ChemistryICSE Class 9 · Chemical Changes and Reactions

Displacement, Double Decomposition and Energy Changes: Predicting Reactions (ICSE Class 9)

Updated 17 October 2026 · Fact-checked

In displacement a more reactive element displaces a less reactive one from its compound (A + BC → AC + B), using the activity series K > Na > Ca > Mg > Al > Zn > Fe > Pb > [H] > Cu > Hg > Ag > Au. In double decomposition two compounds exchange radicals (AB + CD → AD + CB), often forming a precipitate. Exothermic reactions give out heat; endothermic ones take it in.

Understand Displacement, Double Decomposition and Energy Changes in Reactions

**Displacement: $A + B C \to A C + B$ . A more active element displaces a less active one. The activity series** (most to least active):

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

*Metal displaces metal:* $Z n + C u S O_{4} \to Z n S O_{4} + C u$ ; $F e + C u S O_{4} \to F e S O_{4} + C u$ (blue to pale green; reddish-brown copper on the nail); $C u + 2 A g N O_{3} \to C u (N O_{3})_{2} + 2 A g$ .
*Metal displaces hydrogen* (metal above H): $M g + 2 H C l \to M g C l_{2} + H_{2}$ ; metals below H (Cu, Ag) do not react with dilute acids.
*Non-metal displaces non-metal:* $C l_{2} + 2 K B r \to 2 K C l + B r_{2}$ ; $C l_{2} + 2 K I \to 2 K C l + I_{2}$ .
No reaction if the element is less active than the one in the compound: Cu with ZnSO₄.

Predicting from test results. If metal P displaces Q from Q’s salt, P is above Q. Combine several such results to place metals in order.

**Double decomposition: $A B + C D \to A D + C B$ . Two compounds exchange radicals. In precipitation** an insoluble product forms:

Reactants	Precipitate	Colour
$N a_{2} S O_{4} + B a C l_{2} \to B a S O_{4} + 2 N a C l$	barium sulphate	white
$A g N O_{3} + N a C l \to A g C l + N a N O_{3}$	silver chloride	white, curdy
$P b (N O_{3})_{2} + 2 K I \to P b I_{2} + 2 K N O_{3}$	lead iodide	yellow

Neutralisation is also double decomposition: $N a O H + H C l \to N a C l + H_{2} O$ .

Energy changes.

*Exothermic:* heat given out; temperature of surroundings rises: burning, respiration, neutralisation, $C a O + H_{2} O \to C a (O H)_{2} + heat$ .
*Endothermic:* heat taken in: $N_{2} + O_{2} \to 2 N O - heat$ (about 3000 °C in an electric arc or lightning), $C a C O_{3} \to C a O + C O_{2}$ , dissolving ammonium chloride (the tube feels cold).
Light given out (burning Mg), light taken in (photosynthesis, photography); electricity given out (Daniell cell) or taken in (electrolysis).

Mass questions. Balance, write the relative masses with coefficients, and use proportion. In displacement of copper by iron, 56 g of iron goes into solution while 63.5 g of copper is deposited, so the mass of an iron nail rises by 7.5 g for each 56 g of iron dissolved.

Displacement, Double Decomposition and Energy Changes in Reactions: formulas and key facts

Displacement: $A + B C \to A C + B$; A above B in the series.
Double decomposition: $A B + C D \to A D + C B$; Precipitate or water forms.
Precipitates and colours: $B a S O_{4} : white ∣ A g C l : white, curdy ∣ P b I_{2} : yellow$; From the syllabus equations.
Mass in displacement: $F e + C u S O_{4} \to F e S O_{4} + C u : 56 g Fe \to 63.5 g Cu$; Fe = 56, Cu = 63.5.

How to solve Displacement, Double Decomposition and Energy Changes in Reactions questions

Decide the type first, then write and balance.

1Check the activity series for displacement questions.
2For double decomposition swap the radicals using the criss-cross rule and balance.
3Mark the insoluble product as the precipitate and give its colour.
4For masses, take relative masses with coefficients and use proportion.
5Classify the reaction as exothermic or endothermic from the temperature change or the heat term.

Displacement, Double Decomposition and Energy Changes in Reactions 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.

Iron nail in blue vitriol

Suppose an iron nail is left in copper sulphate solution for some time.

What is seen, and what is the equation?

Try it first, then show the solutionSolution

The blue colour fades to pale green; a reddish-brown layer of copper forms on the nail.
$F e + C u S O_{4} \to F e S O_{4} + C u$ (iron is above copper).

Answer: Blue to pale green; copper deposit.

Memory hook: Active metal wins.

Tricks to remember Displacement, Double Decomposition and Energy Changes in Reactions

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

Higher in the series wins
The element higher in the series displaces the one below it.
Use it when: Displacement.
Swap partners
Double decomposition swaps the radicals; one pair is insoluble.
Use it when: Precipitation.
Exo out, endo in
Exothermic: heat exits. Endothermic: heat enters.
Use it when: Energy changes.

Displacement, Double Decomposition and Energy Changes in Reactions: solved examples

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

Example 1: Which metal displaces copper · 1 mark · MCQBoard level

Which of the following metals will displace copper from copper sulphate solution?
(a) silver
(b) gold
(c) zinc
(d) mercury

Try it first, then show the solutionSolution

Zinc is above copper in the series; the others are below it.

Answer: (c) zinc

Example 2: Zinc and copper · 2 marks · Short answerBoard level

What happens when (a) a zinc strip is put in copper sulphate solution, (b) a copper strip is put in zinc sulphate solution? Give the equation where a reaction occurs.

Try it first, then show the solutionSolution

(a) Zinc is above copper: $Z n + C u S O_{4} \to Z n S O_{4} + C u$ ; blue colour fades, copper deposits. (1)
(b) No reaction, because copper is less active than zinc. (1)

Answer: (a) displacement (b) no reaction

Example 3: Placing metals in order · 3 marks · Short answerHarder board level

The following are observed: iron displaces copper from copper sulphate; copper does not displace zinc from zinc sulphate; zinc displaces iron from iron(II) sulphate; copper displaces silver from silver nitrate. Arrange Zn, Fe, Cu and Ag in decreasing order of activity and write the equations.

Try it first, then show the solutionSolution

Order: Zn > Fe > Cu > Ag. (1)
$F e + C u S O_{4} \to F e S O_{4} + C u$ ; $Z n + F e S O_{4} \to Z n S O_{4} + F e$ . (1)
$C u + 2 A g N O_{3} \to C u (N O_{3})_{2} + 2 A g$ . (1)

Answer: Zn > Fe > Cu > Ag

Example 4: Mass of precipitate · 3 marks · Short answerHarder board level

5.2 g of barium chloride (BaCl₂) reacts with excess sodium sulphate solution (Ba = 137, Cl = 35.5, S = 32, O = 16). Write the equation, name the precipitate and its colour, and find its mass.

Try it first, then show the solutionSolution

$B a C l_{2} + N a_{2} S O_{4} \to B a S O_{4} + 2 N a C l$ ; white precipitate of barium sulphate. (1)
$M_{r}$ of BaCl₂ $= 208$ ; of BaSO₄ $= 233$ . (1)
Mass $= 5.2 \times \frac{233}{208} \approx 5.83$ g. (1)

Answer: White BaSO₄; about 5.83 g

Example 5: Hot and cold reactions · 4 marks · Case-basedHOTS / competency

A student adds 5.6 g of quicklime (CaO) to water and the temperature of the mixture rises. In another test, ammonium chloride dissolves in water and the test tube feels cold.
(a) Classify each change as exothermic or endothermic.
(b) Write the equation for CaO with water and find the mass of Ca(OH)₂ formed (Ca = 40, O = 16, H = 1).
(c) Why does the formation of nitric oxide from nitrogen and oxygen need about 3000 °C?
(d) Which of the two changes would suit a self-heating food can?

Try it first, then show the solutionSolution

(a) Quicklime and water: exothermic; dissolving NH₄Cl: endothermic. (1)
(b) $C a O + H_{2} O \to C a (O H)_{2}$ ; $5.6 \times \frac{74}{56} = 7.4$ g. (1½)
(c) The reaction $N_{2} + O_{2} \to 2 N O$ is endothermic, so heat must be supplied continuously (electric arc, lightning). (1)
(d) The CaO and water reaction, because it gives out heat. (½)

Answer: (a) exo; endo (b) 7.4 g (c) endothermic (d) CaO + water

Where students lose marks in Displacement, Double Decomposition and Energy Changes in Reactions

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

Mistake: Reaction predicted for a less active metal
No displacement from a more active metal’s salt.
Wrong working
Cu + ZnSO₄ → CuSO₄ + Zn
Right working
No reaction.
Do this: Check the series first.
Usually costs: 1 to 2 marks.
Mistake: Precipitate colour confused
PbI₂ is yellow; BaSO₄ and AgCl are white.
Wrong working
Lead iodide is white.
Right working
Lead iodide is yellow.
Do this: Learn the three colours.
Usually costs: 1 mark.
Mistake: Exothermic reaction described as one in which heat is absorbed
Exothermic gives out heat.
Wrong working
Slaking of lime absorbs heat.
Right working
Slaking of lime gives out heat.
Do this: Exo = exit.
Usually costs: 1 mark.

Exam tips

Quote the activity series before predicting a displacement reaction.
Give the colour of every precipitate.
Write "no reaction" with a reason when the metal is less active.
Link endothermic or exothermic to the temperature change observed.

Practice questions on Displacement, Double Decomposition and Energy Changes in Reactions

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 · MCQBoard level
The yellow precipitate formed when lead nitrate solution is mixed with potassium iodide solution is:
(a) lead sulphate
(b) lead iodide
(c) silver iodide
(d) potassium nitrate
Hint
Colour table.
Answer
(b) lead iodide
Q2 · 1 mark · Very short answerBoard level
What is the colour of the silver chloride precipitate?
Hint
Curdy.
Answer
White (curdy).
Q3 · 2 marks · Short answerHarder board level
Complete and balance: Cl₂ + KI → ?. Name the type of reaction.
Hint
Chlorine is more reactive than iodine.
Answer
$C l_{2} + 2 K I \to 2 K C l + I_{2}$ ; displacement (iodine is liberated).
Q4 · 3 marks · Short answerHarder board level
Balance and classify: (i) Mg + HCl → MgCl₂ + H₂, (ii) Al + CuCl₂ → AlCl₃ + Cu, (iii) Na₂SO₄ + Pb(NO₃)₂ → PbSO₄ + NaNO₃.
Hint
Two displacements and one double decomposition.
Answer
(i) $M g + 2 H C l \to M g C l_{2} + H_{2}$ (displacement). (ii) $2 A l + 3 C u C l_{2} \to 2 A l C l_{3} + 3 C u$ (displacement). (iii) $N a_{2} S O_{4} + P b (N O_{3})_{2} \to P b S O_{4} + 2 N a N O_{3}$ (double decomposition; white precipitate of PbSO₄).
Q5 · 2 marks · Assertion-reasonHarder board level
Assertion (A): Copper does not react with dilute hydrochloric acid.
Reason (R): Copper is below hydrogen in the activity series.
(a) Both true, R explains A.
(b) Both true, R does not explain A.
(c) A true, R false.
(d) A false, R true.
Hint
Metals above H displace hydrogen.
Answer
(a)
Q6 · 3 marks · Short answerHOTS / competency
An iron nail is left in copper sulphate solution until 5.6 g of iron has dissolved (Fe = 56, Cu = 63.5). Find the mass of copper deposited and the change in the mass of the nail if all the copper sticks to it.
Hint
56 g Fe → 63.5 g Cu.
Answer
Copper deposited $= 5.6 \times \frac{63.5}{56} = 6.35$ g. The nail loses 5.6 g and gains 6.35 g: its mass increases by $0.75$ g.

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.Name the first and last metals in the series given.Show answer
K most active; Au least active.
2.Colour of lead iodide?Show answer
Yellow.
3.Example of an endothermic reaction?Show answer
N₂ + O₂ → 2NO (or CaCO₃ → CaO + CO₂).
4.Is neutralisation exothermic?Show answer
Yes.
5.What is a precipitate?Show answer
An insoluble solid formed in a reaction in solution.

Displacement, Double Decomposition and Energy Changes in Reactions: frequently asked questions

What is a displacement reaction?

A more reactive element displaces a less reactive element from its compound, for example Zn + CuSO₄ → ZnSO₄ + Cu.

What is double decomposition?

A reaction in which two compounds exchange their radicals to form two new compounds, often with an insoluble precipitate.

What is the difference between exothermic and endothermic reactions?

Exothermic reactions give out heat; endothermic reactions take in heat.