ChemistryICSE Class 8 · Chemical Reactions

Combination, Decomposition, Displacement and Double Displacement; the Reactivity Series (ICSE Class 8)

Updated 17 October 2026 · Fact-checked

Chemical reactions are of four kinds: combination (A + B → AB), decomposition (AB → A + B), displacement (A + BC → AC + B, where A is more reactive than B) and double displacement (AB + CD → AD + CB). The reactivity series is K, Na, Ca, Mg, Al, Zn, Fe, Pb, [H], Cu, Hg, Ag, Au.

Understand Types of Chemical Reactions and the Reactivity Series

A chemical reaction turns reactants into new products.

1. Combination (synthesis). Two or more substances give one product: $A + B \to A B$ .
$2 M g + O_{2} \to 2 M g O$ ; $C a O + H_{2} O \to C a (O H)_{2}$ ; $F e + S \to F e S$ .

2. Decomposition. One substance gives two or more simpler ones, usually by heat, light or electricity: $A B \to A + B$ .
Heat: $C a C O_{3} \to C a O + C O_{2}$ . Electricity: $2 H_{2} O \to 2 H_{2} + O_{2}$ .

3. Displacement. A more reactive element pushes a less reactive one out of its compound: $A + B C \to A C + B$ .
$F e + C u S O_{4} \to F e S O_{4} + C u$ (blue solution turns green; brown copper deposited), so iron is more reactive than copper.
$C u + 2 A g N O_{3} \to C u (N O_{3})_{2} + 2 A g$ .
If nothing happens (copper in iron sulphate), the metal is less reactive.

The reactivity series (most reactive first):

K > Na > Ca > Mg > Al > Zn > Fe > Pb > [H] > Cu > Hg > Ag > Au

Metals above hydrogen displace hydrogen from dilute acids: $M g + 2 H C l \to M g C l_{2} + H_{2}$ ; copper does not.

4. Double displacement. Two compounds exchange partners: $A B + C D \to A D + C B$ .

Precipitation: an insoluble product separates, e.g. $B a C l_{2} + N a_{2} S O_{4} \to B a S O_{4} + 2 N a C l$ (white $B a S O_{4}$ ); $A g N O_{3} + N a C l \to A g C l + N a N O_{3}$ (white AgCl).
Neutralisation (next topic) is also a double displacement.

Type	General form	Key feature
Combination	A + B → AB	one product
Decomposition	AB → A + B	one reactant
Displacement	A + BC → AC + B	an element replaces another
Double displacement	AB + CD → AD + CB	partners exchange

Types of Chemical Reactions and the Reactivity Series: formulas and key facts

Four types: Combination A + B → AB
Decomposition AB → A + B
Displacement A + BC → AC + B
Double displacement AB + CD → AD + CB; Name from the pattern.
Reactivity series: $K > N a > C a > M g > A l > Z n > F e > P b > [H] > C u > H g > A g > A u$; Most reactive first.
Displacement example: $F e + C u S O_{4} \to F e S O_{4} + C u$; Iron above copper.
Precipitation example: $B a C l_{2} + N a_{2} S O_{4} \to B a S O_{4} + 2 N a C l$; White precipitate.

How to solve Types of Chemical Reactions and the Reactivity Series questions

Look at the pattern of reactants and products, then use the series for metals.

1One product from two or more reactants: combination. One reactant: decomposition.
2An element and a compound giving a new element and compound: displacement; check the series.
3Two compounds exchanging partners: double displacement; look for a precipitate.
4A metal displaces another only if it is higher in the series.
5Balance the final equation by counting atoms.

Types of Chemical Reactions and the Reactivity Series in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

The iron nail in blue solution

Suppose a clean iron nail is left in copper sulphate solution for 20 minutes.

State the observations, the type of reaction and the equation.

Try it first, then show the solutionSolution

The nail gets a reddish-brown coating of copper and the blue solution turns pale green.
Iron is more reactive than copper, so it displaces copper.
$F e + C u S O_{4} \to F e S O_{4} + C u$ .

Answer: Displacement; Fe + CuSO₄ → FeSO₄ + Cu.

Memory hook: Higher metal pushes out the lower.

The cloudy mixture

Imagine a student mixes solutions of barium chloride and sodium sulphate and sees a white cloudy solid.

Name the solid and the type of reaction.

Try it first, then show the solutionSolution

The solid is barium sulphate, an insoluble precipitate.
The two compounds exchange partners: double displacement.

Answer: Barium sulphate; double displacement (precipitation).

Memory hook: Partners swap, solid appears.

Tricks to remember Types of Chemical Reactions and the Reactivity Series

Short memory aids. Say each one aloud once, then use it on a question the same day.

Pattern spotting
A+B→AB join; AB→A+B split; A+BC→AC+B swap one; AB+CD→AD+CB swap both.
Use it when: Types.
Chant the series
Say the series aloud in order: K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Hg, Ag, Au; make up your own sentence from the first letters.
Use it when: Reactivity series.
Above hydrogen reacts
Only metals above H release hydrogen from dilute acid.
Use it when: Acid reactions.

Types of Chemical Reactions and the Reactivity Series: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Type of reaction · 1 mark · MCQBoard level

$2 M g + O_{2} \to 2 M g O$ is a:
(a) decomposition reaction
(b) displacement reaction
(c) combination reaction
(d) double displacement reaction

Try it first, then show the solutionSolution

Two reactants give one product: combination.

Answer: (c) combination

Example 2: Classify · 2 marks · Short answerBoard level

Name the type of each reaction: (a) $C a C O_{3} \to C a O + C O_{2}$ , (b) $Z n + 2 H C l \to Z n C l_{2} + H_{2}$ , (c) $A g N O_{3} + N a C l \to A g C l + N a N O_{3}$ , (d) $F e + S \to F e S$ .

Try it first, then show the solutionSolution

(a) decomposition; (b) displacement; (c) double displacement; (d) combination. (½ each)

Answer: (a) decomposition (b) displacement (c) double displacement (d) combination

Example 3: Placing the metals · 4 marks · Long answerHarder board level

A student records these observations: (i) zinc in copper sulphate: reaction; (ii) copper in zinc sulphate: no reaction; (iii) iron in copper sulphate: reaction; (iv) zinc in iron sulphate: reaction; (v) magnesium in zinc sulphate: reaction. Arrange Mg, Zn, Fe and Cu in decreasing order of reactivity, with reasons.

Try it first, then show the solutionSolution

(v) Mg displaces Zn: Mg > Zn. (1)
(iv) Zn displaces Fe: Zn > Fe. (1)
(iii) Fe displaces Cu: Fe > Cu. (1)
Order: Mg > Zn > Fe > Cu; (ii) confirms that Cu cannot displace Zn. (1)

Answer: Mg > Zn > Fe > Cu

Example 4: Metals and dilute acid · 3 marks · Short answerHarder board level

Which of Mg, Cu, Zn, Ag and Fe will release hydrogen from dilute hydrochloric acid? Write balanced equations for two of them and give the reason for the others.

Try it first, then show the solutionSolution

Mg, Zn and Fe, which lie above hydrogen. (1)
$M g + 2 H C l \to M g C l_{2} + H_{2}$ ; $Z n + 2 H C l \to Z n C l_{2} + H_{2}$ . (1)
Cu and Ag lie below hydrogen, so they do not react. (1)

Answer: Mg, Zn, Fe react; Cu, Ag do not

Example 5: A white solid forms · 4 marks · Case-basedHOTS / competency

Solutions of barium chloride and sodium sulphate are mixed. (a) State the observation. (b) Write the balanced equation. (c) Name the type of reaction. (d) How could you separate the solid from the mixture? (e) Count the atoms to show the equation is balanced.

Try it first, then show the solutionSolution

(a) A white precipitate forms. (½)
(b) $B a C l_{2} + N a_{2} S O_{4} \to B a S O_{4} + 2 N a C l$ . (1)
(c) Double displacement (precipitation). (½)
(d) Filtration. (½)
(e) Left: Ba 1, Cl 2, Na 2, S 1, O 4; right: Ba 1, S 1, O 4, Na 2, Cl 2. (1½)

Answer: (a) white ppt (b) as above (c) double displacement (d) filtration

Where students lose marks in Types of Chemical Reactions and the Reactivity Series

The slips examiners see most often, and what to write instead.

Mistake: Not checking the series
A metal displaces another only if it is more reactive.
Wrong working
"Copper displaces iron from iron sulphate."
Right working
"No reaction; copper is less reactive than iron."
Do this: Say which is higher.
Mistake: Unbalanced equation
Atoms must be equal on both sides.
Wrong working
$M g + O_{2} \to M g O$ .
Right working
$2 M g + O_{2} \to 2 M g O$ .
Do this: Count atoms.
Mistake: Calling every precipitate a displacement
Two compounds exchanging partners is double displacement.
Wrong working
"BaCl₂ + Na₂SO₄ is displacement."
Right working
"It is double displacement."
Do this: Check element + compound or compound + compound.

Exam tips

Name the type from the pattern, then give the equation.
Give observation for displacement: colour change and deposit.
Always write the series in full when asked to compare metals.
Balance equations and show the atom count.

Practice questions on Types of Chemical Reactions and the Reactivity Series

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · Very short answerBoard level
Name the type of reaction in $2 H_{2} O \to 2 H_{2} + O_{2}$ and the energy that causes it.
Hint
One reactant.
Answer
Decomposition by electricity (electrolysis).
Q2 · 2 marks · Short answerHarder board level
Arrange K, Cu, Fe, Mg and Ag in decreasing order of reactivity.
Hint
Use the series.
Answer
K > Mg > Fe > Cu > Ag.
Q3 · 3 marks · Short answerHarder board level
Why does copper not react with dilute hydrochloric acid, but zinc does? What would you observe with zinc?
Hint
Position relative to hydrogen.
Answer
Copper is below hydrogen in the reactivity series and cannot displace it; zinc is above hydrogen. With zinc, bubbles of hydrogen are seen: $Z n + 2 H C l \to Z n C l_{2} + H_{2}$ .
Q4 · 3 marks · Short answerHarder board level
A strip of copper is placed in silver nitrate solution. Predict what happens, write the balanced equation and name the type.
Hint
Copper is above silver.
Answer
Copper displaces silver (silver deposits, the solution turns blue): $C u + 2 A g N O_{3} \to C u (N O_{3})_{2} + 2 A g$ . Displacement.
Q5 · 4 marks · Long answerHOTS / competency
Iron displaces copper from copper sulphate, but copper does not displace iron from iron sulphate. (a) What does this show? (b) Predict whether lead displaces copper. (c) Name a metal that would react violently with cold water and so cannot be used in such displacement tests in solution.
Hint
Use the series.
Answer
(a) Iron is more reactive than copper. (b) Yes, lead is above copper in the series, so it displaces copper. (c) Sodium (or potassium), which reacts violently with water.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Type: A + B → AB?Show answer
Combination.
2.What causes decomposition?Show answer
Heat, light or electricity.
3.Which metal is above Zn: Fe or Mg?Show answer
Mg.
4.White precipitate from BaCl₂ and Na₂SO₄?Show answer
Barium sulphate.
5.Does Cu react with dilute HCl?Show answer
No.

Types of Chemical Reactions and the Reactivity Series: frequently asked questions

What are the four types of chemical reactions?

Combination, decomposition, displacement and double displacement.

What is the reactivity series?

It is the list of metals in order of reactivity: K, Na, Ca, Mg, Al, Zn, Fe, Pb, [H], Cu, Hg, Ag, Au. A metal displaces another below it from its salt solution.

What is a double displacement reaction?

Two compounds exchange their partners (AB + CD → AD + CB); precipitation and neutralisation are examples.