Where Liquid Nitrogen Comes From: From Air to −196 °C
Editor:adminClick：
Time:2026-10-20 10:37
Many people encountering a liquid nitrogen tank for the first time assume that liquid nitrogen is some rare synthesised substance. In fact it is present in the air we breathe at every moment: nitrogen makes up roughly 78 per cent of air by volume, and its source is abundant.
Turning gas into liquid relies on cryogenic air separation. The air is filtered, compressed and purified to remove moisture, carbon dioxide and impurities, then cooled stage by stage through expansion refrigeration. Once the temperature falls far enough, the air liquefies, and the components are then separated by rectification using their differing boiling points: nitrogen boils at about −196 °C and oxygen at about −183 °C, so the lower-boiling nitrogen is drawn off first to yield high-purity liquid nitrogen. The whole process is physical, and no new substance is created.
Nitrogen's suitability for cryogenic preservation follows from its properties. It is colourless, odourless and tasteless, chemically stable, neither flammable nor an oxidiser, and almost inert towards samples. Its raw material is air itself, which keeps production cost relatively manageable. Together these features strike a workable balance between safety and availability.
Understanding where liquid nitrogen comes from also helps clear up several common misconceptions. Liquid nitrogen is a consumable: the tank does not generate cold, it merely slows the entry of heat, so refills are unavoidable. Purity deserves attention too, as grades differ in impurity content, which may matter subtly for more sensitive samples. And the white mist seen when a lid is opened is not nitrogen, which is colourless and transparent, but tiny droplets formed as water vapour in the air condenses in the cold.
What a liquid nitrogen tank preserves, in the end, is cold drawn from the air around us.
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