Uses of the Gases from Air

Introduction

Once air has been cooled, liquefied and separated into its parts, industry is left with huge amounts of pure oxygen, nitrogen and argon. These gases are not just laboratory curiosities. They keep hospital patients alive, stop crisps from going stale, make welding possible and fill the bulbs and windows in our homes. On this page you will see how each use depends on a property of the gas, which is the key idea examiners look for.

Core explanation

The gases from air are obtained by the fractional distillation of liquid air. This is cheaper than making them by chemical reactions because air is free, is everywhere and already contains the gases in pure form, only mixed. Every use can be explained by a property.

Oxygen (O₂) — supports life and burning.

- Medicine. Patients with breathing difficulties, such as those with pneumonia or lung disease, are given oxygen-enriched air through a mask or nasal tube. Each breath then delivers more oxygen to the blood. Oxygen cylinders are also carried in ambulances. - Breathing apparatus. Divers, firefighters, mountaineers at high altitude and astronauts use oxygen supplies where air is thin, absent or unsafe. - Steelmaking. Pure oxygen is blown through molten iron to burn away impurities such as carbon, which leave as carbon dioxide. This is the largest single use of oxygen. - Welding and cutting. Burning a fuel gas in pure oxygen instead of air gives a much hotter flame, hot enough to melt steel.

Nitrogen (N₂) — unreactive and, as a liquid, very cold.

- Food packaging. Crisp packets and bagged salads are filled with nitrogen instead of air. With little oxygen present, fats do not turn rancid as quickly and many microbes grow more slowly. The gas also cushions the food. - Preventing fires and explosions. Nitrogen is used to flush fuel tanks and pipelines so that flammable vapours cannot mix with oxygen. - Liquid nitrogen. At about −196 °C, liquid nitrogen is used to freeze food rapidly, store biological samples and freeze off warts. - Making ammonia. Nitrogen is the raw material for ammonia, which is turned into fertilisers.

Argon (Ar) — a noble gas, extremely unreactive.

- Lamps. Filament bulbs are filled with argon. The hot tungsten filament would burn in air, but in argon it cannot react. - Welding shield. Argon is blown over hot metal during welding to keep oxygen away, so the weld does not oxidise. - Double glazing. Argon between the panes conducts heat less well than air, improving insulation.

Gas Key property Main uses --- --- --- Oxygen Supports respiration and combustion Medicine, steelmaking, welding Nitrogen Unreactive; very cold as a liquid Food packaging, freezing, ammonia Argon Very unreactive (noble gas) Lamps, welding shield, glazing

Step-by-step reasoning

To explain any use of a gas from air:

1. Name the gas and the use. 2. Identify the property that matters — reactive or unreactive, cold as a liquid, supports burning. 3. Explain what problem the property solves, such as stopping oxidation. 4. Say why that gas was chosen instead of another.

Visual explanation

Picture a crisp packet cut open. Inside, most of the gas is nitrogen, shown as pairs of joined blue spheres that ignore the crisps around them. Compare this with an ordinary air-filled bag where red oxygen molecules are shown attacking the fatty surface of each crisp.

Real-world analogy

Using argon in a light bulb is like putting a precious painting in a sealed display case filled with a harmless gas. Nothing inside the case can attack the painting, so it lasts far longer than if it hung in the open air.

Real-world example

Hospitals often store oxygen as a liquid in large insulated tanks outside the building. The liquid slowly turns to gas and is piped to wards, where a wall outlet beside each bed lets staff give oxygen at a controlled flow rate.

Why?

Why is nitrogen, not argon, normally used in food packaging when argon is even less reactive? Nitrogen makes up about 78% of air, so it is produced in enormous amounts and is much cheaper. It is unreactive enough for the job, so there is no need to pay more for argon.

Common misconception

"The nitrogen in a crisp packet is there to trick customers." The gas does make the bag look full, but its main job is to protect the food from oxidation and to stop the crisps being crushed during transport.

Worked example

Question: A welder uses argon rather than nitrogen or air to shield a weld in aluminium. Explain the choice.

Reasoning: Hot aluminium reacts readily with oxygen in air. At welding temperatures some metals can even react with nitrogen. Argon is a noble gas with a full outer shell, so it does not react even at very high temperatures.

Answer: Argon is chosen because it is completely unreactive, even when very hot, so it keeps oxygen and nitrogen away from the metal.

Quick check

1. Which gas from air is used to help patients who struggle to breathe? Answer: Oxygen, given as oxygen-enriched air.

Exam focus

Always pair each use with a property. "Argon is used in light bulbs" earns little credit alone; add "because it is unreactive, so the hot filament does not react". Learn one use each for oxygen, nitrogen and argon.

Advanced insight

Pure oxygen makes materials burn far more fiercely than they do in air, so oxygen equipment must be kept free of oil and grease and away from flames. Liquid nitrogen is also hazardous in a different way: it boils into a large volume of gas that can displace oxygen in a closed room without any warning smell.

Summary

Oxygen, nitrogen and argon are separated from liquid air and each use depends on a property. Oxygen supports life and burning, so it is used in medicine, steelmaking and welding. Nitrogen is unreactive and cheap, so it protects food and prevents fires; liquid nitrogen freezes things. Argon is very unreactive, so it fills lamps and shields welds.

Practice questions

1. Give one medical use of oxygen and explain why it helps. Answer: Oxygen therapy for patients with lung disease; each breath delivers more oxygen to the blood. 2. Why are bags of salad often filled with nitrogen? Answer: Nitrogen is unreactive and removes oxygen, slowing spoilage and oxidation of the food. 3. Explain why a filament lamp is filled with argon instead of air. Answer: The hot filament would react with oxygen in air and burn out; argon is unreactive so the filament lasts. 4. State why pure oxygen is used in steelmaking. Answer: It reacts with impurities such as carbon, removing them as gases like carbon dioxide.