Displacement Reaction – IGCSE Chemistry Definition and Key Facts

Displacement reactions appear across several topics in the Cambridge 0620 syllabus — metals, halogens, and electrochemistry. They are a reliable way for examiners to test whether you understand the reactivity series and can apply it to predict outcomes.

The 0620 definition

A displacement reaction is a reaction in which a more reactive element takes the place of a less reactive element in a compound. The less reactive element is released as an uncombined substance.

General pattern:

A + BC → AC + B (where A is more reactive than B)

Metal displacement reactions

When a more reactive metal is placed into a solution of a less reactive metal’s salt, displacement occurs.

Reaction	Equation	Observation
Zinc in copper sulfate	Zn + CuSO₄ → ZnSO₄ + Cu	Blue solution fades; brown/reddish solid appears on zinc
Iron in copper sulfate	Fe + CuSO₄ → FeSO₄ + Cu	Blue solution turns green; brown solid forms
Magnesium in zinc sulfate	Mg + ZnSO₄ → MgSO₄ + Zn	Grey deposit forms on magnesium
Copper in magnesium sulfate	No reaction	— (copper is less reactive)

Using the reactivity series to predict reactions

Metal added	CuSO₄ solution	MgSO₄ solution	ZnSO₄ solution
Magnesium	Displaces Cu	No reaction	Displaces Zn
Zinc	Displaces Cu	No reaction	No reaction
Copper	No reaction	No reaction	No reaction

Rule: a reaction happens only if the free metal is above the metal in the compound in the reactivity series.

The key observations examiners want: the colour change of the solution and the appearance of a solid deposit of the displaced metal.

Metal with acid — a special case

When zinc reacts with hydrochloric acid (Zn + 2HCl → ZnCl₂ + H₂), zinc displaces hydrogen. This is technically a displacement reaction because zinc is more reactive than hydrogen.

Halogen displacement reactions

Halogens also undergo displacement. A more reactive halogen displaces a less reactive halogen from a solution of its salt.

Reactivity order: chlorine > bromine > iodine

Reaction	Equation	Observation
Cl₂ + 2KBr	Cl₂ + 2KBr → 2KCl + Br₂	Solution turns orange/brown (bromine released)
Cl₂ + 2KI	Cl₂ + 2KI → 2KCl + I₂	Solution turns brown (iodine released)
Br₂ + 2KI	Br₂ + 2KI → 2KBr + I₂	Solution turns darker brown
Br₂ + 2KCl	No reaction	— (bromine is less reactive than chlorine)

Displacement reactions are redox reactions

Every displacement reaction involves oxidation and reduction.

In Zn + CuSO₄ → ZnSO₄ + Cu:

• Zinc is oxidised (Zn → Zn²⁺ + 2e⁻) — it loses electrons
• Copper ions are reduced (Cu²⁺ + 2e⁻ → Cu) — they gain electrons

Zinc is the reducing agent; Cu²⁺ is the oxidising agent.

Ionic equations (Supplement)

The ionic equation strips away spectator ions:

Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)

The sulfate ions are spectator ions — they appear unchanged on both sides and play no part in the reaction.

Thermit reaction

The thermit reaction is a dramatic displacement:

2Al + Fe₂O₃ → Al₂O₃ + 2Fe

Aluminium displaces iron from iron(III) oxide. It is highly exothermic — enough heat is released to produce molten iron. This reaction is used to weld railway tracks.

Worked exam question

Iron is placed into copper(II) sulfate solution. (a) State two observations. [2] (b) Write the balanced equation. [1] (c) Explain why this reaction occurs. [1]

(a) The blue colour fades / solution turns green [1]; a brown/red-brown solid / coating forms on the iron [1]

(b) Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) [1]

(c) Iron is more reactive than copper / iron is higher in the reactivity series [1]

Common exam mistakes

1. Predicting a reaction when the free metal is less reactive than the metal in the compound — no reaction occurs in that case.
2. Forgetting to describe what you see, not what you know. Write “brown solid forms” not “copper is deposited.”
3. Writing Br instead of Br₂ in halogen displacement equations — halogens are diatomic.
4. Confusing displacement with decomposition. Displacement has two reactants (an element and a compound); decomposition has one reactant that splits apart.