4.Main Content
guidance
Dry ice
Solid carbon dioxide is known as dry ice. It sublimes at -78°C becoming an extremely cold gas. It is often used in theatres to produce clouds (that look a bit like smoke) because it is more dense than the air so it stays low. It cools the air and causes water vapour in the air to condense into tiny droplets – hence the clouds.
It is also useful in the Physics (and Chemistry) laboratory. There are two main methods of getting a source of dry ice.
1. Using a cylinder of carbon dioxide.
To obtain some solid carbon dioxide from the cylinder, fold a piece of closely woven cloth (preferably of dark colour) in the form of a bag. Hold this bag tightly round the nozzle of the cylinder and open the valve at full blast for 5 to 10 seconds. The gas cools on expansion and solidifies, forming dry ice.
Where the cylinder is of the siphon type it should be kept upright. If it is an ordinary cylinder it should be held upside-down during this process.
It is possible to make the solid 'snow' by expansion before the lesson begins and to store it in a wide-necked Thermos flask.
A cylinder of carbon dioxide can be obtained from bottled gas suppliers such as British Oxygen Company (BOC) and refilled by them. A visit to the local fire station may help with refilling it or search out someone who services fire extinguishers. Get the biggest cylinder you can afford because the refilling costs are mainly transport costs and not the cost of the gas.
Remember that the first production of solid carbon dioxide from the cylinder may not produce very much because the cylinder and its attachments have to cool down.
2. Buying blocks or pellets
Blocks of solid carbon dioxide or granulated versions of it can be obtained fairly easily with a search on the internet. Local stage supply shops or Universities may be able to help. It usually comes in expanded foam packing; you can keep it in this packing in a deep freeze for a few days.
The dry ice pellets come in quite large batches. However, they have a number of uses in science lessons so it is worth trying to co-ordinate the activities of different teachers to make best use of your bulk purchase.
Updated 9 Apr 2009