Reduction – IGCSE Chemistry Definition

Reduction is the opposite of oxidation — it always occurs alongside oxidation in a redox reaction. The 0620 syllabus provides three definitions of reduction, all of which you must know and apply in different contexts.

The 0620 definitions

Core definitions

1. Loss of oxygen: e.g. CuO + H₂ → Cu + H₂O (copper oxide is reduced — it loses oxygen)
2. Gain of hydrogen: e.g. in hydrogenation reactions

Supplement definition

3. Gain of electrons: e.g. Cu²⁺ + 2e⁻ → Cu (copper ions are reduced — they gain electrons)

The mnemonic OILRIG:

• Reduction Is Gain (of electrons)

Examples of reduction

Reaction	What is reduced	How we know
CuO + H₂ → Cu + H₂O	CuO	Loses oxygen (CuO → Cu)
Fe₂O₃ + 3CO → 2Fe + 3CO₂	Fe₂O₃	Loses oxygen (Fe₂O₃ → Fe)
Cu²⁺ + 2e⁻ → Cu	Cu²⁺	Gains electrons
Cl₂ + 2e⁻ → 2Cl⁻	Cl₂	Gains electrons
2H⁺ + 2e⁻ → H₂	H⁺	Gains electrons

Reduction in electrolysis

At the cathode (negative electrode), reduction occurs. Cations gain electrons:

• Cu²⁺ + 2e⁻ → Cu (copper deposited)
• Pb²⁺ + 2e⁻ → Pb (lead deposited)
• 2H⁺ + 2e⁻ → H₂ (hydrogen gas produced)

RED CAT: REDuction at the CAThode

Reduction in metal extraction

Extracting metals from their ores involves reducing the metal oxide. The metal ion gains electrons (or loses oxygen) to become the pure metal:

• Fe₂O₃ + 3CO → 2Fe + 3CO₂ (iron oxide is reduced to iron)
• Al₂O₃ → 2Al + 1.5O₂ (by electrolysis — Al³⁺ gains electrons at the cathode)

Worked exam question

Hydrogen gas is passed over hot copper(II) oxide. The black solid turns brown/pink. (a) Name the product formed. (1) (b) State which substance is reduced. (1) (c) Explain your answer using the definition of reduction. (1)

Mark scheme

(a) Copper [1]

(b) Copper(II) oxide / CuO [1]

(c) Copper oxide loses oxygen / oxygen is removed from CuO [1]

Common exam mistakes

• Confusing reduction with oxidation. Reduction is gain of electrons (or loss of oxygen). Oxidation is the opposite.
• Saying “the reducing agent is reduced”. The reducing agent is oxidised — it causes reduction in the other substance.
• Identifying the wrong substance. The substance that loses oxygen or gains electrons is reduced. In CuO + H₂ → Cu + H₂O, CuO is reduced (loses oxygen) and H₂ is oxidised (gains oxygen).