Demonstration

Apparatus and Materials

• Polythene bag or balloons, small

• CO2 cylinder (syphon type)

• Dry ice attachment

Health & Safety and Technical Notes

Whether you use a gas cylinder or solid CO2, you should do a risk assessment.

Solid carbon dioxide must not be handled with bare fingers: use tongs or wear thick gloves. Wear eye protection before breaking a large block (with a hammer and cold chisel).

Read our standard health & safety guidance

You may be able to locate a local supplier for solid CO₂. If you have a dry ice attachment and a CO₂ cylinder, follow the instructions to get about half a teaspoon of carbon dioxide snow.

For further information about dry ice, see the apparatus entry for:

CO2 cylinder (syphon type)

Procedure

If using a balloon, stretch the neck of the balloon and hold it open with several fingers whilst about half a teaspoon of carbon dioxide snow is scraped in. Quickly flatten the balloon and knot the neck firmly. A polythene bag is easier to fill but more awkward to seal well.

Teaching Notes

• Solid carbon dioxide is unusual in that, at room temperature and pressure, it does not turn into a liquid before changing into a gas as it warms up – it sublimes. The change in volume from solid to gas can be seen to be as much as 600 times if the container in which it is expanding is well-sealed and has no holes.
• Alternately you could place a small piece of solid carbon dioxide (say, 0.5 cm³ ) under the mouth of a gas jar or measuring cylinder which is full of water and inverted over a tank of water so that the bubbles of gas can be collected.

This experiment was safety-tested in January 2005