What happens when

(i) Calcium carbonate is heated?

(ii) Silver bromide is exposed to sunlight?

(iii) Lead is added to copper (II) chloride solution?

Write balanced chemical equations of the reactions involved in support of your answer.

Q: What happens when (i) Calcium carbonate is heated? (ii) Silver bromide is exposed to sunlight? (iii) Lead is added to copper (II) chloride solution? Write balanced chemical equations of the reactions involved in support of your answer.

**(i) Calcium carbonate is heated:** Calcium carbonate undergoes thermal decomposition to form calcium oxide (quick lime) and carbon dioxide gas is evolved. This is an important industrial decomposition reaction used in the manufacture of cement, as stated in the NCERT chapter. $$CaCO_3(s) \xrightarrow{\text{Heat}} CaO(s) + CO_2(g)$$ $$\text{(Limestone)} \qquad \text{(Quick lime)}$$ Since a single reactant breaks down into two simpler products on heating, this is a **thermal decomposition reaction**. **(ii) Silver bromide is exposed to sunlight:** White silver bromide decomposes on exposure to sunlight into silver metal and bromine gas, turning grey — the reaction is driven by light energy. The chapter notes this behaves the same way as silver chloride and that such reactions are used in black-and-white photography. $$2AgBr(s) \xrightarrow{\text{Sunlight}} 2Ag(s) + Br_2(g)$$ Since the reactant breaks down under the action of light (not heat), this is a **photochemical (photolytic) decomposition reaction**, and since energy (light) is absorbed, it is also an endothermic reaction. **(iii) Lead is added to copper (II) chloride solution:** Lead, being more reactive than copper, displaces copper from copper(II) chloride solution, forming lead chloride and depositing copper metal. $$Pb(s) + CuCl_2(aq) \rightarrow PbCl_2(aq) + Cu(s)$$ $$\text{(Copper chloride)} \qquad \text{(Lead chloride)}$$ Since a more reactive element (lead) displaces a less reactive element (copper) from its compound, this is a **displacement reaction**.

Chemical Reactions and Equations

Decomposition and Displacement Reactions

Official Answer

(i) Calcium carbonate is heated:

Calcium carbonate undergoes thermal decomposition to form calcium oxide (quick lime) and carbon dioxide gas is evolved. This is an important industrial decomposition reaction used in the manufacture of cement, as stated in the NCERT chapter.

$CaCO_3(s) \xrightarrow{\text{Heat}} CaO(s) + CO_2(g)$

$\text{(Limestone)} \qquad \text{(Quick lime)}$

Since a single reactant breaks down into two simpler products on heating, this is a thermal decomposition reaction.

(ii) Silver bromide is exposed to sunlight:

White silver bromide decomposes on exposure to sunlight into silver metal and bromine gas, turning grey — the reaction is driven by light energy. The chapter notes this behaves the same way as silver chloride and that such reactions are used in black-and-white photography.

$2AgBr(s) \xrightarrow{\text{Sunlight}} 2Ag(s) + Br_2(g)$

Since the reactant breaks down under the action of light (not heat), this is a photochemical (photolytic) decomposition reaction, and since energy (light) is absorbed, it is also an endothermic reaction.

(iii) Lead is added to copper (II) chloride solution:

Lead, being more reactive than copper, displaces copper from copper(II) chloride solution, forming lead chloride and depositing copper metal.

$Pb(s) + CuCl_2(aq) \rightarrow PbCl_2(aq) + Cu(s)$

$\text{(Copper chloride)} \qquad \text{(Lead chloride)}$

Since a more reactive element (lead) displaces a less reactive element (copper) from its compound, this is a displacement reaction.

Marking Scheme

11 mark for correct balanced equation and correct reaction type for calcium carbonate heating (thermal decomposition).
21 mark for correct balanced equation and correct reaction type for silver bromide in sunlight (photochemical decomposition).
31 mark for correct balanced equation and correct reaction type for lead added to copper(II) chloride (displacement reaction).

Hint

Think about what kind of energy (heat, light) or what kind of reactivity comparison drives each change — refer to Activities 1.6 (analogous to CaCO3), 1.8 (AgCl/AgBr), and 1.9 (Fe/Cu, extended to Pb/Cu).

Quick Oral Answer

Calcium carbonate heating gives quick lime and CO2 via thermal decomposition — this is the reverse of a combination reaction. Silver bromide breaks down in sunlight into silver and bromine because light energy supplies the activation energy needed to break the Ag-Br bond; this photochemical decomposition is the basis of black-and-white photography. Lead added to copper(II) chloride solution shows a displacement reaction because lead is more reactive than copper, so it pushes copper out of its salt solution and takes its place, depositing metallic copper.

Analysis & Explanation

Each part tests recognition of a distinct reaction type from the chapter. (i) Only one reactant, $CaCO_3$ , is supplied and heat alone causes it to split into $CaO$ and $CO_2$ — exactly analogous to the chapter's Eq. 1.20, so it is classified as thermal decomposition. (ii) $AgBr$ , like $AgCl$ in Activity 1.8/Eq. 1.23, decomposes only when exposed to sunlight (not heat), splitting into elemental silver and bromine gas; this is photochemical decomposition and also endothermic since light energy is absorbed. (iii) Lead is added to a solution of a different metal's salt ( $CuCl_2$ ); because lead is more reactive than copper (as established for zinc and lead in the chapter's displacement reaction section), it displaces copper from solution, forming $PbCl_2$ and depositing metallic copper — a single displacement reaction, not a double displacement, since only one element is exchanged rather than a pair of ions.

Common Mistakes

1Forgetting to balance the AgBr decomposition equation with coefficient 2 on both sides.
2Calling the AgBr reaction a thermal decomposition instead of photochemical decomposition — the energy source is sunlight, not heat.
3Writing lead chloride's formula incorrectly (should be $PbCl_2$ , not $PbCl$ ) — lead in copper(II) chloride displacement forms $Pb^{2+}$ .
4Mislabeling the displacement reaction as a double displacement reaction — only one element (copper) is being replaced by another element (lead), not an exchange of ion pairs.

Interesting Facts

Silver chloride and silver bromide both undergo the same type of light-driven decomposition, which is the chemical basis of traditional (non-digital) black-and-white photographic film.

Thermal decomposition of limestone ( $CaCO_3$ ) to quick lime ( $CaO$ ) is one of the largest-scale industrial decomposition reactions, central to cement manufacturing.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2026?

This question carries 3 marks in the CBSE Class 10 Science 2026 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Chemical Reactions and Equations" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Decomposition and Displacement Reactions" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2026 paper?

This is a Short Answer question from Section B in the CBSE Class 10 Science 2026 paper.