The common thermos flask which most people use for keeping drinks hot was actually invented by James Dewar in 1892 to keep condensed gases cold. In recent years liquid nitrogen, which boils at – 196 ^OC has become more readily available as celebrity chefs use it for preparing food. You might consider trying to get hold of some to demonstrate some of the properties of gases, particularly the ideal gas equation, which is covered in Topic 1: Quantitative chemistry.

If you are unable to get your own liquid nitrogen then you could use the embedded video by Dr Roy Lowry of Plymouth University which demonstrates the explosive expansion of liquid nitrogen to shower a sports hall with ping pong balls.

Although fun to watch you could ask students to estimate the amount of liquid nitrogen he puts into the drinks bottle and what volume this would occupy once it has turned into a gas at say a temperature of 60 ^oC which is roughly the temperature of the warm water he uses. You could also go on to ask how liquid nitrogen could be used to retrieve other gases present in air as oxygen has a higher boiling point (– 183 ^OC) than nitrogen.

If you do manage to get your own liquid nitrogen then you can carefully pour a little over the back of your hand. Because it forms an insulating layer of gas between your hand and the liquid nitrogen it only stings mildly and does not cause any damage. To show the very real danger of submerging your finger actually in liquid nitrogen you could put a banana in it then bang the frozen banana against the edge of a desk so students can see it shatter.  As well as in chemical research which requires low temperatures liquid nitrogen is used medically to remove warts and mild skin cancers. Celebrity chefs such as Heston Blumenthal have increasingly been using liquid nitrogen for instantly freezing food and drinks, or creating an impressive cloud of vapour or fog when exposed to air. Exotic dishes such as nitro-scrambled egg and bacon ice-cream and other ‘super smooth’ ice-creams are all prepared using liquid nitrogen. In knowledgeable hands cryogenic cooking using liquid nitrogen is safe to use as long as the two possible dangers, frostbite and asphyxiation due to lack of oxygen when it turns into a gas, are avoided. In the wrong hands though, it can be extremely dangerous.  Last month the Guardian newspaper reported that a teenager, Gaby Scanlon, drank a cocktail given to her in a bar in Lancashire, UK. Liquid nitrogen had been used to cool the cocktail and make it smoke and some still remained while she drank it. She needed to be rushed to hospital and had to have most of her stomach removed as the expansion of the liquid gas had caused it to rupture.