Sleeping bubbles

BubbleOver the next two weeks, activities will use frozen carbon dioxide – better known as ‘dry ice’. This material can be purchased through some party supply companies or direct from BOC Gases or Air Liquide.

Warning: Dry ice is not like water ice. Do not allow it to touch your skin; the extreme cold can cause harm. Wear thick gloves and safety goggles when doing this activity.

You will need

  • Dry ice
  • Water
  • Plastic cup
  • Safety goggles
  • Cotton gardening gloves
  • An empty fish tank or other large container
  • Bubble mix (See this Science by Email activity to make your own)
  • Bubble wand

What to do

  1. While wearing your gloves and safety goggles, scatter several cups of dry ice on the bottom of your empty fish tank.
  2. Pour two or three cups of water over the ice to make it sublime (turn straight from an ice into a gas without turning into liquid in between). You should see it turn into a thick fog.
  3. Stand at one end of the fish tank and blow a bubble. It might take a few attempts to get one to drift down into the tank, so keep blowing.
  4. Watch what happens as the bubble drops into the fog. Can you get it to hover in one spot?

What’s happening?

Carbon dioxide (CO2) is a molecule made up of a carbon atom and two oxygen atoms. Normally, the surrounding air only contains a tiny amount of this gas mixed in with a lot of nitrogen and oxygen. However, if you had a litre of CO2 gas (at room temperature and a pressure of one atmosphere), it would weigh a hefty 1.98 grams, compared with just 1.25 grams for a litre of nitrogen. Frozen, a litre of CO2 weighs a massive 1600 grams because the frozen molecules pack together tighter than the gas molecules, so there are lots more in a litre.

Carbon dioxide is a lot denser than the gases that make up most of the air we breathe. When it sublimes, it’s still quite cold, which increases its density even more. All of these heavy carbon dioxide molecules packed close together will sink below the nitrogen and oxygen in the air in the fish tank, forming a layer. The air in your bubble is also less dense than the carbon dioxide and so it’s able to float on the pool of cool CO2.


Our atmosphere is a mixture of many different gases, not to mention a haze of dust and soot particles. Of these gases, only a tiny 0.04% is carbon dioxide. It’s not very much, but it can do something nitrogen and oxygen can’t – scatter infrared radiation. This makes it an important gas in our atmosphere, helping reflect heat back to the surface.

The atmosphere isn’t all the same, however. Most of the carbon dioxide sits close to the Earth’s surface in a layer called the troposphere. A gas called ozone mostly floats near the bottom of a region called the stratosphere, which starts roughly 10 kilometres up (where most international airliners fly).

The outermost part of the Earth’s atmosphere is called the ‘exosphere’, and it starts at about 690 kilometres – higher than the international space station, even. In this zone, gas particles are so far apart that the air doesn’t behave like a fluid any more. Molecules can whiz about for hundreds of kilometres without bumping into anything.

Dry ice
Melt the dry ice with water in a large container.
Bubble mix
Blow bubbles over the container.
Watch your bubbles float on a pond of carbon dioxide.