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Blowing bubbles

Air and fluids · level 1, lesson 3 of 6

A question to start

Dip a wand in soapy water and blow. Out floats a shiny, round bubble. What is inside it, and why does it pop?

The idea

A soap bubble is a very thin skin of soapy water with air inside. When you blow, your breath stretches the skin into a ball. Then the skin closes up around the air.

  • Bubbles are round, because the stretchy skin pulls in the same on every side.
  • They drift with the wind, because they are so light.
  • They slowly sink, because the watery skin is a little heavier than the air.
  • They pop when the skin gets too thin, dries out, or touches something dry.
airthin soapy skin

You can also see bubbles of air in water. Blow through a straw into a glass of water: bubbles of air rush up to the top, because air is much lighter than water.

Worked examples

1. A big bubble. Blow slowly and gently, and more air goes in before the skin closes, so the bubble is bigger.

2. A bubble that lands on a wet hand can sit there for a while. Land it on a dry sleeve and it pops at once.

3. Bubbles in a fish tank come from a pump pushing air into the water. They always go up.

Try it

  1. What is inside a soap bubble?
  2. Why are bubbles round?
  3. Why do bubbles in water go up?
  4. Give one reason a bubble pops.
Answers
  1. Air: your breath.
  2. The stretchy skin pulls in the same on every side.
  3. Air is much lighter than water.
  4. Any one: the skin gets too thin, dries out, or touches something dry.

Common mistake

"Bubbles are made of soap." The skin is soapy water, but almost all of a bubble is the air inside.

Where it leads