ChemistryICSE Class 9 · Chemical Changes and Reactions

Direct Combination and Decomposition Reactions: Equations, Observations and Mass Ratios (ICSE Class 9)

Updated 17 October 2026 · Fact-checked

In direct combination, two or more substances combine to form one new substance (A + B → AB), as in 2Mg + O₂ → 2MgO. In decomposition, one compound breaks into two or more simpler substances (AB → A + B) by heat, electricity or light, as in CaCO₃ → CaO + CO₂. Both follow the law of conservation of mass.

Count the products: many reactants giving one product is combination, one reactant giving many is decomposition, and total mass never changes.

Illustration drawn for QuestionBuddy.

Understand Direct Combination and Decomposition Reactions: Equations, Observations and Masses

Chemical change. New substances form, usually permanently; signs include a colour change, a gas, a precipitate, a temperature change, light or sound.

**Direct combination: $A + B \to A B$ .**

$2 M g + O_{2} \to 2 M g O$ (dazzling white light, white ash)
$F e + S \to F e S$ (the mixture glows and a black solid forms)
$H_{2} + C l_{2} \to 2 H C l$ (in sunlight)
$C a O + H_{2} O \to C a (O H)_{2}$ (hissing, much heat)
$N H_{3} + H C l \to N H_{4} C l$ (dense white fumes)
$C O_{2} + H_{2} O \to H_{2} C O_{3}$ ; $2 S O_{2} + O_{2} ⇌ 2 S O_{3}$

**Decomposition: $A B \to A + B$ .** Energy must be supplied.

*Thermal (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 to black); $2 H g O \to 2 H g + O_{2}$ (red solid, silvery droplets); $2 P b (N O_{3})_{2} \to 2 P b O + 4 N O_{2} + O_{2}$ (crackling, reddish-brown $N O_{2}$ , yellow residue); $(N H_{4})_{2} C r_{2} O_{7} \to C r_{2} O_{3} + N_{2} + 4 H_{2} O$ (orange to green).
*Electrolytic (electricity):* $2 H_{2} O \to 2 H_{2} + O_{2}$ (acidified water).
*Photolytic (light):* $2 A g C l \to 2 A g + C l_{2}$ , $2 A g B r \to 2 A g + B r_{2}$ (silver salts darken in light; used in photography).

Identify by pattern. One product from several reactants: combination. One reactant giving several products: decomposition. Name decomposition by the energy used.

Gas tests. Carbon dioxide turns lime water milky: $C O_{2} + C a (O H)_{2} \to C a C O_{3} + H_{2} O$ . Oxygen relights a glowing splint. Reddish-brown fumes indicate nitrogen dioxide.

Mass relations from a balanced equation. The coefficients and the relative atomic masses give ratios of masses. For $2 M g + O_{2} \to 2 M g O$ (Mg = 24, O = 16): 48 parts of Mg + 32 parts of O₂ → 80 parts of MgO; the total mass is conserved. Use proportion to scale to the actual mass given.

Steps: (1) balance the equation; (2) write the relative mass of each substance wanted, multiplied by its coefficient; (3) scale by the ratio of the given mass to the corresponding relative mass.

Direct Combination and Decomposition Reactions: Equations, Observations and Masses: formulas and key facts

Combination: $A + B \to A B$; 2Mg + O₂ → 2MgO.
Decomposition: $A B \to A + B$; Heat, electricity or light.
Thermal decomposition of lead nitrate: $2 P b (N O_{3})_{2} \to 2 P b O + 4 N O_{2} + O_{2}$; Reddish-brown NO₂; yellow PbO.
Mass proportion: $\frac{mass wanted}{relative mass wanted} = \frac{mass given}{relative mass given}$; Use coefficients × Mr.

How to solve Direct Combination and Decomposition Reactions: Equations, Observations and Masses questions

Balance first, then use the balanced equation as a recipe.

1Write and balance the equation.
2Compute relative masses of the substances involved, multiplied by coefficients.
3Check they add up on both sides.
4Set up the proportion with the mass given.
5Report the answer with units and check conservation of mass.

Direct Combination and Decomposition Reactions: Equations, Observations and Masses 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.

Quicklime and water

Suppose a mason slakes 11.2 g of quicklime (CaO) with water (Ca = 40, O = 16, H = 1).

What mass of slaked lime Ca(OH)₂ forms, and is the reaction exothermic?

Try it first, then show the solutionSolution

$C a O + H_{2} O \to C a (O H)_{2}$ : 56 g gives 74 g.
$11.2 \times \frac{74}{56} = 14.8$ g.
Much heat is given out: exothermic.

Answer: 14.8 g; exothermic.

Memory hook: Ratio of relative masses.

Tricks to remember Direct Combination and Decomposition Reactions: Equations, Observations and Masses

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

One out of many, many out of one
Combination: many into one. Decomposition: one into many.
Use it when: Classifying.
Name by energy
Heat: thermal. Light: photolytic. Electricity: electrolytic.
Use it when: Decomposition types.
Brown fumes mean nitrate
Reddish-brown NO₂ on heating a nitrate.
Use it when: Observations.

Direct Combination and Decomposition Reactions: Equations, Observations and Masses: solved examples

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

Example 1: Classify the reaction · 1 mark · MCQBoard level

The reaction 2HgO → 2Hg + O₂ is an example of:
(a) direct combination
(b) decomposition
(c) displacement
(d) double decomposition

Try it first, then show the solutionSolution

One compound gives two simpler substances: decomposition.

Answer: (b) decomposition

Example 2: Burning magnesium · 2 marks · Short answerBoard level

Write the balanced equation for burning magnesium in air and state two observations.

Try it first, then show the solutionSolution

$2 M g + O_{2} \to 2 M g O$ . (1)
A dazzling white flame (light) and a white ash of magnesium oxide. (1)

Answer: 2Mg + O₂ → 2MgO; white light, white ash

Example 3: Heating lead nitrate · 3 marks · Short answerHarder board level

6.62 g of lead nitrate is heated (Pb = 207, N = 14, O = 16). Write the equation and find the masses of lead oxide, nitrogen dioxide and oxygen formed. Show that mass is conserved.

Try it first, then show the solutionSolution

$2 P b (N O_{3})_{2} \to 2 P b O + 4 N O_{2} + O_{2}$ : $662 \to 446 + 184 + 32$ (relative masses). (1)
Scale factor $= \frac{6.62}{662} = 0.01$ : PbO $= 4.46$ g, NO₂ $= 1.84$ g, O₂ $= 0.32$ g. (1)
$4.46 + 1.84 + 0.32 = 6.62$ g: mass conserved. (1)

Answer: 4.46 g PbO; 1.84 g NO₂; 0.32 g O₂

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

6 g of magnesium burns completely in air (Mg = 24, O = 16). Find the mass of magnesium oxide formed and the mass of oxygen used.

Try it first, then show the solutionSolution

$2 M g + O_{2} \to 2 M g O$ : 48 g Mg gives 80 g MgO. (1)
MgO $= 6 \times \frac{80}{48} = 10$ g. (1)
Oxygen used $= 10 - 6 = 4$ g (or $6 \times \frac{32}{48} = 4$ g). (1)

Answer: 10 g MgO; 4 g oxygen

Example 5: An unknown green solid · 4 marks · Case-basedHOTS / competency

A green solid X turns black on heating and gives a colourless gas that turns lime water milky.
(a) Identify X and the black solid.
(b) Write the equation for the heating.
(c) Write the equation for the test of the gas.
(d) Name the type of reaction and state whether it is exothermic or endothermic.

Try it first, then show the solutionSolution

(a) X is copper carbonate, CuCO₃; the black solid is copper oxide, CuO. (1)
(b) $C u C O_{3} \to C u O + C O_{2}$ . (1)
(c) $C O_{2} + C a (O H)_{2} \to C a C O_{3} + H_{2} O$ (calcium carbonate makes the lime water milky). (1)
(d) Thermal decomposition; endothermic (heat is taken in). (1)

Answer: (a) CuCO₃; CuO (b) CuCO₃ → CuO + CO₂ (c) CO₂ + Ca(OH)₂ → CaCO₃ + H₂O (d) thermal decomposition, endothermic

Where students lose marks in Direct Combination and Decomposition Reactions: Equations, Observations and Masses

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

Mistake: Equation not balanced before mass calculations
Ratios come from coefficients.
Wrong working
Pb(NO₃)₂ → PbO + NO₂ + O₂ used as is.
Right working
2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂.
Do this: Balance first.
Usually costs: Whole numerical.
Mistake: Relative mass not multiplied by the coefficient
Two formula units give twice the mass.
Wrong working
PbO taken as 223 in the ratio.
Right working
2PbO = 446.
Do this: Write coefficient × Mr.
Usually costs: The answer.
Mistake: Reddish-brown gas named as chlorine
It is nitrogen dioxide.
Wrong working
Chlorine is evolved when lead nitrate is heated.
Right working
Nitrogen dioxide (reddish brown).
Do this: Nitrate gives NO₂.
Usually costs: 1 mark.

Exam tips

Give the balanced equation and one colour or gas observation for every reaction.
Name the energy for decomposition (thermal, electrolytic, photolytic).
Show the relative masses with coefficients in mass questions.
Test for CO₂ with lime water and write the equation.

Practice questions on Direct Combination and Decomposition Reactions: Equations, Observations and Masses

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
Which of the following is a decomposition reaction?
(a) $2 M g + O_{2} \to 2 M g O$
(b) $C a C O_{3} \to C a O + C O_{2}$
(c) $H_{2} + C l_{2} \to 2 H C l$
(d) $C a O + H_{2} O \to C a (O H)_{2}$
Hint
One reactant.
Answer
(b)
Q2 · 1 mark · Very short answerBoard level
What colour change is seen when copper carbonate is heated?
Hint
Green solid.
Answer
Green to black.
Q3 · 2 marks · Short answerHarder board level
Balance and name the type of reaction: Mg + N₂ → Mg₃N₂.
Hint
Balance Mg first.
Answer
$3 M g + N_{2} \to M g_{3} N_{2}$ ; direct combination.
Q4 · 3 marks · Short answerHarder board level
3.76 g of silver bromide decomposes in light (Ag = 108, Br = 80). Write the equation and find the masses of silver and bromine formed.
Hint
2 AgBr = 376.
Answer
$2 A g B r \to 2 A g + B r_{2}$ : 376 g gives 216 g Ag and 160 g Br₂. Scale $\frac{3.76}{376} = 0.01$ : silver $= 2.16$ g, bromine $= 1.60$ g.
Q5 · 2 marks · Assertion-reasonHarder board level
Assertion (A): Photographic film is kept away from light.
Reason (R): Silver bromide on the film decomposes in light.
(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
Photolytic decomposition.
Answer
(a)
Q6 · 3 marks · Short answerHOTS / competency
20 g of calcium carbonate is heated strongly (Ca = 40, C = 12, O = 16) and the residue is found to weigh 14 g. (a) What would the residue weigh if all the carbonate had decomposed? (b) What mass of carbon dioxide escaped? (c) What mass of calcium carbonate has decomposed and what percentage of the sample is that?
Hint
CaCO₃ = 100, CaO = 56, CO₂ = 44.
Answer
(a) $20 \times \frac{56}{100} = 11.2$ g. (b) $20 - 14 = 6$ g. (c) $6 \times \frac{100}{44} \approx 13.6$ g, i.e. about 68% of the sample: decomposition was incomplete.

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.What is direct combination?Show answer
Two or more substances combine to form a single new substance.
2.What is decomposition?Show answer
A compound breaks down into two or more simpler substances.
3.Gas given off on heating lead nitrate?Show answer
Reddish-brown nitrogen dioxide (and oxygen).
4.Photolytic decomposition example?Show answer
2AgBr → 2Ag + Br₂.
5.Test for carbon dioxide?Show answer
It turns lime water milky.

Direct Combination and Decomposition Reactions: Equations, Observations and Masses: frequently asked questions

What is a decomposition reaction?

One compound breaks down into two or more simpler substances on heating, passing electricity or exposure to light.

What are examples of direct combination?

2Mg + O₂ → 2MgO, Fe + S → FeS, CaO + H₂O → Ca(OH)₂ and NH₃ + HCl → NH₄Cl.

What is photolytic decomposition?

Decomposition by light, such as 2AgCl → 2Ag + Cl₂, used in black-and-white photography.