So increasing pressure of a gas causes warming and decreasing pressure causes cooling. You'll notice the same thing if you let the air rush out of an inflated tyre. 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 have almost certainly noticed this while spraying deodorant from an aerosol bottle under your armpits. The second thing you might recall is that a drop in gas pressure causes cooling. You might have noticed that the pump and valve both get warmer as you pump air in. The first thing to remember is that pumping air into a bicycle or car tyre increases its temperature. A dense white cloud forms instantaneously inside the bottle. When you suddenly release the pressure, something rather stunning and surprising happens. When you start pumping air in, the internal pressure rises and the air inside the bottle becomes noticeably clearer. While the temperature and pressure inside the bottle remain steady, nothing much happens. If that happens in mid-air (as opposed to on a surface), the tiny droplets produce what we call a cloud (or fog). Like water vapour, that invisible isopropyl alcohol vapour can condense again to become visible liquid isopropyl alcohol. Like water, isopropyl alcohol evaporates to form an invisible vapour. This trick demonstrates the phenomenon behind those sayings. You've probably heard the phrase 'high pressure, fair weather' or that a 'falling barometer' means rain approaching. 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. As you pump, the thick white fog vanishes right before your eyes and the air turns crystal clear. If you're feeling fancy, you can fire off a few nifty fog rings at this stage by gently squeezing the bottle.įor yet more "wow", reinsert the pump and re-pressurise the bottle while it is still full of dense white fog. The bottle will instantly fill with a dense, opaque white fog. Remove the pump swiftly so that the air pressure inside the bottle drops suddenly and rapidly. Pump air into the bottle until it feels 'full'. Insert the pump to form a seal between the poster putty and the neck of the bottle. Rotate and wiggle the bottle to swirl the alcohol around for 10 or 20 seconds to 'encourage' it to evaporate. If you're a young whipper-snapper, please remember you need adult supervision when handling these liquids. Pour 10 to 20 millilitres of antispetic medicinal rubbing alcohol (isopropyl alcohol) or methylated spirits into an empty soft drink bottle. The putty will form a seal with the neck of the bottle. Wind a strip of reusable adhesive poster hanging putty around the needle. 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). 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). Video: Cloud in a bottle (Science Online Video)Ĭan't see the video? Download an mp4 version of this video.
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