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  The Materials

  You will need:

  • Soap bubble solution (or washing-up liquid)
  • A wand for blowing the bubbles
  • Half a cup of baking soda (sodium bicarbonate)
  • One cup of vinegar
  • A big transparent container with an open top (a large empty fish tank works perfectly)
  • A shallow glass dish such as a baking dish to fit inside the large container

• Now you need to make some scientific observations about the soap bubbles:

  • What colour are they?
  • Do the colours change?
  • Does the size of each bubble change?
  • Do the bubbles sink or rise?

The Floating Bubbles

If you like playing with soap bubbles you will enjoy this experiment you can do in your own kitchen. Soap bubbles are beautiful and captivating, but because they are so light and fragile it can be very hard to observe them closely or for more than a few seconds. They blow away on the breeze, or if you are making bubbles in still air they soon settle down on a surface and pop.

But it is possible to examine soap bubbles more closely, and see properties that are not otherwise easily seen, if we can get them to float on a gas that is more dense than the air that’s inside them. One such gas is carbon dioxide, which we will be creating in this experiment.

What to do

Place the big transparent container on a table away from any draughts and where you can easily see through its sides. Put the glass dish inside the big container.

Pour the baking soda into the glass dish, then pour the vinegar into the dish. The mixture will immediately start to fizz as the liquid and powder react and form carbon dioxide gas.

The carbon dioxide is more dense than air, so it will stay inside the big container so long as it is not disturbed by any draughts of air.

Carbon dioxide is colourless, so you won’t be able see it. But you can detect its presence with your soap bubbles.

After the mixture has fizzed away in the dish for about a minute or so, carefully blow some soap bubbles over the opening of the big container, so that they float down into it. Don’t blow directly into the container, in case you blow the carbon dioxide out of it.

Look at the soap bubbles: instead of sinking to the bottom, as they would in air, they float on the invisible cushion of carbon dioxide that you have created.

The colours of the bubbles come from reflections of the white light that falls on them. Science tells us that white light, whether from the Sun or from a lightbulb, contains light of all colours.

Reflected light separates into the colours of the rainbow. So the bubbles will show a spectrum of colours — red, orange, yellow, green, blue, indigo and violet — just as a rainbow does.

Notice now the light hits the bubbles and is reflected off the top and bottom of their surfaces. As the bubbles get thinner, the colours become redder.