by Chris Woodford. Last updated: June 24, 2018.
Pssssssssssssst! Everyone loves the convenience of aerosol cans. They make it easy to paint, polish, and lubricate—and lots of other household chemicals, from deodorants and hairsprays to air fresheners and detergents, come in them too. Let's take a quick look at the two main kinds of aerosol dispenser (cans and misters) and find out how they work.
Photo: An aerosol emerging from an aerosol can. Aerosol cans are a simple and convenient way to package all kinds of household chemicals. It's always a good idea to shake them before you spray them so the chemical you want to release and the propellant (the gas that carries it) are properly mixed together.
What are aerosols?
Photo: Aerosols drifting over my home. Clouds are aerosols of water vapor dispersed through air. Interestingly aerosols (naturally created ones such as smoke from forest fires as well as the ones we spray) have interesting and very complex effects on cloud formation. You can read a little bit more about that from NASA.
Aerosols aren't aerosols at all. No, really, let's be clear about this. An aerosol is really the cloud of liquid and gas that comes out of an aerosol can, not the can itself. In fact, to be strictly correct about it, an aerosol is a fine mist of liquid, or lots of solid particles, widely and evenly dispersed throughout a gas. So clouds, fog, and steam from your kettle are all examples of aerosols, because they're made up of water droplets dispersed through a much bigger volume of air. Smoke is an aerosol too, though unlike those other examples (which are liquids dispersed in gases) it's made up of solid particles of unburned carbon (soot) mixed through a cloud of warm, rising air. Even candles make aerosols: the smoky steam swirling above a candle flame consists of soot and water vapor dispersed through hot air.
How do aerosol cans work?
Photo: Many aerosols contain flammable propellants, such as butane or propane. Always read and follow the warnings on the back of the can. Don't point aerosols into your face and don't breathe in the spray.
Now you know what an aerosol is, you can see what an aerosol can is all about: it's a mechanism designed to turn a liquid, such as paint or polish, into a finely dispersed mist. So how does it work?
If you've ever read the back of an aerosol can, you'll have noticed messages such as "pressurized container" and "contents stored under pressure." What's that all about? To ensure that something like paint comes out evenly when you press the button on the top of an aerosol can, the manufacturers have to squeeze the contents inside with a pump or compressor (a bit like inflating a bicycle tire). Typically, the contents of an aerosol are stored at 2–8 times normal atmospheric pressure (and usually the lower end of that range). That's why aerosols really rush out when you press their buttons.
Now we can't easily pressurize liquids, so just pumping something like liquid paint into a can isn't going to make an aerosol that works properly. Fortunately, we can pressurize gases very easily. So, in practice, aerosol cans contain two different substances: the liquid product you're interested in releasing (the paint, detergent, hairspray, or whatever it might be) and a pressurized gas called a propellant that helps to push the liquid product into the air and turn it into an aerosol cloud. The propellant gas usually turns into a liquid when it's forced inside the can at high pressure during manufacturing. That makes the propellant and the product mix together (and you can help to ensure they do so by shaking the can before you use it). The propellant turns back to a gas (evaporates) when you push the nozzle and the pressure is released. It disappears into the air leaving behind the product you're really interested in.
Evaporation is the main reason why aerosols feel really cold when you spray them on or near your body. In the case of a typical aerosol (such as a deoderant or perfume mister), the contents aren't gases but (usually) volatile liquids (ones that evaporate at everyday temperatures). When they leave the nozzle, they immediately evaporate. The molecules of liquid have to be ripped apart from one another and separated to turn them into a gas, which takes lots of energy (technically known as the heat of vaporization). This energy is sucked into the liquid/gas from its surroundings, causing them to cool. Evaporative cooling also makes aerosol or mister spray feel cold when you blow it on your skin: your body provides the energy that turns the spray from liquid to gas. The pressure inside the can is reduced slightly each time you spray some contents out of it, which means there's some evaporation and cooling going on inside as well as outside, causing the can to cool down too.
Artwork: Why aerosols feel cold. When you spray liquid droplets on your arm, they quickly evaporate. For a liquid (left) to become a gas (right), its molecules (shown here as yellow dots) have to be pulled apart and separated, which takes energy. That energy is sucked from your arm, making it feel cold.
Until the 1980s, chlorofluorocarbons (CFCs) were widely used as the propellants in aerosol cans, but they were banned after scientists discovered conclusively that they damaged Earth's ozone layer. (No wonder, really, when you consider that something like 10 billion aerosol cans are used and thrown away each year.) Now other chemicals are used as propellants instead, including the gases propane and butane. Although these gases don't damage the ozone layer, they do have other drawbacks: they can be harmful to inhale and they are highly flammable.
An aerosol can would be entirely useless if there weren't some way of allowing its contents to escape in a very controlled way. That job is done is by the valve at the top of the can—just underneath the button you press—which has a spring to stop it staying permanently open. When you force the button down against the pressure of the spring, the valve opens and reduces the pressure at the top of the can, allowing the contents to escape as an aerosol. Release the button and the spring closes the valve again.