Science Puzzle
Why Does a Cold Can Sweat?
Take a can of drink out of the fridge on a warm, muggy day and, within minutes, its outside is covered in beads of water. Leave it for a while and a small puddle forms around its base. The can is sealed and isn't leaking.
Where does the water on the outside of the can come from?
The Answer
From the air. The room is full of invisible water vapour; the cold can turns some of it back into liquid water.
Air always carries some water as a gas, water vapour, which you can't see. How much vapour air can hold depends on its temperature: warm air can hold a lot, cold air much less. When the warm kitchen air touches the cold can, the thin layer of air against the metal cools right down. Once it drops below a certain temperature, called the dew point, it can no longer hold all its vapour, and the extra condenses into droplets on the can.
You've seen the same thing many times. Mist forms on a bathroom mirror after a hot shower, dew appears on grass after a clear, cool night, and your breath turns cloudy on a frosty morning. Clouds themselves form this way, when rising air cools until its water vapour condenses into tiny droplets.
That's also why a cold can sweats a lot on a humid day and hardly at all in a dry desert: the more vapour the air holds, the more there is to condense.
The principle: Condensation and the dew point. Air holds invisible water vapour, and when it's cooled below its dew point the vapour condenses into liquid water on cold surfaces.