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|Two objects are placed in liquid nitrogen, a banana and a flower (a). |
Liquid oxygen is also used in demonstrations:
The banana becomes so hard and crystalline that it cracks when struck on a metal plate. The flower becomes so brittle upon freezing that it can crumble into dust with a slight hand squeeze.
The glowing ball of steel wool and the lit cigarrette both explode when they hit liquid oxygen.
Gases must be subjected to very low temperatures in order to become liquids. These liquids, by their very nature, end up being very cold themselves. Solids become more crystalline when they are cooled (in this case, when they are placed in liquid nitrogen). The banana becomes a hammer and the flower becomes as delicate as thin glass for this reason.
The gaseous oxygen in the air is dispersed among other gases. Liquid oxygen, on the other hand, is pure. Also, liquid oxygen is much more dense than gaseous oxygen (as a general rule, the volume of a given sample of gas decreases by a factor of about 800 when it forms a liquid). For these reasons, combustion in a liquid oxygen environment is faster and more violent than in a standard atmoshperic environment. A heated piece of steel wool produces a fireball, and a normal cigarette explodes.
Objects that are normally inflammable may react violently when ignited in the presence of liquid oxygen, so appropriate safety measures must be taken when dealing with it. Of particular note: clothing that comes in contact with liquid oxygen can burst into flames in the presence of a spark for several hours after the liquid has evaporated.
An NSF funded Project