Oxidation: Gaining Oxygen

Introduction

Oxygen makes up about one-fifth of the air, and it is a very reactive element. Many substances slowly or quickly join with it. A freshly cut apple goes brown, a shiny bicycle chain turns rusty, and a match flares into flame. In each case something is combining with oxygen. Chemists call this type of reaction oxidation , and it is one of the most common kinds of chemical change in everyday life.

Core explanation

Definition. At this level, oxidation means gaining oxygen . When an element combines with oxygen, the product is an oxide :

magnesium + oxygen → magnesium oxide 2Mg + O₂ → 2MgO

The magnesium has gained oxygen, so it has been oxidised.

Oxidation of metals. Most metals react with oxygen to form metal oxides. How fast this happens depends on the metal's reactivity:

Metal Behaviour with oxygen Product --- --- --- Sodium Tarnishes within seconds when cut sodium oxide Magnesium Burns with a brilliant white flame when heated magnesium oxide Iron Burns as sparks when powdered; rusts slowly with water present iron oxides Copper Forms a black coating when heated copper oxide Gold Does not react none

Metal oxides are usually solids, and those that dissolve in water form alkaline solutions. They are described as basic oxides .

Oxidation of non-metals. Non-metals also form oxides. Carbon burns to form carbon dioxide; sulfur burns with a blue flame to form sulfur dioxide; hydrogen burns to form water, which is hydrogen oxide. Non-metal oxides are often gases, and those that dissolve in water form acidic solutions — they are acidic oxides .

Fast and slow oxidation. Burning (combustion) is rapid oxidation that releases energy as heat and light. Corrosion, such as rusting, is slow oxidation of a metal surface over days or years. Respiration in living cells is a controlled oxidation of glucose. All three are exothermic: they release energy.

Reduction. The opposite of oxidation is reduction , the loss of oxygen. When copper oxide is heated with carbon, the carbon takes the oxygen: copper oxide + carbon → copper + carbon dioxide. Copper oxide is reduced, and carbon is oxidised. Oxidation and reduction always happen together, and such reactions are called redox reactions.

Step-by-step reasoning

To decide whether a substance has been oxidised:

1. Write the word or symbol equation. 2. Look at the substance before the reaction: does it contain oxygen? 3. Look at what it has become: does it now contain more oxygen? 4. If it has gained oxygen, it has been oxidised. If it has lost oxygen, it has been reduced.

Visual explanation

Imagine a particle diagram: grey spheres (metal atoms) packed in a block, and pairs of red spheres (O₂ molecules) approaching. After the reaction, each red sphere sits bonded among grey spheres in a new arrangement. Red spheres have moved from free molecules into the solid, showing oxygen being gained.

Real-world analogy

Oxidation is like a magnet gathering paper clips. The substance being oxidised picks up oxygen as it goes. A very reactive metal is like a strong magnet that grabs clips instantly; a less reactive metal is like a weak magnet that picks them up slowly; gold is like a piece of wood that picks up none.

Real-world example

Sparklers and fireworks contain powdered metals such as magnesium, aluminium or iron. When lit, the metal particles are rapidly oxidised, releasing heat and bright light. Iron filings produce the golden branching sparks, while magnesium and aluminium give intense white light.

Why?

Why does magnesium ribbon get heavier when it burns? The product, magnesium oxide, contains the original magnesium atoms plus oxygen atoms taken from the air. Mass is conserved overall, but the solid gains the mass of the oxygen it has combined with.

Common misconception

"Oxidation only happens when something burns." Burning is just a fast example. Rusting, tarnishing, the browning of cut fruit and respiration are all oxidation reactions that happen slowly and without flames.

Worked example

Question: When copper is heated in air it forms a black solid. Name the product, write the word equation and state which substance is oxidised.

Reasoning: Copper combines with oxygen from the air to make an oxide.

Answer: Copper oxide; copper + oxygen → copper oxide; copper is oxidised because it gains oxygen.

Quick check

1. Name the product when sulfur is oxidised by burning in air. Answer: Sulfur dioxide.

Exam focus

Be ready to define oxidation as gain of oxygen and reduction as loss of oxygen, to identify which substance is oxidised in a given equation, and to write word and balanced symbol equations for elements burning in oxygen. Remember that metal oxides are basic and non-metal oxides are often acidic.

Advanced insight

Chemists later broaden the definition of oxidation to the loss of electrons. When magnesium reacts with oxygen, each magnesium atom loses two electrons to become Mg²⁺. This wider definition includes reactions that involve no oxygen at all, such as magnesium burning in chlorine.

Summary

Oxidation is the gain of oxygen, and the products are oxides. Metals form basic oxides; many non-metals form acidic oxides. Oxidation may be fast (burning), slow (corrosion) or controlled (respiration), and it releases energy. Reduction is the loss of oxygen, and the two always occur together in redox reactions.

Practice questions

1. Define oxidation in terms of oxygen. Answer: Oxidation is the gain of oxygen by a substance. 2. Write the word equation for carbon burning completely in oxygen. Answer: carbon + oxygen → carbon dioxide 3. In the reaction copper oxide + hydrogen → copper + water, which substance is oxidised and which is reduced? Answer: Hydrogen is oxidised (gains oxygen to form water); copper oxide is reduced (loses oxygen). 4. Give one example of slow oxidation and one of fast oxidation. Answer: Slow: rusting of iron. Fast: burning of magnesium or a fuel.