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Boiling and steam

Heat and matter · level 2, lesson 3 of 6

A question to start

Heat a pan of water and, after a while, big bubbles rise and burst. What is inside those bubbles?

The idea

When water gets hot enough, it boils. Bubbles of steam form deep in the water, rise and burst at the top. Steam is water as a gas.

In a pan in your kitchen, water boils at about 100 °C. However long you keep heating it, boiling water does not get any hotter; the heat goes into making more steam.

steam bubblescloud of tiny dropssteam you cannot see

You cannot see real steam. The white cloud above a kettle is made of tiny drops of water: the steam has cooled in the air and turned back into liquid.

Be careful. Steam and boiling water burn badly. Only grown-ups should handle a boiling kettle.

Worked examples

1. A kettle. It heats water to 100 °C. The water boils, and the kettle switches itself off.

2. Pasta. Cooks boil it in water at 100 °C. Turning the heat up only makes the water boil harder, not hotter.

3. The white cloud. Look closely at a kettle's spout. Right at the spout there is a clear gap: real steam. The cloud starts a little way off.

Try it

  1. What is steam?
  2. At about what temperature does water boil in a pan?
  3. What is the white cloud above a kettle made of?
  4. Why is steam dangerous?
Answers
  1. Water as a gas.
  2. About 100 °C.
  3. Tiny drops of liquid water.
  4. It is very hot and can burn your skin.

Common mistake

"The white cloud is steam." Real steam is invisible. The cloud is steam that has already turned back into tiny drops of water.

Where it leads