condensation nuclei in there added by the smoke. They form tiny water vapour droplets and that is the clouds that we see every day over our heads

# Cloud in a bottle

If you try this out for yourself at home, please be careful, particularly around the potential fire risk when using the matches. We recommend you complete the experiment well away from flammable materials, in a room with a high ceiling, or even better, do the experiment outside.

There’s a worksheet to accompany the video demonstration in the files section below.

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