Why is it that, when spraying an aerosol can of deodorant, it suddenly becomes very cold when it runs out?
David Jackson
Gosport, Hampshire, UK
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Many aerosols use a mixture of the hydrocarbon gases propane and butane as their propellant 鈥� the material that drives the contents out. This is because these gases are liquids at a couple of atmospheres of pressure, which is easily contained in a metal can.
Anyone who has handled a transparent gas lighter will have seen this, where the butane fuel is visible as a liquid with gas above it. The same is true in the aerosol can, with the product mixed in with the liquefied gas.
When the can button is pressed, the pressure is released and the gas and product spray out. Meanwhile, some of the liquid propellant turns to gas to maintain the pressure, so the can always seems to have the same spraying ability. This causes a slight, but much less noticeable, temperature drop unless the can is used for a long time.
When the liquid product has all gone, the propellant can come out much faster, as the gas doesn鈥檛 have the product 鈥渋n the way鈥�. The remaining liquid propellant therefore evaporates faster and cools more noticeably.
As an aside, propane and butane aren鈥檛 directly greenhouse gases, though once in the atmosphere they oxidise to water and carbon dioxide, and the latter is, of course, a greenhouse gas.
David Muir
Edinburgh, UK
When you sweat, the sweat evaporates. The phase change from liquid to gas requires heat to break the intermolecular bonds between the molecules of liquid. This heat is supplied by your body, which is why sweating cools you down.
Aerosol propellants readily boil at room temperature and are only liquid inside the can due to pressure. When the nozzle is pressed, the pressure is released and the propellant boils, taking heat from its surroundings, mainly the liquid in the can. This then cools the can slightly 鈥� not easy to notice with a short spray.
With continual spraying, condensation may form on the outside of the can and might even freeze. This shouldn鈥檛 be tried indoors, as some propellants are highly flammable.
When the can is almost empty, the last 鈥渟coosh鈥� requires as much heat as the first, and all that is left to heat the propellant is the can. The resultant temperature drop feels cold to your sensitive fingers.
Guy Cox
Sydney, Australia
Your aerosol consists of a liquid 鈥� the deodorant 鈥� and the propellant, a compressed gas.
When a gas expands, in the absence of external heat, it cools. That is because work is done moving the molecules further apart, and the energy for that has to come from the temperature of the gas. Hence air is thinner, and cooler, in the mountains.
So every time you use your spray, it will actually be cooled, but the amount of gas stays the same and the amount of liquid decreases, so initially the effect won鈥檛 be noticeable.
Of course, after use, everything comes back to room temperature. But once there is only a little liquid left, the cooling effect makes itself felt.
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