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Cloud in a bottle


Cloud in a bottle

Creating a cloud in a bottle. (Source: Ruben Meerman/ABC Science Online)

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You'll need medicinal rubbing alcohol (isopropyl alcohol, available from pharmacies and supermarkets) or methylated spirits for this experiment (so adult supervision is required for young children).

Grab a hand operated air pump with a needle valve for inflating sports balls (soccer balls, basket balls, footballs etc.)

Note: you can use a bicycle pump instead of a ball pump but you will also need the valve from an old tyre tube and then modify it to fit the neck of a bottle (eg by drilling a hole through a rubber stopper or cork).

Wind a strip of reusable adhesive poster hanging putty around the needle. The putty will form a seal with the neck of the bottle.

Pour 10 to 20 millilitres of antispetic medicinal rubbing alcohol (isopropyl alcohol) or methylated spirits into an empty soft drink bottle.

If you're a young whipper-snapper, please remember you need adult supervision when handling these liquids.

Rotate and wiggle the bottle to swirl the alcohol around for 10 or 20 seconds to 'encourage' it to evaporate.

Insert the pump to form a seal between the poster putty and the neck of the bottle. Pump air into the bottle until it feels 'full'.

Remove the pump swiftly so that the air pressure inside the bottle drops suddenly and rapidly. The bottle will instantly fill with a dense, opaque white fog.

If you're feeling fancy, you can fire off a few nifty fog rings at this stage by gently squeezing the bottle.

For yet more "wow", reinsert the pump and re-pressurise the bottle while it is still full of dense white fog. As you pump, the thick white fog vanishes right before your eyes and the air turns crystal clear.

If you want to make another thick, dense cloud, you'll need to shake and jiggle the bottle again before repeating the demonstration from the top.


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What's going on?

You've probably heard the phrase 'high pressure, fair weather' or that a 'falling barometer' means rain approaching. This trick demonstrates the phenomenon behind those sayings.

Like water, isopropyl alcohol evaporates to form an invisible vapour. Like water vapour, that invisible isopropyl alcohol vapour can condense again to become visible liquid isopropyl alcohol. If that happens in mid-air (as opposed to on a surface), the tiny droplets produce what we call a cloud (or fog).

While the temperature and pressure inside the bottle remain steady, nothing much happens. When you start pumping air in, the internal pressure rises and the air inside the bottle becomes noticeably clearer. When you suddenly release the pressure, something rather stunning and surprising happens. A dense white cloud forms instantaneously inside the bottle. But why?

The first thing to remember is that pumping air into a bicycle or car tyre increases its temperature. You might have noticed that the pump and valve both get warmer as you pump air in. The second thing you might recall is that a drop in gas pressure causes cooling. You have almost certainly noticed this while spraying deodorant from an aerosol bottle under your armpits. The compressed gas moves from the region of high pressure inside the aerosol can to a region of much lower pressure outside the can, which causes the gas temperature to plummet. You'll notice the same thing if you let the air rush out of an inflated tyre.

So increasing pressure of a gas causes warming and decreasing pressure causes cooling. But there's one more thing. Pumping air into your soft drink bottle increases the internal temperature and therefore also increases the rate of evaporation of the isopropyl alcohol (and some water) from the wet surfaces inside. Releasing that high pressure suddenly causes a rapid and dramatic fall of temperature inside the bottle causing the isopropyl alcohol vapour (and some water vapour) to condense into tiny droplets of visible liquid. Each of those droplets reflects visible light forming a dense white fog, or cloud.

Pumping air back into the bottle causes the internal temperature to rise again so those droplets evaporate once more and the air inside turns crystal clear once more.

The same fundamental principle produces the clear skies and fair weather we associate with high pressure weather systems that form over land. The air in a high pressure system warms as it descends so the skies tend to remain cloud-free. The relatively warm rising air of a low pressure weather system, however, cools as it ascends into the atmosphere causing the invisible water vapour it carries to condense into beautiful, fluffy clouds.

Tags: physics

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Published 21 May 2013

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