ChemistryICSE Class 9 · Chapter 2

Chemical Changes and Reactions ICSE Class 9: Types of Reactions and Energy Changes

Updated 15 October 2026 · Fact-checked

A chemical change makes new substances. ICSE Class 9 studies four types: direct combination, decomposition, displacement and double displacement (such as sodium sulphate with barium chloride). Reactions that give out heat, light or electricity are exothermic; those that take them in are endothermic.

What Chemical Changes and Reactions is about

In a chemical change the substances present are turned into new substances with new properties. It is usually permanent, and is shown by signs such as a colour change, a gas given off, a precipitate, a change of temperature, or light and sound.

The CISCE 2028 syllabus (Class IX, topic 2) covers: the types of chemical change, direct combination, decomposition, displacement and double decomposition (double displacement), with the three named double-displacement reactions, sodium sulphate with barium chloride, silver nitrate with sodium chloride and lead nitrate with potassium iodide; and energy changes: exothermic and endothermic reactions involving heat, light and electricity. All reactions are to be learnt with balanced equations and observations.

Weightage: CISCE publishes no chapter weightage for ICSE Chemistry. Chemical Changes and Reactions is topic 2 of the Class IX part of the 2028 syllabus; the Class IX paper (80 marks, 2 hours) is set and marked by the school.Source: CISCE ICSE Chemistry (Science Paper 2) syllabus, examination year 2028 (Class IX)

Key concepts

Direct combination

Two or more substances (elements or compounds) combine to form a single new substance: $A + B \to A B$ .

Element + element:

$F e + S \to F e S$ (grey mixture glows and forms a black solid)
$2 M g + O_{2} \to 2 M g O$ (dazzling white light, white ash)
$H_{2} + C l_{2} \to 2 H C l$ (in sunlight)

Compound + compound or element:

$C a O + H_{2} O \to C a (O H)_{2}$ (hissing, much heat: slaking of lime)
$N H_{3} + H C l \to N H_{4} C l$ (dense white fumes)
$C O_{2} + H_{2} O \to H_{2} C O_{3}$
$2 S O_{2} + O_{2} ⇌ 2 S O_{3}$

Worked example

Quicklime is added to water in a beaker. State two observations and write the equation. Name the type of reaction.

Try it first, then show the solutionSolution

A hissing sound is heard and the beaker becomes very hot.
$C a O + H_{2} O \to C a (O H)_{2}$
Two substances form one product: direct combination (and exothermic).

Answer: Hissing and heat; direct combination.

Decomposition

A single compound breaks down into two or more simpler substances: $A B \to A + B$ . Energy is needed, so it is named by the energy used.

Thermal decomposition (heat):

$C a C O_{3} \to C a O + C O_{2}$
$C u C O_{3} \to C u O + C O_{2}$ (green → black)
$Z n C O_{3} \to Z n O + C O_{2}$ (white; residue yellow when hot, white when cold)
$2 H g O \to 2 H g + O_{2}$ (red → silvery droplets)
$2 P b (N O_{3})_{2} \to 2 P b O + 4 N O_{2} + O_{2}$ (crackling sound, reddish brown gas, residue yellow)
$(N H_{4})_{2} C r_{2} O_{7} \to C r_{2} O_{3} + N_{2} + 4 H_{2} O$ (orange → green, volcano-like)

Electrolytic decomposition (electricity): $2 H_{2} O \to 2 H_{2} + O_{2}$ (acidified water)

Photolytic decomposition (light): $2 A g C l \to 2 A g + C l_{2}$ and $2 A g B r \to 2 A g + B r_{2}$ (white or pale yellow turns grey; used in black-and-white photography)

Worked example

Lead nitrate crystals are heated in a dry test tube. Give three observations and the equation.

Try it first, then show the solutionSolution

Crackling sound (decrepitation) as the crystals burst.
Reddish brown gas (nitrogen dioxide) is given off.
A residue is left that is reddish brown when hot and yellow when cold (lead monoxide).
$2 P b (N O_{3})_{2} \to 2 P b O + 4 N O_{2} + O_{2}$

Answer: As above.

Displacement

A more active element displaces a less active element from its compound: $A + B C \to A C + B$ . Which metal wins is shown by the activity (reactivity) series:

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

Metal displaces metal:

$Z n + C u S O_{4} \to Z n S O_{4} + C u$ (blue colour fades; reddish brown copper coats the zinc)
$F e + C u S O_{4} \to F e S O_{4} + C u$ (blue → pale green; iron nail turns reddish brown)
$C u + 2 A g N O_{3} \to C u (N O_{3})_{2} + 2 A g$ (shiny silver crystals; solution turns blue)

Metal displaces hydrogen (metals above H): $Z n + 2 H C l \to Z n C l_{2} + H_{2}$

Non-metal displaces non-metal: $C l_{2} + 2 K B r \to 2 K C l + B r_{2}$ and $C l_{2} + 2 K I \to 2 K C l + I_{2}$

No reaction if the element is less active: $C u + Z n S O_{4}$ gives nothing.

Worked example

Will silver react with copper sulphate solution? Will iron react with zinc sulphate solution? Give reasons.

Try it first, then show the solutionSolution

Silver is below copper in the activity series, so it cannot displace copper.
Iron is below zinc, so it cannot displace zinc.

Answer: No reaction in either case.

Double decomposition (double displacement)

Two compounds exchange their radicals (ions) to form two new compounds: $A B + C D \to A D + C B$ . Usually one product is an insoluble precipitate (precipitation) or water (neutralisation).

The three named in the syllabus, all precipitation reactions:

Reactants	Equation	Observation
Sodium sulphate + barium chloride	$N a_{2} S O_{4} + B a C l_{2} \to B a S O_{4} + 2 N a C l$	white precipitate of barium sulphate
Silver nitrate + sodium chloride	$A g N O_{3} + N a C l \to A g C l + N a N O_{3}$	white, curdy precipitate of silver chloride
Lead nitrate + potassium iodide	$P b (N O_{3})_{2} + 2 K I \to P b I_{2} + 2 K N O_{3}$	yellow precipitate of lead iodide

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

Demonstration: add one solution to the other in a test tube and watch the precipitate form; filter it off and the other product stays in the filtrate.

Worked example

Name the precipitate and give its colour when potassium iodide solution is added to lead nitrate solution. Write the equation.

Try it first, then show the solutionSolution

Radicals exchange: Pb²⁺ pairs with I⁻, K⁺ with NO₃⁻.
$P b (N O_{3})_{2} + 2 K I \to P b I_{2} + 2 K N O_{3}$
Lead iodide is insoluble and yellow.

Answer: Yellow lead iodide.

Exothermic and endothermic reactions: heat

Every chemical change involves an energy change.

Exothermic: energy is given out; the surroundings get hotter. $C + O_{2} \to C O_{2} + heat$ ; $C a O + H_{2} O \to C a (O H)_{2} + heat$ ; burning fuels; respiration; neutralisation.
Endothermic: energy is taken in; the surroundings get colder or heat must be supplied. $N_{2} + O_{2} \to 2 N O - heat$ (needs about 3000 °C in an electric arc or lightning); $C a C O_{3} + heat \to C a O + C O_{2}$ ; dissolving ammonium chloride in water (the test tube feels cold).

Most combination reactions are exothermic and most decomposition reactions are endothermic, but there are exceptions: $N_{2} + O_{2}$ is a combination that is endothermic.

Worked example

Ammonium chloride is dissolved in water in a test tube held in the hand. What is felt and why?

Try it first, then show the solutionSolution

The tube feels cold.
Dissolving ammonium chloride takes in heat from the water, the glass and the hand.

Answer: Cold: the change is endothermic.

Light and electricity in chemical changes

Energy may also be in the form of light or electricity.

Light given out: $2 M g + O_{2} \to 2 M g O$ (dazzling white light; used in fireworks and old flash bulbs); burning candles and fuels.

Light taken in: photosynthesis $6 C O_{2} + 6 H_{2} O \to C_{6} H_{12} O_{6} + 6 O_{2}$ (sunlight, chlorophyll); $2 A g B r \to 2 A g + B r_{2}$ (photography).

Electricity given out: in a cell, a displacement reaction such as $Z n + C u S O_{4} \to Z n S O_{4} + C u$ (Daniell cell) or the reactions in a dry cell drive a current.

Electricity taken in: electrolysis, e.g. $2 H_{2} O \to 2 H_{2} + O_{2}$ and electroplating.

Worked example

Classify as giving out or taking in energy, and name the form: (a) photosynthesis, (b) a torch battery working, (c) electrolysis of acidified water.

Try it first, then show the solutionSolution

(a) Takes in light.
(b) Gives out electricity.
(c) Takes in electricity.

Answer: As above.

Chemistry in real life: Chemical Changes and Reactions

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.

Exothermic and endothermic packs

Suppose a sports first-aid kit has an 'instant cold pack' containing ammonium nitrate and water in separate pouches, and a camping kit has a 'self-heating' pack using quicklime and water.

Explain why one pack gets cold and the other hot when the pouches are mixed.

Try it first, then show the solutionSolution

Dissolving ammonium nitrate in water takes in heat from the surroundings: endothermic, so the pack feels cold.
Quicklime and water combine: $C a O + H_{2} O \to C a (O H)_{2}$ , giving out a lot of heat: exothermic.

Answer: Cold pack: endothermic dissolving. Hot pack: exothermic combination.

Memory hook: Heat in, cold pack; heat out, hot pack.

Iron container for copper sulphate

Suppose a farmer stores copper sulphate solution (a fungicide spray) in an iron bucket overnight and finds a reddish brown coating inside and a greenish solution the next morning.

Explain the change with an equation and say what container should be used.

Try it first, then show the solutionSolution

Iron is more active than copper.
$F e + C u S O_{4} \to F e S O_{4} + C u$ : copper deposits, and iron(II) sulphate makes the solution pale green.
The spray is spoiled and the bucket corroded; a plastic container should be used.

Answer: Displacement; store it in plastic.

Memory hook: A more active metal steals the place of a less active one.

Mass in a decomposition

Suppose a lime kiln worker heats 20 kg of pure limestone until it stops losing mass: $C a C O_{3} \to C a O + C O_{2}$ .

Using relative molecular masses, find the mass of quicklime left and of carbon dioxide released. (Ca = 40, C = 12, O = 16)

Try it first, then show the solutionSolution

Mr: CaCO₃ = 100, CaO = 56, CO₂ = 44.
100 parts of CaCO₃ give 56 parts of CaO and 44 parts of CO₂.
CaO = 20 × 56/100 = 11.2 kg; CO₂ = 20 × 44/100 = 8.8 kg (total 20 kg).

Answer: 11.2 kg quicklime; 8.8 kg carbon dioxide.

Memory hook: Mass is conserved: products add up to the reactant.

Topics in this chapter

Chemical Changes and Reactions formula sheet

Barium chloride with sodium sulphate: $N a_{2} S O_{4} + B a C l_{2} \to B a S O_{4} + 2 N a C l$; White precipitate.
Silver nitrate with sodium chloride: $A g N O_{3} + N a C l \to A g C l + N a N O_{3}$; White curdy precipitate.
Lead nitrate with potassium iodide: $P b (N O_{3})_{2} + 2 K I \to P b I_{2} + 2 K N O_{3}$; Yellow precipitate.
Heating lead nitrate: $2 P b (N O_{3})_{2} \to 2 P b O + 4 N O_{2} + O_{2}$; Reddish brown gas; yellow residue.
Zinc in copper sulphate: $Z n + C u S O_{4} \to Z n S O_{4} + C u$; Blue fades.
Nitrogen with oxygen: $N_{2} + O_{2} \to 2 N O$; Endothermic combination.

Chemical Changes and Reactions: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

Heating lead nitrate in a test tube

Draw the set-up for the thermal decomposition of lead nitrate and label the observations.

Draw

A dry hard-glass test tube clamped at an angle, held by a test-tube holder or clamp.
Lead nitrate crystals at the bottom; a burner flame under them.
Reddish brown fumes rising from the mouth.

Label

lead nitrate
heat
reddish brown NO₂
yellow residue (PbO)

Watch out: Point the mouth away from people; show the residue colour.

Precipitation in a test tube

Show the double-displacement reaction of lead nitrate with potassium iodide.

Draw

A test tube with colourless lead nitrate solution.
A dropper adding colourless potassium iodide solution.
A yellow solid settling at the bottom; clear liquid above.

Label

lead nitrate solution
potassium iodide added
yellow precipitate (PbI₂)
filtrate (KNO₃ solution)

Watch out: Shade the precipitate; label both products.

How the school exam asks Chemical Changes and Reactions

The question patterns to expect, and the method that works for each.

Question type	How it is asked	Method
Classify the reaction	Name the type of each of 4-5 equations; give one example of each type.	Count reactants and products; look for exchange of radicals.
Observations	State what is seen when lead nitrate is heated, barium chloride is added to a sulphate, zinc is put in copper sulphate.	Write colour, gas, precipitate, sound or heat: not just the equation.
Double displacement equations	Complete and balance the three named reactions; name and colour the precipitate.	Exchange partners; balance; mark the insoluble product.
Energy changes	Classify as exothermic or endothermic; give examples involving heat, light and electricity.	Given out = exo; taken in = endo; say which form.
Activity series predictions	Will metal X displace metal Y? Explain an unexpected observation.	Higher in the series displaces lower.

Tricks to remember Chemical Changes and Reactions

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

Pattern letters
Combination A + B → AB; Decomposition AB → A + B; Displacement A + BC → AC + B; Double displacement AB + CD → AD + CB.
Use it when: Identifying the type at a glance.
Precipitate colours: 'Barium Bleaches, Silver Snows, Lead Lemons'
BaSO₄ white, AgCl white curdy, PbI₂ yellow.
Use it when: The three syllabus double-displacement reactions.
Activity series: 'Please Stop Calling Me A Zebra, I Like Her Calling Me A Sweet Goat'
K Na Ca Mg Al Zn Fe Pb H Cu Hg Ag Au.
Use it when: Displacement predictions.
EXO exits, ENDO enters
Exothermic: energy exits. Endothermic: energy enters.
Use it when: Energy classification.

Chemical Changes and Reactions: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

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

The reaction $A g N O_{3} + N a C l \to A g C l + N a N O_{3}$ is:
(a) direct combination
(b) decomposition
(c) displacement
(d) double displacement

Try it first, then show the solutionSolution

Both compounds exchange radicals.

Answer: (d)

Example 2: Endothermic combination · 1 mark · Assertion-reasonHarder exam level

Assertion (A): The reaction of nitrogen with oxygen to form nitric oxide is a combination reaction.
Reason (R): All combination reactions are exothermic.
(a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
(c) (A) is true and (R) is false.
(d) (A) is false and (R) is true.

Try it first, then show the solutionSolution

Two reactants give one product, so A is true.
This very reaction is endothermic, so R is false.

Answer: (c)

Example 3: Three double displacements · 3 marks · Short answerExam level

Write balanced equations and give the colour of the precipitate when (a) barium chloride is added to sodium sulphate, (b) sodium chloride is added to silver nitrate, (c) potassium iodide is added to lead nitrate.

Try it first, then show the solutionSolution

(a) $N a_{2} S O_{4} + B a C l_{2} \to B a S O_{4} + 2 N a C l$ : white.
(b) $A g N O_{3} + N a C l \to A g C l + N a N O_{3}$ : white, curdy.
(c) $P b (N O_{3})_{2} + 2 K I \to P b I_{2} + 2 K N O_{3}$ : yellow.

Answer: As above.

Example 4: Classify five reactions · 4 marks · Short answerHarder exam level

Classify, with a reason: (a) $2 H g O \to 2 H g + O_{2}$ (b) $C l_{2} + 2 K I \to 2 K C l + I_{2}$ (c) $N H_{3} + H C l \to N H_{4} C l$ (d) $N a O H + H C l \to N a C l + H_{2} O$

Try it first, then show the solutionSolution

(a) Thermal decomposition: one compound breaks into two substances.
(b) Displacement: chlorine, more active, displaces iodine.
(c) Direct combination: two compounds form one.
(d) Double displacement (neutralisation): radicals exchange.

Answer: As above.

Example 5: Activity series reasoning · 3 marks · Short answerHarder exam level

Metal P displaces copper from copper sulphate but not iron from iron(II) sulphate. Metal Q displaces both. Arrange P, Q, Fe and Cu in decreasing order of activity and suggest what P and Q might be.

Try it first, then show the solutionSolution

P is above Cu but below Fe: Fe > P > Cu.
Q is above Fe.
Order: Q > Fe > P > Cu. P could be lead; Q could be zinc.

Answer: Q > Fe > P > Cu (for example Q = Zn, P = Pb).

Example 6: Energy forms · 3 marks · Short answerExam level

Give one example, with an equation, of a reaction in which (a) light is given out, (b) light is taken in, (c) electricity is taken in.

Try it first, then show the solutionSolution

(a) $2 M g + O_{2} \to 2 M g O$
(b) $2 A g B r \to 2 A g + B r_{2}$ (in light)
(c) $2 H_{2} O \to 2 H_{2} + O_{2}$ (electrolysis)

Answer: As above.

Example 7: Mass of precipitate · 3 marks · Short answerHOTS / competency

3.31 g of lead nitrate is dissolved in water and excess potassium iodide solution is added. Calculate the mass of lead iodide precipitated. (Pb = 207, N = 14, O = 16, K = 39, I = 127)

Try it first, then show the solutionSolution

$P b (N O_{3})_{2} + 2 K I \to P b I_{2} + 2 K N O_{3}$ : 1 formula of lead nitrate gives 1 of lead iodide.
Mr Pb(NO₃)₂ = 207 + 2(14 + 48) = 331; Mr PbI₂ = 207 + 254 = 461.
Mass of PbI₂ = 3.31 × 461/331 = 4.61 g.

Answer: 4.61 g

Example 8: Identify the unknowns · 3 marks · Short answerHOTS / competency

A white solid X, on heating, gives a gas that turns lime water milky, and leaves a residue Y that is yellow when hot and white when cold. (a) Identify X and Y. (b) Write the equation. (c) Is the change exothermic or endothermic?

Try it first, then show the solutionSolution

Yellow when hot, white when cold is the mark of zinc oxide.
X = zinc carbonate; Y = zinc oxide; gas = CO₂.
$Z n C O_{3} \to Z n O + C O_{2}$ ; heat must be supplied: endothermic.

Answer: X = ZnCO₃, Y = ZnO; endothermic.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 1 + 2 + 2 marks.

Three colourless solutions

Suppose a lab assistant has three unlabelled bottles, P, Q and R, known to be sodium chloride, sodium sulphate and potassium iodide solutions. She has barium chloride, silver nitrate and lead nitrate solutions to test them.

(i) · 1 mark
Lead nitrate gives a yellow precipitate with R. What does R contain, and what is the precipitate?
Answer
Answer: R is potassium iodide; the precipitate is yellow lead iodide, PbI₂.
(ii) · 2 marks
Barium chloride gives a white precipitate with Q. Write the equation.
Answer
Answer: Q is sodium sulphate: $N a_{2} S O_{4} + B a C l_{2} \to B a S O_{4} + 2 N a C l$
(iii) · 2 marks
Silver nitrate gives a white curdy precipitate with P. Name P and the precipitate, and give the type of reaction.
Answer
Answer: P = sodium chloride; silver chloride; double displacement (precipitation).

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Give one observation each when (a) zinc granules are put in copper sulphate solution, (b) barium chloride solution is added to sodium sulphate solution. (2 marks) (2 marks)

Step	Marks
(a) Blue colour fades; reddish brown deposit on zinc	1
(b) White precipitate forms	1

Lead nitrate is heated. Write the equation and two observations. (3 marks) (3 marks)

Step	Marks
$2 P b (N O_{3})_{2} \to 2 P b O + 4 N O_{2} + O_{2}$ (balanced)	1
Reddish brown gas evolved	1
Crackling sound, or residue yellow when cold	1

Define exothermic and endothermic reactions and give one example of each with an equation. (4 marks) (4 marks)

Step	Marks
Exothermic: heat (energy) given out	1
e.g. $C + O_{2} \to C O_{2}$ + heat	1
Endothermic: heat (energy) absorbed	1
e.g. $C a C O_{3} \to C a O + C O_{2}$ (heat absorbed)	1

Where students lose marks in Chemical Changes and Reactions

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

Mistake: Writing the equation instead of the observation
CISCE examiners' comments on the ICSE Class 10 paper, 2025: asked for an observation when barium chloride was added to a sulphate solution, some candidates wrote the equation or the products instead of the white precipitate, and a few confused white with blue.
Wrong working
Observation: BaSO₄ and ZnCl₂ are formed.
Right working
A white precipitate is formed.
Do this: An observation is what you see, hear, smell or feel.
Usually costs: 1 mark.
Mistake: Vague colour of the gas
CISCE examiners' comments on the ICSE Class 10 paper, 2025: for heating lead nitrate, some candidates wrote only 'brown gas' without specifying it fully.
Wrong working
Brown gas is seen.
Right working
Reddish brown gas (nitrogen dioxide) is evolved.
Do this: Learn colours exactly: reddish brown NO₂, yellow PbO when cold.
Usually costs: 1 mark.
Mistake: Not recognising double displacement
CISCE examiners' comments on the ICSE Class 10 paper, 2025: in an MCQ on the best pair of reactants for making lead(II) sulphate, some candidates did not see that double displacement (precipitation), sodium sulphate + lead nitrate, is the method for an insoluble salt.
Wrong working
Lead + sulphuric acid is a double displacement.
Right working
Two compounds exchange radicals: $N a_{2} S O_{4} + P b (N O_{3})_{2} \to P b S O_{4} + 2 N a N O_{3}$
Do this: Double displacement needs two compounds, both swapping partners.
Usually costs: 1 mark.
Mistake: Assuming every decomposition or combination has a fixed energy sign
Students learn 'combination = exothermic' as a rule.
Wrong working
N₂ + O₂ → 2NO is exothermic because it is a combination.
Right working
It is endothermic: it needs an electric arc or lightning.
Do this: Learn the exceptions named in your notes.
Usually costs: 1 mark.
Mistake: Displacement in the wrong direction
Activity series not checked.
Wrong working
$C u + F e S O_{4} \to C u S O_{4} + F e$
Right working
No reaction: copper is less active than iron.
Do this: Check that the free element is above the one in the compound.
Usually costs: 1 mark.

Practice questions on Chemical Changes and 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 · MCQExam level
Which reaction is a photochemical decomposition?
(a) $C a C O_{3} \to C a O + C O_{2}$
(b) $2 A g B r \to 2 A g + B r_{2}$
(c) $2 H_{2} O \to 2 H_{2} + O_{2}$
(d) $F e + S \to F e S$
Hint
Light.
Answer
(b)
Q2 · 1 mark · MCQExam level
When iron filings are added to copper sulphate solution, the solution turns:
(a) colourless
(b) pale green
(c) deep blue
(d) yellow
Hint
Iron(II) sulphate.
Answer
(b)
Q3 · 3 marks · Short answerExam level
Complete and balance: (a) $C u + A g N O_{3} \to$ (b) $(N H_{4})_{2} C r_{2} O_{7} \to$ (c) $C l_{2} + K B r \to$
Hint
Displacement; decomposition; displacement.
Answer
(a) $C u + 2 A g N O_{3} \to C u (N O_{3})_{2} + 2 A g$ (b) $(N H_{4})_{2} C r_{2} O_{7} \to C r_{2} O_{3} + N_{2} + 4 H_{2} O$ (c) $C l_{2} + 2 K B r \to 2 K C l + B r_{2}$
Q4 · 3 marks · Short answerExam level
Give one example of each, with an equation: (a) combination of two compounds, (b) decomposition by electricity, (c) displacement of hydrogen.
Hint
Lime; water; zinc and acid.
Answer
(a) $C a O + H_{2} O \to C a (O H)_{2}$ (b) $2 H_{2} O \to 2 H_{2} + O_{2}$ (c) $Z n + 2 H C l \to Z n C l_{2} + H_{2}$
Q5 · 2 marks · Short answerHarder exam level
Explain why (a) silver chloride is stored in dark bottles, (b) a copper vessel can hold zinc sulphate solution but a zinc vessel cannot hold copper sulphate solution.
Hint
Light; activity series.
Answer
(a) Light decomposes it: $2 A g C l \to 2 A g + C l_{2}$ . (b) Copper is less active than zinc and does not displace it; zinc displaces copper from copper sulphate, so the vessel would be eaten away.
Q6 · 2 marks · Short answerHarder exam level
Copper carbonate is heated. State the colour change and write the equation. Is the reaction exothermic or endothermic?
Hint
Green to black.
Answer
Green → black. $C u C O_{3} \to C u O + C O_{2}$ ; endothermic.
Q7 · 3 marks · Short answerHarder exam level
Sodium sulphate solution is mixed with lead nitrate solution. (a) Predict the type of reaction and write the equation. (b) How would you separate the two products?
Hint
Lead sulphate is insoluble.
Answer
(a) Double displacement: $N a_{2} S O_{4} + P b (N O_{3})_{2} \to P b S O_{4} + 2 N a N O_{3}$ (white precipitate). (b) Filter: lead sulphate stays on the paper; evaporate the filtrate to get sodium nitrate.
Q8 · 3 marks · Short answerHarder exam level
Mercury(II) oxide is heated. (a) Write the equation. (b) What two observations are made? (c) What does this reaction tell you about the stability of oxides of less active metals?
Hint
Red powder, silvery drops.
Answer
(a) $2 H g O \to 2 H g + O_{2}$ (b) The red powder disappears and silvery droplets of mercury collect on the cooler part; the gas relights a glowing splint. (c) Oxides of less active metals decompose easily on heating.
Q9 · 3 marks · Short answerHarder exam level
Explain how electricity is produced when zinc and copper are placed in their salt solutions joined by a wire, and name the reaction type.
Hint
Zinc is more active.
Answer
Zinc, being more active, loses electrons and goes into solution; the electrons flow through the wire to copper, where copper ions gain them and deposit as copper. The overall change $Z n + C u S O_{4} \to Z n S O_{4} + C u$ is displacement, and the electron flow is the current.
Q10 · 3 marks · Short answerHOTS / competency
1.0 g of sodium chloride is dissolved and treated with excess silver nitrate solution. What mass of silver chloride forms? (Na = 23, Cl = 35.5, Ag = 108)
Hint
1 NaCl → 1 AgCl.
Answer
Mr NaCl = 58.5, AgCl = 143.5. Mass = 1.0 × 143.5/58.5 = 2.45 g.
Q11 · 3 marks · Short answerHOTS / competency
A student heats a blue crystalline solid. Droplets of water form at the mouth of the tube and a white powder remains; adding water to the cooled powder turns it blue and makes the tube warm. (a) Name the solid. (b) Classify both changes as exothermic or endothermic. (c) Write the formulae involved.
Hint
Blue vitriol.
Answer
(a) Copper(II) sulphate pentahydrate. (b) Heating to drive off water is endothermic; adding water back is exothermic. (c) CuSO₄·5H₂O → CuSO₄ + 5H₂O, and the reverse.

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.Colour of lead iodide precipitate?Show answer
Yellow.
2.Precipitate from sodium sulphate + barium chloride?Show answer
Barium sulphate, white.
3.Gas from heating lead nitrate?Show answer
Nitrogen dioxide (reddish brown) and oxygen.
4.Type of reaction: Zn + CuSO₄?Show answer
Displacement.
5.An endothermic combination?Show answer
N₂ + O₂ → 2NO.
6.Decomposition by light?Show answer
2AgCl → 2Ag + Cl₂ (or AgBr).
7.Is neutralisation exothermic or endothermic?Show answer
Exothermic.
8.Oxide that is yellow when hot, white when cold?Show answer
Zinc oxide.

Best order to study it

Learn the four reaction patterns with two examples each.
Learn the three named double-displacement reactions with colours.
Learn the thermal decompositions with observations.
Learn the activity series and practise displacement predictions.
Learn exothermic and endothermic examples for heat, light and electricity.

Quick revision checklist

The night before the exam: can you do each of these without looking?

Can you write and balance the three syllabus double-displacement reactions?
Can you give three observations for heating lead nitrate?
Can you predict whether a metal will displace another?
Can you give an exothermic and an endothermic example for heat, light and electricity?
Can you name an endothermic combination and explain why it is unusual?

Chemical Changes and Reactions: frequently asked questions

Is neutralisation a double displacement reaction?

Yes. The acid and base exchange radicals to form a salt and water, for example NaOH + HCl \rightarrow NaCl + H_2O.

Why is lead nitrate used to show thermal decomposition?

It gives clear observations: a crackling sound, a coloured gas and a coloured residue, and it has no water of crystallisation to confuse the result.

Is dissolving ammonium chloride a chemical change?

Dissolving is usually treated as a physical change, but it still shows an energy change. It is used here only as a simple example of heat being absorbed.

Do I need to know the full activity series?

You should know the order of the common metals, K Na Ca Mg Al Zn Fe Pb (H) Cu Hg Ag Au, to predict displacement reactions and the reactions of metals with water and acids in the hydrogen chapter.

Based on: CISCE ICSE 2028 Science (52) Chemistry (Science Paper 2) syllabus, Classes IX and X; CISCE Analysis of Pupil Performance, ICSE (Class 10) Chemistry (Science Paper 2), 2025