Oxygen: The Gas for Life and Burning

Introduction

Oxygen makes up only about one-fifth of the air, yet almost every animal on Earth dies within minutes without it, and no ordinary fire can burn in its absence. Unlike nitrogen, oxygen is a highly reactive gas. On this page you will see what oxygen is like, how it takes part in respiration and combustion, and why these two processes are really close chemical cousins.

Core explanation

Oxygen in air. Dry air contains about 21% oxygen by volume. Oxygen exists as diatomic molecules, O₂, in which two oxygen atoms share two pairs of electrons in a double bond. It is colourless and has no smell or taste. It is slightly soluble in water, which is just enough to let fish and other aquatic organisms breathe using their gills.

Physical properties. Oxygen boils at about −183 °C, so it is a gas at all everyday temperatures. It is slightly denser than air. Liquid oxygen is pale blue and is attracted to a magnet.

Oxygen is reactive. The O=O double bond is much weaker than the N≡N triple bond in nitrogen, so oxygen reacts with a huge range of elements and compounds. Reactions that add oxygen to a substance are called oxidation reactions. Rusting of iron, the browning of a cut apple and the burning of a candle are all oxidation reactions.

Combustion. When a substance burns, it reacts rapidly with oxygen and releases energy as heat and light. Burning is an exothermic reaction. Some examples:

- Carbon burns to form carbon dioxide: C + O₂ → CO₂ - Magnesium burns with a brilliant white flame to form magnesium oxide: 2Mg + O₂ → 2MgO - Methane (natural gas) burns to form carbon dioxide and water: CH₄ + 2O₂ → CO₂ + 2H₂O

A fire needs three things, often drawn as the fire triangle : fuel, heat and oxygen. Remove any one and the fire goes out. This is why fire blankets and carbon dioxide extinguishers work — they cut off the oxygen supply.

Respiration. Inside every living cell, glucose from food reacts with oxygen to release the energy the cell needs. This is aerobic respiration :

glucose + oxygen → carbon dioxide + water (+ energy)

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Respiration is a step-by-step oxidation controlled by enzymes, so energy is released gradually rather than as a flame.

Where the oxygen comes from. Almost all the oxygen in air has been produced by photosynthesis in green plants, algae and some bacteria. Photosynthesis and respiration run in opposite directions, which helps to keep the oxygen level in air steady.

Step-by-step reasoning

To predict the products when a substance burns completely in oxygen:

1. List the elements in the fuel. 2. Each element combines with oxygen to form its oxide: carbon gives CO₂, hydrogen gives H₂O, sulfur gives SO₂, a metal gives a metal oxide. 3. Write the word equation, then the symbol equation. 4. Balance the equation so that the numbers of each type of atom are equal on both sides.

Visual explanation

Imagine a jar of air as a crowd of 100 molecules: 78 blue nitrogen molecules drifting past untouched, 21 red oxygen molecules, and one grey argon atom. Drop a burning candle into the jar and only the red molecules join the reaction, pairing up with carbon and hydrogen from the wax. When the red ones are used up, the flame dies even though the jar is still mostly full of gas.

Real-world analogy

Oxygen is like the busiest worker on a building site. Nitrogen stands around doing nothing, but oxygen joins almost every job going — slow ones like rusting and respiration, and fast, dramatic ones like burning. Take oxygen away and most of the work stops.

Real-world example

Hospitals give patients with breathing difficulties air enriched with extra oxygen, supplied from cylinders or piped from a liquid oxygen store. Because oxygen makes materials burn far more fiercely, strict rules ban smoking and open flames near oxygen equipment, and oily or greasy materials are kept away from oxygen fittings.

Why?

Why does a candle burn more brightly in pure oxygen than in air? In air, only about one molecule in five that hits the wax vapour is oxygen. In pure oxygen, every collision can be with an oxygen molecule, so the reaction happens much faster and releases energy more quickly, giving a hotter, brighter flame.

Common misconception

"Oxygen itself burns." Oxygen does not burn; it is the substance that fuels react with when they burn. Oxygen supports combustion. A glowing splint relights in oxygen because the wood burns faster, not because the oxygen is catching fire.

Worked example

Question: Write a balanced symbol equation for the complete combustion of hydrogen in oxygen, and name the product.

Reasoning: Hydrogen (H₂) combines with oxygen (O₂) to form its oxide, water. Unbalanced: H₂ + O₂ → H₂O. There are two oxygen atoms on the left and one on the right, so put 2 in front of H₂O; this gives four hydrogen atoms on the right, so put 2 in front of H₂.

Answer: 2H₂ + O₂ → 2H₂O; the product is water.

Quick check

1. What approximate percentage of dry air is oxygen? Answer: About 21%, roughly one-fifth.

Exam focus

Be ready to write word and balanced symbol equations for combustion and for aerobic respiration. Know the three sides of the fire triangle and use them to explain how fire extinguishers work. Say that oxygen supports combustion — do not say that it burns.

Advanced insight

Oxygen was not always present in Earth's air. It built up gradually after photosynthetic bacteria evolved, more than two billion years ago. Oxygen can also exist as ozone, O₃, a molecule of three oxygen atoms. High in the atmosphere, ozone absorbs harmful ultraviolet radiation; near the ground it is an irritating pollutant.

Summary

Oxygen, O₂, makes up about 21% of dry air. It is colourless, odourless and far more reactive than nitrogen because its double bond is weaker. Oxygen supports combustion, in which fuels react rapidly with it releasing heat and light, and it is used in aerobic respiration, a controlled oxidation of glucose in living cells. Photosynthesis replaces the oxygen that is used.

Practice questions

1. Name the three things needed for a fire to burn. Answer: Fuel, heat and oxygen (the fire triangle). 2. Write the word equation for aerobic respiration. Answer: Glucose + oxygen → carbon dioxide + water (with energy released). 3. Magnesium burns in oxygen to form magnesium oxide. Write the balanced symbol equation. Answer: 2Mg + O₂ → 2MgO. 4. Explain why a fire blanket puts out a small fire in a pan. Answer: It covers the fire and cuts off the supply of oxygen, removing one side of the fire triangle, so combustion stops. 5. Why is oxygen much more reactive than nitrogen? Answer: The double bond in O₂ is much weaker than the triple bond in N₂, so oxygen molecules are more easily broken apart to react.