Class 10 chemical reactions: balancing equations and reaction types
A chemical equation shows reactants changing into products. Balance it by keeping the number of atoms of each element equal on both sides.
Coefficients and subscripts do different jobs
In 2H₂O, the coefficient 2 multiplies the entire formula: there are four hydrogen atoms and two oxygen atoms. The subscript in H₂O describes the substance itself. Changing H₂O to H₂O₂ changes water into a different substance; it is not a way to balance an equation.
Worked example: balance Fe + O₂ → Fe₂O₃
- Count atoms: the left side has 1 Fe and 2 O; the right has 2 Fe and 3 O.
- The least common multiple of 2 and 3 is 6. Put 3 before O₂ and 2 before Fe₂O₃.
- The right side now contains 4 Fe atoms, so put 4 before Fe.
- Check again: 4Fe + 3O₂ → 2Fe₂O₃ has 4 Fe and 6 O on each side.
Balancing reflects conservation of atoms in a chemical reaction. Use the smallest whole-number coefficients and include states or conditions when the question asks for them.
Recognise common reaction types
- Combination: reactants form one product. Example: CaO + H₂O → Ca(OH)₂.
- Decomposition: one compound breaks into simpler substances. On heating, CaCO₃ → CaO + CO₂.
- Displacement: a more reactive element replaces a less reactive one in a compound. Example: Zn + CuSO₄ → ZnSO₄ + Cu.
- Double displacement: ions exchange partners. Example: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq). Insoluble barium sulfate forms a precipitate.
These categories can overlap with descriptions such as oxidation, reduction, exothermic or endothermic. A reaction can fit more than one description.
Oxidation and reduction
In terms of oxygen transfer, oxidation is gaining oxygen and reduction is losing oxygen. In CuO + H₂ → Cu + H₂O, copper(II) oxide loses oxygen and is reduced to copper; hydrogen gains oxygen and is oxidised. Oxidation and reduction occur together.
Common mistakes
- Changing a chemical formula instead of its coefficient.
- Counting only the first element in a bracket: Ca(OH)₂ has two O and two H atoms.
- Assuming an equation is balanced without checking every element.
- Omitting a stated reaction condition, such as heating or light.
Try these questions
1. Balance Al + O₂ → Al₂O₃.
4Al + 3O₂ → 2Al₂O₃. Each side contains 4 aluminium atoms and 6 oxygen atoms.
2. Classify 2AgCl → 2Ag + Cl₂ in sunlight.
It is a decomposition reaction driven by light: one compound breaks into simpler substances. It is also called photochemical decomposition.
3. How many oxygen atoms are represented by 3Ca(OH)₂?
There are 6 oxygen atoms. Each formula unit contains two oxygen atoms, and the coefficient multiplies that by three.
Build a revision check
For each equation, name the reactants and products, balance the atoms, then explain the reaction type. Use the CBSE and NCERT study workflow to return to the relevant chapter. Consult your prescribed textbook for the full content and school laboratory guidance.
These are original paper-based revision examples, not experiment instructions or official exam questions.
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