If liquid nitrogen is so cold, why does it boil?

(Lansing State Journal, April 13, 1994)

Matter can exist in three forms or states: solid, liquid or gas.

The state of any given material depends on the amount of energy it has. The more energy it has the more active its individual molecules. When solids gain enough energy they melt into liquids. As liquids gain more and more energy, they boil into gases.

We are familiar with these states, but do not commonly see changes from one form to the other.

The exception is water. We commonly see water in all three states: as ice, liquid water and steam. The change from one state to another generally is associated with the addition or removal of heat; we boil water or freeze it. However, pressure and volume also can have an effect on change of state.

Under ordinary circumstances - normal temperature and pressure at the Earth’s surface - nitrogen is gas. (In fact, nitrogen makes up nearly four-fifths of the atmosphere!) That is to say, the molecules of nitrogen are so energetic that they cannot stay together, either as a liquid or solid.

As a gas, molecules are much more dispersed and occupy more than a thousand times as much space as the liquid or solid forms.

We can prepare liquid nitrogen by removing some of the molecular energy, using a process called liquidfication. The liquid has very little energy and is very cold. But when the liquid nitrogen is exposed to ordinary conditions, the molecules quickly absorb heat energy from the environment. This causes the liquid to change state, from a liquid back into gas.

We see this change take place as the liquid nitrogen boils at temperatures very much below zero.

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