ScienceCBSE Class 10 · Chemical Reactions and Equations

Combination and Decomposition Reactions

Updated 14 October 2026 · Fact-checked

In a combination reaction two or more substances join to form one product, like quick lime and water giving slaked lime. In a decomposition reaction one substance breaks into two or more simpler ones, using heat, light or electricity. Combination reactions often release heat (exothermic); decomposition reactions absorb energy (endothermic).

Understand Combination and Decomposition Reactions

Reactions are sorted into types by what happens to the reactants. Two of the simplest types are mirror images of each other.

Combination reaction. Two or more reactants, elements or compounds, join to give a single product. The pattern is $A + B \to A B$ . NCERT's key example is quick lime (calcium oxide) added to water: $C a O (s) + H_{2} O (l) \to C a (O H)_{2} (a q)$ . The beaker becomes very hot, and the product, slaked lime, is used for whitewashing. Burning of coal, $C (s) + O_{2} (g) \to C O_{2} (g)$ , forming water from hydrogen and oxygen, $2 H_{2} (g) + O_{2} (g) \to 2 H_{2} O (l)$ , and burning a magnesium ribbon, $2 M g + O_{2} \to 2 M g O$ , are combination reactions too. The test is simple: count the products. Exactly one product means combination.

Exothermic and endothermic. A reaction that gives out heat as the products form is exothermic. Quick lime with water, burning of natural gas, respiration ( $C_{6} H_{12} O_{6} + 6 O_{2} \to 6 C O_{2} + 6 H_{2} O$ + energy) and the rotting of vegetable matter into compost are exothermic. A reaction that takes in energy is endothermic.

Decomposition reaction. A single reactant splits into two or more simpler products: $A B \to A + B$ . Breaking bonds needs energy, so decomposition reactions are usually endothermic. The energy can come in three forms:

Heat (thermal decomposition): green ferrous sulphate crystals lose water, change colour and then give ferric oxide, sulphur dioxide and sulphur trioxide with a smell of burning sulphur. Limestone gives quick lime and carbon dioxide, an important step for making cement. Lead nitrate gives off brown fumes of nitrogen dioxide.
Light (photolytic): white silver chloride turns grey in sunlight as silver forms; silver bromide behaves the same way. These reactions are used in black and white photography.
Electricity (electrolytic): passing current through water (with a few drops of dilute sulphuric acid) gives hydrogen and oxygen. The volume of hydrogen collected is twice that of oxygen, because a water molecule contains two hydrogen atoms for each oxygen atom.

This is why NCERT calls decomposition the opposite of combination: $2 H_{2} + O_{2} \to 2 H_{2} O$ builds water, while electrolysis takes it apart. Board questions ask you to identify the type, write the balanced equation with the condition on the arrow, describe what you observe, and say whether the change gives out or takes in energy.

Combination and Decomposition Reactions: formulas and key facts

Combination (general): $A + B \to A B$; Many reactants, one product. Example: $C a O (s) + H_{2} O (l) \to C a (O H)_{2} (a q)$ , strongly exothermic.
Decomposition (general): $A B \to A + B$; One reactant, two or more products; energy from heat, light or electricity.
Thermal decomposition of ferrous sulphate: $2 F e S O_{4} (s) heat F e_{2} O_{3} (s) + S O_{2} (g) + S O_{3} (g)$; Green crystals change colour; smell of burning sulphur.
Thermal decomposition of limestone: $C a C O_{3} (s) heat C a O (s) + C O_{2} (g)$; Quick lime is used in making cement.
Decomposition by light: $2 A g C l (s) sunlight 2 A g (s) + C l_{2} (g)$; White to grey; also $2 A g B r \to 2 A g + B r_{2}$ . Used in black and white photography.
Decomposition by electricity: $2 H_{2} O (l) electricity 2 H_{2} (g) + O_{2} (g)$; Hydrogen : oxygen by volume = 2 : 1.

How to solve Combination and Decomposition Reactions questions

For 'identify the type' and 'write the equation' questions on these two reaction types, follow this order.

1Count reactants and products in the word statement. Several in, one out: combination. One in, several out: decomposition.
2For decomposition, name the energy source (heat, light or electricity) and write it above the arrow.
3Write correct formulae, then balance with coefficients only.
4Add state symbols if asked; gases given off are (g), residues are (s).
5State the observation the examiner expects: heat felt, colour change, brown fumes, a gas that burns with a pop, a grey deposit.
6Classify the energy change: heat given out is exothermic; energy taken in is endothermic. Decompositions are usually endothermic.

Combination and Decomposition 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.

Combination reaction and exothermic change

Suppose a painter mixes a sack of quick lime with water in a large tub before whitewashing your school wall. The tub becomes too hot to touch, and a few days after painting the walls look shiny.

Name the reaction in the tub, write its balanced equation and say why the tub heats up. Then explain, with an equation, why the wall becomes shiny after two or three days, and say whether that second reaction is a combination reaction.

Try it first, then show the solutionSolution

In the tub, calcium oxide combines with water to form a single product, calcium hydroxide (slaked lime): $C a O (s) + H_{2} O (l) \to C a (O H)_{2} (a q)$ .
A large amount of heat is released as the product forms, so it is an exothermic combination reaction.
On the wall, calcium hydroxide slowly reacts with carbon dioxide from the air: $C a (O H)_{2} (a q) + C O_{2} (g) \to C a C O_{3} (s) + H_{2} O (l)$ . A thin layer of calcium carbonate gives the shine.
This second reaction gives two products, calcium carbonate and water, so it is not a combination reaction.

Answer: Tub: exothermic combination, $C a O + H_{2} O \to C a (O H)_{2}$ . Wall: $C a (O H)_{2} + C O_{2} \to C a C O_{3} + H_{2} O$ forms a shiny calcium carbonate layer; it has two products, so it is not a combination reaction.

Memory hook: Hot tub, shiny wall: lime combines with water, then meets carbon dioxide from the air.

Tricks to remember Combination and Decomposition Reactions

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

Count the right-hand side
One product = combination; one reactant = decomposition.
Use it when: Instant identification in MCQs.
Three energies to break things: HLE
Heat, Light, Electricity. Heat: $F e S O_{4}$ , $C a C O_{3}$ , $P b (N O_{3})_{2}$ . Light: $A g C l$ , $A g B r$ . Electricity: water.
Use it when: 'Give one example each of decomposition by heat, light and electricity.'
Exo = exit
In EXOthermic heat EXITs into the surroundings (beaker feels hot); in ENDOthermic heat goes in (beaker feels cold).
Use it when: Classifying reactions from what you feel or measure.

Combination and Decomposition Reactions: solved examples

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

Example 1: Three decompositions, three energy sources · 3 marks · Short answerBoard level

Write one balanced equation each for decomposition reactions in which energy is supplied as (a) heat, (b) light and (c) electricity.

Try it first, then show the solutionSolution

(a) Heat: $C a C O_{3} (s) heat C a O (s) + C O_{2} (g)$ (1 mark).
(b) Light: $2 A g B r (s) sunlight 2 A g (s) + B r_{2} (g)$ (1 mark).
(c) Electricity: $2 H_{2} O (l) electricity 2 H_{2} (g) + O_{2} (g)$ (1 mark).

Answer: (a) Limestone to quick lime and carbon dioxide; (b) silver bromide to silver and bromine; (c) water to hydrogen and oxygen.

Example 2: Heating ferrous sulphate crystals · 3 marks · Short answerHarder board level

A student heats green ferrous sulphate crystals in a dry boiling tube. (a) List two observations. (b) Write the balanced equation. (c) Name the type of reaction and state whether the change is exothermic or endothermic.

Try it first, then show the solutionSolution

(a) The green colour of the crystals changes (they first lose their water of crystallisation), and a smell of burning sulphur is noticed (1 mark).
(b) $2 F e S O_{4} (s) heat F e_{2} O_{3} (s) + S O_{2} (g) + S O_{3} (g)$ (1 mark).
(c) One reactant gives three products using heat: thermal decomposition. Heat must be supplied continuously, so it is endothermic (1 mark).

Answer: (a) Colour change of the crystals; smell of burning sulphur. (b) $2 F e S O_{4} \to F e_{2} O_{3} + S O_{2} + S O_{3}$ . (c) Thermal decomposition, endothermic.

Example 3: Electrolysis of water · 4 marks · Short answerHarder board level

In the electrolysis of water, gas collected in tube A has double the volume of gas in tube B. (a) Name the gases in A and B. (b) Why is the volume in A twice that in B? (c) How would you confirm each gas with a burning candle? (d) Why are a few drops of dilute sulphuric acid added?

Try it first, then show the solutionSolution

(a) A: hydrogen; B: oxygen (1 mark).
(b) $2 H_{2} O \to 2 H_{2} + O_{2}$ : two molecules of hydrogen form for every one of oxygen, so its volume is double (1 mark).
(c) Hydrogen burns with a pop sound when a burning candle is brought near; oxygen makes the candle burn more brightly (1 mark).
(d) Pure water conducts electricity very poorly; the acid makes it conduct so that the decomposition takes place (1 mark).

Answer: A: hydrogen, B: oxygen; ratio 2 : 1 from the equation; pop test for hydrogen, brighter flame for oxygen; acid helps water conduct.

Example 4: Classify from observations · 3 marks · Short answerHarder board level

Identify the type of each reaction and whether it is exothermic or endothermic.
(a) White silver chloride turns grey when left in sunlight.
(b) A magnesium ribbon burns with a dazzling white flame leaving a white powder.
(c) Brown fumes come off when lead nitrate powder is heated.
Write the balanced equation for (c).

Try it first, then show the solutionSolution

(a) Decomposition by light; energy (light) is absorbed, so endothermic (1 mark).
(b) Combination ( $2 M g + O_{2} \to 2 M g O$ ); heat and light are given out, so exothermic (1 mark).
(c) Thermal decomposition, endothermic: $2 P b (N O_{3})_{2} (s) heat 2 P b O (s) + 4 N O_{2} (g) + O_{2} (g)$ ; the brown gas is nitrogen dioxide (1 mark).

Answer: (a) Photolytic decomposition, endothermic. (b) Combination, exothermic. (c) Thermal decomposition, endothermic; $2 P b (N O_{3})_{2} \to 2 P b O + 4 N O_{2} + O_{2}$ .

Example 5: Is respiration a combination reaction? · 3 marks · Short answerHOTS / competency

Respiration is described as an exothermic process. (a) Write the equation for respiration of glucose. (b) Why is it exothermic? (c) A student calls it a combination reaction because glucose 'combines' with oxygen. Is the student right? Justify.

Try it first, then show the solutionSolution

(a) $C_{6} H_{12} O_{6} (a q) + 6 O_{2} (a q) \to 6 C O_{2} (a q) + 6 H_{2} O (l)$ + energy (1 mark).
(b) Glucose from digested carbohydrates reacts with oxygen in our cells and energy is released, which keeps us alive; energy given out means exothermic (1 mark).
(c) No. A combination reaction gives a single product, but respiration gives two products, carbon dioxide and water. The word 'combines' describes glucose reacting with oxygen, not the reaction type (1 mark).

Answer: (a) $C_{6} H_{12} O_{6} + 6 O_{2} \to 6 C O_{2} + 6 H_{2} O$ + energy. (b) Energy is released in cells. (c) No; it has two products, so it is not a combination reaction.

Where students lose marks in Combination and Decomposition Reactions

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

Mistake: Missing the energy condition on the arrow
Students write a decomposition as if it happens by itself.
Wrong working
$C a C O_{3} \to C a O + C O_{2}$ as the answer to 'decomposition by heat'.
Right working
$C a C O_{3} (s) heat C a O (s) + C O_{2} (g)$
Do this: Always write heat, sunlight or electricity over the arrow in decomposition questions.
Usually costs: ½ mark per equation.
Mistake: Calling every reaction with oxygen a combination
The word 'combine' is used loosely.
Wrong working
Burning methane is a combination reaction.
Right working
$C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O$ has two products, so it is not combination; it is exothermic.
Do this: Count products before naming the type.
Usually costs: 1 mark in classification questions.
Mistake: Mixing up the gases in electrolysis
Students assume equal volumes or swap the tubes.
Wrong working
The larger volume is oxygen because water has more oxygen.
Right working
The larger volume is hydrogen; ratio $H_{2} : O_{2} = 2 : 1$ from $2 H_{2} O \to 2 H_{2} + O_{2}$ .
Do this: Read the ratio straight off the balanced equation.
Usually costs: 1-2 marks in activity-based questions.
Mistake: Wrong product for ferrous sulphate
Students write $F e O$ or forget one of the two sulphur gases.
Wrong working
$F e S O_{4} \to F e O + S O_{3}$
Right working
$2 F e S O_{4} \to F e_{2} O_{3} + S O_{2} + S O_{3}$
Do this: Learn this one as a whole: ferric oxide plus both sulphur dioxide and sulphur trioxide.
Usually costs: The equation mark.

Exam tips

When asked to 'identify the type', name it and give the reason in a few words (single product formed, single reactant broken).
Activity-based questions expect observations: heat felt, colour change from green, brown fumes, grey colour, bubbles at both electrodes.
For exothermic or endothermic, link to the energy: heat given out versus heat, light or electricity taken in.
Keep the NCERT examples ready with balanced equations: quick lime and water, coal, hydrogen and oxygen, ferrous sulphate, limestone, lead nitrate, silver chloride, silver bromide, water.
In case-based questions, a reaction and its reverse often appear together; label each one separately.

Practice questions on Combination and Decomposition 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
Which of the following is a combination reaction?
(a) $C a C O_{3} \to C a O + C O_{2}$
(b) $2 H_{2} + O_{2} \to 2 H_{2} O$
(c) $2 A g C l \to 2 A g + C l_{2}$
(d) $Z n + C u S O_{4} \to Z n S O_{4} + C u$
Hint
Look for exactly one product.
Answer
(b). Hydrogen and oxygen form a single product, water.
Q2 · 2 marks · Very short answerBoard level
A solution of substance X is used for whitewashing. (a) Name X and write its formula. (b) Write the reaction of the substance from which X is made with water, and name the reaction type.
Hint
X is made by adding water to quick lime.
Answer
(a) X is calcium hydroxide (slaked lime), $C a (O H)_{2}$ . (b) $C a O (s) + H_{2} O (l) \to C a (O H)_{2} (a q)$ : an exothermic combination reaction.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Decomposition reactions are generally endothermic.
Reason (R): Energy is needed to break a reactant into simpler products.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
What do heat, light and electricity do in these reactions?
Answer
(a). Heat, light or electricity supplies the energy to break the reactant, so energy is absorbed.
Q4 · 5 marks · Long answerHarder board level
Zinc carbonate on heating gives zinc oxide and carbon dioxide. (a) Write the balanced equation with state symbols. (b) Name the reaction type. (c) How would you show that the gas is carbon dioxide? (d) Why are decomposition reactions called the opposite of combination reactions? Give a pair of equations to support your answer.
Hint
For (d) choose one substance that can be both formed and broken down, such as water.
Answer
(a) $Z n C O_{3} (s) heat Z n O (s) + C O_{2} (g)$ (1 mark). (b) Thermal decomposition (1 mark). (c) Pass the gas through lime water; it turns milky (1 mark). (d) In combination, two or more substances form one product; in decomposition one substance gives two or more products, the reverse direction. Example: $2 H_{2} + O_{2} \to 2 H_{2} O$ (combination) and $2 H_{2} O electricity 2 H_{2} + O_{2}$ (decomposition) (2 marks).
Q5 · 3 marks · Short answerHOTS / competency
Suppose you add equal amounts of four solids to four beakers of water (or copper sulphate solution for the last) and record the temperature before and after: A common salt: hardly any change; B ammonium nitrate: falls by several degrees; C anhydrous copper sulphate: rises; D iron filings in copper sulphate solution: rises slightly. Classify each change as exothermic, endothermic or neither, and explain how a falling thermometer reading shows that energy is absorbed.
Hint
Heat flowing out of the mixture warms it; heat taken in from the water cools it.
Answer
A: hardly any heat change, so treat it as neither. B: endothermic. C: exothermic. D: exothermic (displacement of copper by iron). A fall in temperature means the process takes heat from the water and beaker, so the surroundings cool; energy has been absorbed.
Q6 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
Suppose a cement factory heats crushed limestone in a kiln. The solid left behind is a white powder that reacts vigorously with water, and a colourless gas leaves through a chimney.
(a) Write the balanced equation for the reaction in the kiln. (1 mark)
(b) Name the white powder and give one use of it from NCERT. (1 mark)
(c) The white powder is mixed with water at a building site and the mixture gets hot. Write the equation and classify this reaction in two ways (by type and by energy change). (2 marks)
Hint
The kiln reaction and the building-site reaction are opposite kinds.
Answer
(a) $C a C O_{3} (s) heat C a O (s) + C O_{2} (g)$ . (b) Calcium oxide (quick lime); used in the manufacture of cement. (c) $C a O (s) + H_{2} O (l) \to C a (O H)_{2} (a q)$ ; a combination reaction that is exothermic.
Q7 · 2 marks · Very short answerHarder board level
Silver chloride is stored in dark-coloured bottles in the laboratory. Explain why, with a balanced equation, and state the colour change you would see if a sample were left on a sunny window sill.
Hint
Which form of energy decomposes silver chloride?
Answer
Light decomposes it: $2 A g C l (s) sunlight 2 A g (s) + C l_{2} (g)$ . Dark bottles keep light out. On the sill the white solid would turn grey as silver forms.

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.Define a combination reaction.Show answer
A reaction in which two or more substances combine to form a single product.
2.What is the common name of $C a O$ and of $C a (O H)_{2}$ ?Show answer
Quick lime; slaked lime.
3.What colour fumes come off when lead nitrate is heated?Show answer
Brown fumes of nitrogen dioxide.
4.Why does silver chloride turn grey in sunlight?Show answer
Light decomposes it into silver (grey) and chlorine.
5.Give two exothermic processes named in NCERT besides burning.Show answer
Respiration; decomposition of vegetable matter into compost.
6.Volume ratio of hydrogen to oxygen in electrolysis of water?Show answer
2 : 1.

Combination and Decomposition Reactions: frequently asked questions

What is the difference between combination and decomposition reactions?

A combination reaction joins two or more substances into one product. A decomposition reaction breaks one substance into two or more products. One is the reverse of the other.

What are the three types of decomposition reaction in Class 10?

Thermal decomposition (by heat), decomposition by light, and electrolytic decomposition (by electricity). Examples: limestone, silver chloride and water.

Are all combination reactions exothermic?

The Class 10 examples (quick lime with water, burning coal, burning magnesium, forming water) all give out heat, but the definition of combination is about one product, not about heat. Decide the energy change separately.

What happens when ferrous sulphate crystals are heated?

They lose water and their green colour changes, then they decompose into ferric oxide, sulphur dioxide and sulphur trioxide, with a smell of burning sulphur.

Why is respiration called exothermic?

Glucose reacts with oxygen in our cells to give carbon dioxide and water, and energy is released in the process.