NDA Chemistry · Chemical Reactions

Types of Reactions — Combination, Decomposition, Displacement

Every reaction in the bank falls into one of four shapes — combination, decomposition, displacement or double displacement — and recognising the shape from the equation is most of the marks.

All Chemical Reactions notes NDA Chemistry strategy

Why this matters

The largest non-redox subtopic (7 PYQs) and the home of the match-list questions, where you classify four equations in one go. Get the four shapes cold and you can also do the addition/hydrogenation and 'which statement is NOT correct' variants the bank slips in.

Concept 1 of 3: The four reaction shapes

Look at how many reactants and products there are, and whether atoms swap partners. One product from many = combination. Many products from one = decomposition. One element kicks another out = displacement. Two compounds swap ions = double displacement.

Definition

The four shapes by their equation pattern:

Combination (A + B → AB): two or more reactants make ONE product. Example: C + O₂ → CO₂ (burning coal); CaO + H₂O → Ca(OH)₂.
Decomposition (AB → A + B): ONE reactant splits into two or more products, usually on heating, electrolysis or light. Example: 2H₂O → 2H₂ + O₂ (electrolysis); CaCO₃ → CaO + CO₂.
Displacement (A + BC → AC + B): a more reactive element displaces a less reactive one. Example: Fe + CuSO₄ → FeSO₄ + Cu.
Double displacement (AB + CD → AD + CB): two compounds exchange ions, often forming a precipitate. Example: BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl.

Type	Pattern	Example
Combination	A + B → AB	C + O₂ → CO₂ (burning coal)
Decomposition	AB → A + B	2H₂O → 2H₂ + O₂ (electrolysis of water)
Displacement	A + BC → AC + B	Fe + CuSO₄ → FeSO₄ + Cu
Double displacement	AB + CD → AD + CB	BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl Double displacement = ions swap partners; a precipitate or water often forms.

Practice this concept4 quick reps

The same idea in a real exam question:

NDA · Sep 2023 · Q62Hard

Example 1 · Chemical Reactions · Types of Reactions: Combination, Decomposition, Displacement

Match List I with List II and select the correct answer using the code given below the Lists:

List I (Chemical process)	List II (Reaction)
A. Electrolysis of water	1. Double displacement reaction
B. Burning of coal	2. Combination reaction
C. Iron nail immersed in copper sulphate solution	3. Decomposition reaction
D. Addition of barium chloride solution to aluminium sulphate solution	4. Displacement reaction

Code: A-B-C-D

Decomposition vs double displacement in match-lists

Electrolysis of water is DECOMPOSITION (one → many), while BaCl₂ + a sulphate is DOUBLE displacement (two compounds swap ions). Both feel like 'breaking up', but only one reactant breaks in decomposition.

Single vs double displacement

Single (simple) displacement has a free ELEMENT kicking another out (Fe + CuSO₄). Double displacement has TWO compounds swapping ions (Na₂SO₄ + BaCl₂) — no free element is involved.

Drill 3 more on the four reaction shapes

Concept 2 of 3: Displacement and metal reactivity

In a displacement reaction the MORE reactive metal pushes the less reactive one out of its salt. An iron nail in blue copper sulphate goes brown because iron (more reactive) displaces copper (less reactive) and a reddish-brown copper coating deposits on the nail.

Definition

A metal displaces another metal from its salt solution only if it is higher in the reactivity (activity) series:

Reactivity order (high to low): K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Ag > Au.
Fe + CuSO₄ → FeSO₄ + Cu: iron is above copper, so iron displaces copper; the blue solution fades to green and a brown copper layer forms.
This is also a redox reaction — the more reactive metal is oxidised (loses electrons), the displaced metal ion is reduced.
A metal cannot displace one ABOVE it: copper cannot displace iron, so 'copper is more reactive than iron' is always false.

Reactivity (activity) series — selected metals

\text{K} > \text{Na} > \text{Ca} > \text{Mg} > \text{Al} > \text{Zn} > \text{Fe} > \text{Pb} > \text{Cu} > \text{Ag}

Worked example

An iron nail is dipped in blue copper sulphate solution and turns brown. State the reaction type and which statement is false: (a) Fe displaces Cu, (b) it is a displacement reaction, (c) copper is more reactive than iron.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

NDA · Apr 2019 · Q82Easy

Example 2 · Chemical Reactions · Types of Reactions: Combination, Decomposition, Displacement

Which one of the following statements is NOT correct for the given reaction?

\text{Fe(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{FeSO}_4\text{(aq)} + \text{Cu(s)}

'Copper is more reactive than iron' is always false

If iron displaces copper from CuSO₄, iron MUST be the more reactive metal. Any statement that copper is more reactive than iron is incorrect — it would mean copper could displace iron, which it cannot.

Drill 2 more on displacement and metal reactivity

Concept 3 of 3: Addition reactions and hydrogenation of oils

An addition reaction adds atoms across a double bond, turning an unsaturated molecule into a saturated one — no atoms are lost. Hardening vegetable oil into vanaspati ghee is exactly this: hydrogen adds across the C=C double bonds of the oil over a nickel catalyst.

Definition

Key points about addition reactions:

An addition reaction adds atoms across a double or triple bond — the unsaturated compound becomes saturated; nothing is released.
Hydrogenation of vegetable oils (liquid, unsaturated) with H₂ over a nickel (Ni) catalyst gives a solid fat (vanaspati / margarine). It is an addition reaction.
It is NOT a displacement or decomposition — the whole H₂ molecule is added, none of the oil is broken off.

Worked example

Hydrogenation of vegetable oils using a nickel catalyst converts liquid oil into solid fat. What type of reaction is this?

Practice this concept3 quick reps

The same idea in a real exam question:

NDA · Apr 2021 · Q82Easy

Example 3 · Chemical Reactions · Types of Reactions: Combination, Decomposition, Displacement

Hydrogenation of vegetable oils using nickel catalyst is an example of

Hydrogenation is addition, not displacement

In hydrogenation the whole H₂ molecule ADDS across a double bond — nothing leaves the oil. So it is an addition reaction, not displacement (which requires one element to leave).

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Displacement and metal reactivity
Reactivity (activity) series — selected metals
$\text{K} > \text{Na} > \text{Ca} > \text{Mg} > \text{Al} > \text{Zn} > \text{Fe} > \text{Pb} > \text{Cu} > \text{Ag}$

Reference tables (1)

The four reaction shapes4 rows

Type	Pattern	Example
Combination	A + B → AB	C + O₂ → CO₂ (burning coal)
Decomposition	AB → A + B	2H₂O → 2H₂ + O₂ (electrolysis of water)
Displacement	A + BC → AC + B	Fe + CuSO₄ → FeSO₄ + Cu
Double displacement	AB + CD → AD + CB	BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl Double displacement = ions swap partners; a precipitate or water often forms.

Watch out for (4)

Decomposition vs double displacement in match-lists
→ The four reaction shapes
Single vs double displacement
→ The four reaction shapes
'Copper is more reactive than iron' is always false
→ Displacement and metal reactivity
Hydrogenation is addition, not displacement
→ Addition reactions and hydrogenation of oils

Test yourself on Chemical Reactions

15 past NDA questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

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