NDA Chemistry · Chemical Reactions

Thermal and Photochemical Decomposition

A decomposition reaction splits one compound into two or more products; the energy for the split comes from heat (thermal) or light (photochemical).

All Chemical Reactions notes NDA Chemistry strategy

Why this matters

A small but reliable subtopic (3 PYQs) testing which oxides break on heating, which salts break in light, and the physical states of the products. The recurring catches are that silver salts are the light-sensitive ones and that mercury oxide gives a LIQUID metal.

Concept 1 of 1

Heat-driven vs light-driven decomposition

Intuition

Some compounds need heat to break apart (thermal decomposition); a few — mainly silver salts — break apart in sunlight (photochemical decomposition), which is the basis of black-and-white photography. The bank asks which decomposes, by which trigger, and into what states.

Definition

The decompositions the bank tests:

Thermal decomposition (driven by heat): 2HgO →(Δ) 2Hg + O₂ — solid HgO gives liquid mercury and gaseous oxygen; 2Ag₂O →(Δ) 4Ag + O₂; CaCO₃ →(Δ) CaO + CO₂; 2Pb(NO₃)₂ →(Δ) 2PbO + 4NO₂ + O₂.
Photochemical decomposition (driven by sunlight): 2AgCl →(sunlight) 2Ag + Cl₂ and 2AgBr →(sunlight) 2Ag + Br₂ — silver halides darken in light (used in photography).
Thermally stable (do NOT decompose on heating): ZnO, MgO — these are stable oxides.
Decomposition is the reverse of combination and usually needs energy IN (endothermic).

Reaction	Trigger	Product states / note
2HgO → 2Hg + O₂	Heat	Solid → liquid Hg + gas O₂ Mercury is the metal that comes off as a LIQUID — states are solid, liquid, gas.
2Ag₂O → 4Ag + O₂	Heat	Silver oxide decomposes on heating
2AgCl → 2Ag + Cl₂	Sunlight	Photochemical — silver chloride darkens in light Silver halides (AgCl, AgBr) decompose in SUNLIGHT, not heat — the basis of photography.
ZnO, MgO	—	Thermally STABLE — do not decompose on heating

Practice this concept4 quick reps

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
What triggers the decomposition 2AgCl → 2Ag + Cl₂?
2.
Heating HgO gives mercury in which state?
3.
Which of ZnO, MgO and Ag₂O decomposes on heating?
4.
Is decomposition usually endothermic or exothermic?

From the bank · past-year question

Example 1Chemical ReactionsMODERATE

Consider the following reaction :

2\text{HgO} \xrightarrow{\Delta} 2\text{Hg} + \text{O}_2

The respective state of HgO, Hg and

\text{O}_2

in the above reaction is

[Q65 · Sep · 2023]

Silver halides break in LIGHT, not heat

2AgCl → 2Ag + Cl₂ is a PHOTOchemical decomposition — it happens in sunlight, not on heating. The light-sensitivity of silver salts is what made black-and-white photography work.

HgO gives liquid mercury

In 2HgO → 2Hg + O₂ the product states are solid (HgO), LIQUID (Hg) and gas (O₂). Mercury is a liquid metal at room temperature, so do not write it as a solid.

ZnO and MgO are thermally stable

Not every oxide decomposes on heating. ZnO and MgO are stable; silver oxide (Ag₂O) and mercury oxide (HgO) are the ones that break down.

Drill 2 more on heat-driven vs light-driven decomposition

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (1)

Heat-driven vs light-driven decomposition4 rows

Reaction	Trigger	Product states / note
2HgO → 2Hg + O₂	Heat	Solid → liquid Hg + gas O₂ Mercury is the metal that comes off as a LIQUID — states are solid, liquid, gas.
2Ag₂O → 4Ag + O₂	Heat	Silver oxide decomposes on heating
2AgCl → 2Ag + Cl₂	Sunlight	Photochemical — silver chloride darkens in light Silver halides (AgCl, AgBr) decompose in SUNLIGHT, not heat — the basis of photography.
ZnO, MgO	—	Thermally STABLE — do not decompose on heating

Watch out for (3)

Silver halides break in LIGHT, not heat
→ Heat-driven vs light-driven decomposition
HgO gives liquid mercury
→ Heat-driven vs light-driven decomposition
ZnO and MgO are thermally stable
→ Heat-driven vs light-driven decomposition

Mastery check — 2 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1Chemical ReactionsMODERATE

Which of the following compounds undergoes/undergo thermal decomposition?

[Q72 · Sep · 2022]

Example 2Chemical ReactionsEASY

The chemical reaction

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

takes place

[Q61 · Sep · 2024]

Drill every past-year question on this subtopic

3 questions from the bank — paginated, with cart and Word-export support.

Drill the 3 questions