Skip to content

Condensation

Heat and matter · level 2, lesson 5 of 6

A question to start

Take a cold drink out of the fridge. Soon the outside of the glass is covered in tiny drops of water. The glass does not leak. So where do the drops come from?

The idea

The air always has some water vapour in it, a gas you cannot see. When the vapour touches something cold, it cools down and turns back into liquid water. This is condensation: the opposite of evaporation.

water vapourin the aircold glass

You see condensation in many places:

  • a bathroom mirror after a hot shower;
  • your breath on a cold window;
  • dew on the grass on a cold morning;
  • clouds, which are tiny drops that formed as damp air rose and cooled.

Worked examples

1. The cold glass. The glass cools the air touching it. The water vapour in that air condenses into drops.

2. A steamy mirror. The shower fills the room with water vapour. It condenses on the cool mirror.

3. Dew. At night the grass cools down. By morning, drops have formed on it from the air.

Try it

  1. What is condensation?
  2. Where do the drops on a cold glass come from?
  3. Why does your breath make a cold window misty?
  4. Is condensation the opposite of melting or of evaporation?
Answers
  1. A gas turning into a liquid when it cools.
  2. From the water vapour in the air.
  3. Your breath is warm and damp; the vapour condenses on the cold glass.
  4. Of evaporation.

Common mistake

"The glass is leaking." The water comes from the air outside the glass, not from the drink inside. A dry, cold, empty glass gets wet too.

Where it leads