Photo: Thomas Hawk (flickr)
The chilling effect of aerosol cans.
We’re not talking here about the effect of aerosol on the environment, but about something you’ll only notice if you use up most or all of the can in one long burst, like spray-painting a large surface.
By the time you’re finished, the can will be noticeably cooler. In fact, on a already cold day, an aerosol can could get cold enough to cause frostbite.
To see why this works we need to think of the temperature of the can as a measure of the energy contained in the molecules of gas and paint that are inside it. The more energy in the can, the warmer it feels. When you take the can off the shelf, the molecules inside have roughly the same temperature as the surrounding air, so the can doesn’t feel especially warm or cold.
Now, aim that can at the surface you want to paint, and press the button. Out comes a jet of colorful paint.
It takes energy to propel the paint out through the air and onto that surface, just as it takes energy to throw a ball or fly an airplane. The energy to propel the paint comes from inside the can, and so along with the paint, you’ve also used up some of the can’s energy. As the amount of energy contained in the can drops, its temperature drops as well.
Of course the aerosol can won’t stay at that temperature. Like anything that’s colder than the surrounding air, the can absorbs heat, or energy, from the air until eventually it’s back at room temperature.