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Nitrogen is needed to make proteins,which are vital to life. Plants and animals recycle nitrogen through the air and soil in a process called the NITROGEN CYCLE . As a gas, nitrogen makes up 78 per cent of air. At everyday temperatures it is very unreactive. It is used in place of air in crisp packets, for example, so the contents do not go stale. Nitrogen is also used to make industrial chemicals such as fertilizers and explosives.
When nitrogen gas is cooled to -196ºC (-320ºF), it turns to a liquid. Liquid nitrogen is so cold that it can freeze a substance in seconds. In hospitals, it is used to preserve blood and body parts for transplant. The material to be preserved is placed in a special, sealed container that is filled with liquid nitrogen. Because nitrogen is so unreactive, it does not alter the preserved materials in any way.
The heat produced by lightning forces nitrogen molecules in the air to split. Nitrogen atoms bond with oxygen to form nitrogen oxides, which dissolve in water to create nitric acid. Weak nitric acid falls to the soil, where it splits apart to form the compounds nitrates and nitrites. These compounds are essential to lifefor plants and micro-organisms.
Farmers often use fertilizers to help their crops grow well. Many fertilizers contain nitrogen in the form of nitrates, because this is the form that plants can use. Natural fertilizers are made from compost and manure. Synthetic fertilizers are made by combining nitrogen from the air with hydrogen from natural gas.
Nitrogen compounds are used to make explosives. These compounds contain chemicals that break apart easily to release huge volumes of gases extremely quickly. They can be used in a controlled way to demolish a building without harming other buildings nearby. The explosive TNT (trinitrotoluene) releases hydrogen, carbon monoxide, and nitrogen, and carbon powder, which produces black smoke.
All living things need nitrogen, but most cannot use nitrogen gas directly from the air. The nitrogen has to be fixed (combined) with other elements to form nitrites and nitrates. This is done by lightning andby nitrogen-fixing bacteria. The nitrates are taken up by plants, which are eaten by animals. This starts the continual cycle of nitrogen called the nitrogen cycle.
Nitrogen from the air is fixed to make nitrates in the soil by nitrifyingbacteria. The nitrates are taken up byplants to build plant protein. When an animal eats a plant, it turns the plant protein into animal protein. Denitrifying bacteria convert the nitrogen contained in animal waste and in decaying plant and animal material back into nitrogen gas again.
Nitrifying bacteria are a key part of the nitrogen cycle. Some live in the root nodules of legumes (peas and beans), like this nodule from the rootof a pea plant. Others live free in the soil. Bacteria in the soil make nitratesfrom nitrites and other nitrogen molecules. Bacteria in legume root nodules take up nitrates from the soil.

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