Decomposition reaction
- CBSE · NCERT Class 10, Ch 1 Chemical Reactions and Equations
A decomposition reaction is one in which a single compound breaks down into two or more simpler substances.
It is the opposite of a combination reaction. One substance goes in, and more than one comes out. The energy to break it up can come from heat, from light or from electricity.
- Heat: calcium carbonate breaks into calcium oxide and carbon dioxide.
- Light: silver chloride turns grey in sunlight as silver forms and chlorine is given off.
- Electricity: water splits into hydrogen and oxygen.
A small example
CaCO₃ → CaO + CO₂ on heating. Count the atoms. The left side has 1 Ca, 1 C and 3 O. The right side has 1 Ca and 1 O in CaO, and 1 C and 2 O in CO₂. That is 1 Ca, 1 C and 3 O again, so it is balanced. Both products here are compounds, and it is still decomposition.
Where marks go
Question 29 of CBSE's 2026-27 Science sample paper asks who is right. Daljeet says the products are two compounds. Avik says they are two elements. The 2026-27 Science marking scheme says both are partly right.
It gives 2 marks for the rule: one compound breaks down, and the products can be elements or compounds. It gives the third mark for water, which gives two elements and is also decomposition. An answer that picks one student and stops would miss marks. See how step marking works.
Sources
Facts last checked against these sources on 30 September 2026.
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