Question
Classify chemical reactions into combination, decomposition, displacement, double displacement, and redox reactions. How do you identify each type from a given equation?
(CBSE Class 10 Science — guaranteed 3-5 marks in board exams)
Solution — Step by Step
Two or more substances combine to form a single product.
Identification tip: Look for fewer products than reactants. Often exothermic.
A single compound breaks down into two or more simpler substances. Requires energy input (heat, light, or electricity).
Types: thermal decomposition, electrolytic decomposition, photolytic decomposition (by light, e.g., AgBr in photography).
A more reactive element displaces a less reactive element from its compound.
Identification tip: One element and one compound as reactants. The free element replaces the other.
Two compounds exchange their ions to form two new compounds. One product is usually a precipitate, gas, or water.
Identification tip: Two compounds react, and their positive/negative ions swap.
A reaction where oxidation and reduction happen simultaneously. One species loses electrons (oxidised), another gains them (reduced).
Here, CuO is reduced (loses oxygen), H₂ is oxidised (gains oxygen).
flowchart TD
A[Given chemical equation] --> B{How many reactants and products?}
B -->|"A + B → C (many → one)"| C[Combination]
B -->|"A → B + C (one → many)"| D[Decomposition]
B -->|"A + BC → AC + B"| E{Is A a free element?}
E -->|Yes| F[Displacement]
B -->|"AB + CD → AD + CB"| G[Double Displacement]
A --> H{Oxidation number change?}
H -->|Yes| I[Redox Reaction]Why This Works
Chemical reactions are classified by what happens to the atoms — do they join together, break apart, swap places, or transfer electrons? This classification is not mutually exclusive. A combination reaction can also be a redox reaction (e.g., is both).
The key to identification is counting: count the number of reactants and products, identify if elements or compounds are reacting, and check if oxidation numbers change.
Alternative Method
For CBSE board exams, here is the quick identification trick:
- Only one product? → Combination
- Only one reactant? → Decomposition
- A free element reacts with a compound? → Displacement
- Two compounds swap ions? → Double displacement
- Oxidation numbers change? → Redox
Check for redox separately — it can overlap with the other types.
Common Mistake
Students treat these categories as mutually exclusive. Many reactions belong to more than one type. For example, is both a displacement reaction AND a redox reaction (Zn is oxidised, Cu²⁺ is reduced). When a board exam asks “identify the type of reaction,” mention ALL applicable types for full marks.