The idea for this experiment came from my wife’s camera. She loves taking photos at the beach, but sand keeps finding its way into the camera. I got a can of compressed air to blow the grit away. As I used it, I knew that it would make a great science activity.
To try this, you will need;
- a spray can of something that you don't mind wasting a bit of
We will spray the spray can for a few seconds. If you have a project that requires spraying that is perfect. Otherwise, take the can outside, where you will not make a mess.
First, notice the temperature of the can. It probably does not feel particularly warm or cool. Point the can away from anything that might get messy (including younger brothers and sisters). Press the button and spray for about ten seconds. Quickly notice the temperature of the can again. Its cold!
Why would spraying make the can cold? Actually, there are two things at work here. First, the can contains compressed gas. When you compress a gas, it gets hotter. You can test that the next time you put air in a tire. When you finish adding air, feel the valve. It will be quite warm. The reverse is also true. When a gas expands, it gets cooler. These two ideas are the basis of your refrigerator and air conditioner. A gas is squeezed by a device called a compressor. This makes the gas quite hot. The hot gas moves through metal coils that let the heat move from the gas to the surrounding air. That is why warm air comes from under your refrigerator. Then the gas is moved to the area that you want chilled. There, it is allowed to expand. That causes it to absorb heat from its surroundings, leaving things there much cooler.
But expanding gas is only part of the cooling. Shake the spray can. You should feel something sloshing around inside. Under enough pressure, a gas can change to a liquid. As the pressure is reduced by spraying, some of the liquid evaporates and becomes a gas again. This process absorbs a lot of heat. You can experience that by placing some water on the back of your hand. Blow on the wet skin and it will feel cold. Blowing speeds up the evaporation of the water. As it evaporates, it absorbs heat and cools your skin. That is one reason that we sweat. As it evaporates, it cools our bodies.
The cooling of the spray can is caused by a combination of expanding gas and evaporation. Both processes cause heat to be absorbed, so the surrounding area is chilled. The longer you spray, the colder the can will get. Spray cans don't contain much gas, so even if you sprayed the entire can, it would not get cold enough to freeze your fingers. For that you need something really cold, like a very large bowl of ice cream, preferably with hot fudge sauce (To help prevent frost bite, of course.)