Condensation Misconception

As a kid I could never understand why water droplets formed/appeared on the outside of a pop can or glass of water.

I used to think that somehow the water from inside was leaking through the bottle.  But that didn't make sense because I didn't always see water droplets form on the outside of a bottle or glass.  It was only under certain conditions that this occurred, usually with very cold water.

So what's going on here?  Obviously water isn't leaking through the glass, so where does it come from?  The water comes from the air.  There is water moisture in the air that condenses onto the glass/bottle surface.  The reason this happens is that heat transfers from warmer objects to colder objects.  If the water is especially cold (with ice in it), heat is transferred from the surrounding air to the cold glass/bottle.  When heat is transferred, the temperature of the surrounding air decreases.  If it decreases enough, the water vapor in the air will change phase from a gas to a liquid and water droplets will develop on the glass/bottle.

The same thing can happen to windows in your house or car.

The next time you see condensation on a glass/bottle or on your windows, take a moment to explain to your kids what is going on.  Don't let them think (like I once did) that water is leaking through the glass!