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Science · 2025 · 3 marks
Chemical Reactions and EquationsCombination and Decomposition Reactions3 marksUnderstandshort answer
CBSE 2025 · Region 2 · Set 1 · Q27
(a) (i) Define the term decomposition reaction. Write one chemical equation each for decomposition reaction where energy is supplied in the form of…
(a)(i)
Define the term decomposition reaction. Write one chemical equation each for decomposition reaction where energy is supplied in the form of heat, light or electricity.
(ii)
Decomposition of vegetable matter into compost is considered an exothermic reaction. Why ?
OR
(b)
Why are decomposition reactions called the opposite of combination reactions ? Write one chemical equation each for these two types of reactions mentioning the name of the reactant(s) and the product(s) involved in the reactions.
Marking-scheme solution
(a) (i) A single reactant (substance) breaks down to give two or more products.
\(\displaystyle \mathrm{CaCO}_{3}(\mathrm{~s}) \xrightarrow{\text { Heat }} \mathrm{CaO}(\mathrm{s})+\mathrm{CO}_{2}(\mathrm{~g})\)
2AgCl (s) \(\displaystyle \xrightarrow{\text { Sunlight }} 2 \mathrm{Ag}(\mathrm{s})+\mathrm{Cl}_{2}(\mathrm{~g})\)
\(\displaystyle 2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \xrightarrow[\text { Current }]{\text { Electric }} 2 \mathrm{H}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g})\)
(ii) because energy (heat) is released.
(b)
In combination reaction single product (substance) is formed from two or more reactants (substances) whereas in decomposition reaction a single reactant (substance) breaks down to give two or more products (substances). So, the two are opposite.
Example of combination reaction
\[\mathrm{C}(\mathrm{~s})+\mathrm{O}_{2}(\mathrm{~g}) \longrightarrow \mathrm{CO}_{2}(\mathrm{~g})+\text { Heat }
\] Carbon Oxygen Carbon dioxide
Example of decomposition reaction
\[\mathrm{CaCO}_{3}(\mathrm{~s}) \quad \xrightarrow{\text { Heat }} \mathrm{CaO}(\mathrm{~s}) \quad+\quad \mathrm{CO}_{2}(\mathrm{~g})
\] Calcium carbonate Calcium Oxide Carbon dioxide
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CBSE Class 10 Science past-paper question from the 2025 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.