Decomposition Reactions

Heat a scoop of green ferrous sulphate crystals in a dry test tube and something dramatic happens: the green solid turns brown, a sharp choking smell fills the room, and the single substance you started with has become three different ones. This is a decomposition reaction — a single compound breaking apart into two or more simpler substances under the push of some outside energy. It is the mirror image of a combination reaction, where separate substances join into one; here, one substance splits. CBSE Class 10 asks you to recognise three flavours of decomposition based on what kind of energy drives the breakup — heat, electricity, or light — and to write balanced equations for the standard examples: limestone roasting into quicklime, water splitting into hydrogen and oxygen, and silver chloride darkening in sunlight. This article walks through the general pattern, each type with its equation, a fully solved worked example, and the traps examiners love to set around them.

The core insight

A decomposition reaction breaks one reactant ( $AB \rightarrow A + B$ ) into simpler products by absorbing heat, electricity, or light, and CBSE classifies every example into exactly one of these three energy-source types.

The core idea

A decomposition reaction always has exactly one reactant on the left and two or more products on the right:

AB \rightarrow A + B

where: $AB$ is the single compound that breaks apart, and $A$ , $B$ are the simpler substances (elements or compounds) it splits into. Because chemical bonds must be broken to pull $AB$ apart, energy has to be supplied from outside — so almost all decomposition reactions are endothermic, absorbing heat, light, or electrical energy rather than releasing it. This is the opposite tendency of most combination reactions, which tend to release energy as bonds form. CBSE sorts decomposition reactions into three types by the form of energy that drives the split:

Thermal decomposition uses heat. The textbook example is calcium carbonate (limestone), which on strong heating gives calcium oxide (quicklime) and carbon dioxide gas:

\text{CaCO}_3(s) \xrightarrow{\Delta} \text{CaO}(s) + \text{CO}_2(g)

The symbol $\Delta$ written over the arrow is chemistry's shorthand for "heat is applied," and this exact reaction is the industrial basis of lime kilns, which supply quicklime for cement and mortar.

Electrolytic decomposition uses electricity, usually passed through a molten or dissolved compound. Passing a current through slightly acidified water splits it into hydrogen and oxygen gas in a fixed 2:1 volume ratio:

2\text{H}_2\text{O}(l) \xrightarrow{\text{electricity}} 2\text{H}_2(g) + \text{O}_2(g)

The acid itself is not consumed — it is added only to make pure water (a poor conductor on its own) carry current well enough for electrolysis to proceed.

Photolytic decomposition uses light. Silver chloride, ordinarily a white or pale-yellow solid, darkens when placed in sunlight because it decomposes into grey metallic silver and chlorine gas:

2\text{AgCl}(s) \xrightarrow{\text{sunlight}} 2\text{Ag}(s) + \text{Cl}_2(g)

Silver bromide undergoes the same kind of photolytic breakdown, and both reactions underlie traditional black-and-white photographic film, where light striking the silver-halide coating creates the image.

Figure

Three arrows showing a single reactant breaking into two products under heat electricity and sunlight labelled thermal electrolytic and photolytic decomposition

The one habit worth building now is spotting the number of reactants first. If you see two or more separate substances reacting, it cannot be a decomposition reaction, no matter how the products look. Decomposition is always a single-in, multiple-out pattern.

Derivation ABalancing a thermal decomposition equation by atom conservation

The goal here is to turn the unbalanced skeletal equation for a real CBSE example — the strong heating of ferrous sulphate crystals — into a correctly balanced, state-symbol-complete equation, using only the conservation of atoms (no coefficients guessed by trial and error, each one is forced by a count). Ferrous sulphate crystals ( $\text{FeSO}_4\cdot7\text{H}_2\text{O}$ ) are green; on strong heating they first lose their water of crystallisation, then the anhydrous ferrous sulphate itself decomposes, the colour turning brown as ferric oxide forms and the sharp smell of sulphur oxide gases appearing.

Step 1. Write the skeletal (unbalanced) equation for the anhydrous decomposition, listing every product you observe. Ferrous sulphate breaks into ferric oxide, sulphur dioxide, and sulphur trioxide:

\text{FeSO}_4 \rightarrow \text{Fe}_2\text{O}_3 + \text{SO}_2 + \text{SO}_3

As written this is unbalanced: the left side has one iron atom while the right side's $\text{Fe}_2\text{O}_3$ needs two.

Step 2. Fix iron first, since it appears in only one product. Placing a coefficient of $2$ in front of $\text{FeSO}_4$ supplies two iron atoms to match $\text{Fe}_2\text{O}_3$ :

2\text{FeSO}_4 \rightarrow \text{Fe}_2\text{O}_3 + \text{SO}_2 + \text{SO}_3

Step 3. Verify sulphur and oxygen atom-by-atom rather than assuming the equation is now balanced just because iron worked out. The left side has $2$ sulphur atoms (one in each of the two $\text{FeSO}_4$ units) and $8$ oxygen atoms ( $4$ oxygens per $\text{FeSO}_4$ , times $2$ ). The right side has $1$ sulphur in $\text{SO}_2$ plus $1$ sulphur in $\text{SO}_3$ , totalling $2$ sulphur atoms — matching. Oxygen on the right is $3$ (from $\text{Fe}_2\text{O}_3$ ) plus $2$ (from $\text{SO}_2$ ) plus $3$ (from $\text{SO}_3$ ), totalling $8$ oxygen atoms — also matching.

Step 4. Since iron, sulphur, and oxygen all balance with the single coefficient of $2$ , no further adjustment is needed. Add the state symbols and the heat condition above the arrow to complete the equation:

2\text{FeSO}_4(s) \xrightarrow{\Delta} \text{Fe}_2\text{O}_3(s) + \text{SO}_2(g) + \text{SO}_3(g)

One solid reactant has produced three distinct products under heat, confirming this as a thermal decomposition reaction — the same atom-counting method balances every decomposition equation you will meet on this syllabus.

Worked example

Water is decomposed by passing electricity through it in a voltameter fitted with two electrodes. Write the balanced equation, name the gases collected at each electrode, and state the volume ratio observed.

Step 1. Identify the energy source and hence the type. Electrical energy is driving the breakup of water, so this is electrolytic decomposition — a small amount of acid is usually added first, only to make the water conduct current, not as a reactant.

Step 2. Write the skeletal equation. Water breaks into its two elements:

\text{H}_2\text{O} \rightarrow \text{H}_2 + \text{O}_2

Step 3. Balance by counting atoms. The left side has $2$ hydrogen and $1$ oxygen atom; the right side (before balancing) already has $2$ hydrogen but $2$ oxygen. Placing a coefficient of $2$ in front of $\text{H}_2\text{O}$ fixes the oxygen mismatch:

2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2

Checking: the left side now has $4$ hydrogen and $2$ oxygen atoms; the right side has $4$ hydrogen (two $\text{H}_2$ molecules) and $2$ oxygen (one $\text{O}_2$ molecule). Balanced.

Step 4. Add state symbols and the electricity condition:

2\text{H}_2\text{O}(l) \xrightarrow{\text{electricity}} 2\text{H}_2(g) + \text{O}_2(g)

Step 5. State the observation. Hydrogen gas collects at one electrode and oxygen at the other, in a volume ratio of $2:1$ — hydrogen occupies exactly twice the volume of oxygen, matching the $2:1$ mole ratio in the balanced equation. A burning splint held near the hydrogen-collecting tube gives a characteristic "pop," while a glowing splint held near the oxygen-collecting tube rekindles into flame. Answer: the balanced equation is $2\text{H}_2\text{O}(l) \xrightarrow{\text{electricity}} 2\text{H}_2(g) + \text{O}_2(g)$ , with hydrogen and oxygen collected in a $2:1$ volume ratio.

Common traps

Confusing decomposition with displacement or combination — decomposition always starts from a single reactant breaking apart; two separate substances reacting rules it out immediately.
Assuming every decomposition reaction needs heat — electrolytic decomposition uses electricity and photolytic decomposition uses light, even though thermal decomposition is usually the first example students meet.
Getting the energy direction backwards — decomposition reactions generally absorb energy (endothermic) to break bonds, the opposite tendency of most combination reactions.
Forgetting the condition written above the arrow ( $\Delta$ , "electricity," or "sunlight") — this label is often specifically graded and is the only thing that distinguishes the three sub-types from each other in an equation.

Ask the tutor: unsure whether a given reaction is thermal, electrolytic, or photolytic decomposition — ask the AI tutor to help you spot the energy source stated in the equation and classify it correctly.

How do I quickly tell a decomposition reaction apart from a displacement reaction in an exam question?

Count the reactants first. A decomposition reaction always starts with exactly one reactant that breaks into two or more products, written as $AB \rightarrow A + B$ . A displacement reaction always starts with two reactants — an element and a compound — where the element takes the place of another element in the compound. If you see only one substance on the left of the arrow, it has to be decomposition; if you see two, it cannot be.

Why does adding acid to water before electrolysis not count as a reactant in the equation?

The acid is added purely to make pure water conduct electricity well, since pure water on its own is a very poor conductor. The acid itself is not consumed and does not appear as a separate reactant or product in the balanced equation $2\text{H}_2\text{O}(l) \xrightarrow{\text{electricity}} 2\text{H}_2(g) + \text{O}_2(g)$ — only water is shown decomposing, because only water's atoms end up rearranged into hydrogen and oxygen gas.

Are all three types of decomposition reaction always endothermic, or can some release energy instead?

Within this syllabus, treat all three types — thermal, electrolytic, and photolytic — as endothermic, since breaking the bonds of a single compound into separate products always requires an outside supply of heat, electricity, or light. You are not expected to calculate or compare energy values here, only to recognise that decomposition reactions absorb energy, which is the reverse tendency of most combination reactions you studied alongside this topic.

Decomposition Reactions

The core idea

Worked example

Common traps

Before this

Where this leads