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The first chapter of Class 10 chemistry looks easy, and that is why it costs marks. Students learn five reaction types as a list, then meet an equation that fits two types at once, or an unbalanced equation they fix by changing a formula. This post covers what the syllabus asks, how to balance any equation in the chapter, how to classify reactions, and a practice routine.
What does the CBSE syllabus ask for?
The CBSE Secondary Science syllabus for 2025–26 lists, for Class 10 Unit I: "Chemical reactions, Chemical equation, Balanced chemical equation, types of chemical reactions: combination, decomposition, displacement, double displacement, precipitation, endothermic exothermic reactions, oxidation and reduction." All the examples below come from Chapter 1 of the NCERT Class 10 Science textbook. Check the syllabus for your own exam year before you revise.
How do you balance an equation step by step?
NCERT calls this the hit-and-trial method. Its own example is iron reacting with steam: Fe + H₂O → Fe₃O₄ + H₂.
Count atoms on each side. Left: 1 Fe, 2 H, 1 O. Right: 3 Fe, 2 H, 4 O.
Start with the compound that has the most atoms. That is Fe₃O₄. Balance its oxygen first: put 4 in front of H₂O. Now there are 4 O on each side.
Fix the hydrogen you just changed. The left now has 8 H, so put 4 in front of H₂: Fe + 4H₂O → Fe₃O₄ + 4H₂.
Balance what is left. Three Fe on the right, so put 3 in front of Fe.
Check every element and add state symbols: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g).
A harder one. Heating lead nitrate gives brown fumes of nitrogen dioxide: Pb(NO₃)₂ → PbO + NO₂ + O₂.
Nitrogen: 2 on the left, so write 2NO₂. Oxygen is now 6 on the left and 1 + 4 + 2 = 7 on the right.
Odd numbers are awkward, so double the lead nitrate: 2Pb(NO₃)₂. Then lead needs 2PbO and nitrogen needs 4NO₂.
Oxygen: left 12, right 2 + 8 + 2 = 12. Balanced: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂.
Why must you never change a subscript?
Changing H₂O to H₂O₂ to "fix" the oxygen turns water into hydrogen peroxide. The equation balances, but it is now about a different substance. Yarroch (1985) interviewed high school students and found that all of them could balance the four simple equations given, yet seven of the twelve could not draw diagrams consistent with what they had written. Those same students had a poor idea of what a subscript means and were willing to break the rule against changing subscripts. Being able to balance is not the same as understanding what the numbers say.
A quick check: say each formula in words. "4H₂O" is four molecules, each with two hydrogen atoms and one oxygen atom. If you cannot say it, you may be pushing numbers around.
How do you tell the reaction types apart?
Use what happens to the reactants, with NCERT's own examples:
Combination: two or more reactants form one product. CaO + H₂O → Ca(OH)₂, quick lime with water, which also releases heat (exothermic).
Decomposition: one reactant breaks into simpler products. Heat does it in 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ and CaCO₃ → CaO + CO₂. Light does it in 2AgCl → 2Ag + Cl₂, which NCERT links to black-and-white photography. These reactions absorb energy (endothermic).
Displacement: a more reactive element pushes out a less reactive one. Fe + CuSO₄ → FeSO₄ + Cu, where the blue solution fades and the nail turns brownish. Zinc and lead do the same with copper salts.
Double displacement: two compounds swap ions. Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl, where the white BaSO₄ is a precipitate. Any reaction that forms a precipitate is also a precipitation reaction.
Oxidation and reduction: one substance gains oxygen and another loses it. In CuO + H₂ → Cu + H₂O, copper oxide is reduced and hydrogen is oxidised. In ZnO + C → Zn + CO, carbon is oxidised and zinc oxide is reduced.
Can one reaction belong to two types?
Yes, and this is where marks go. Fe + CuSO₄ is a displacement reaction and also a redox reaction: iron loses electrons to become Fe²⁺ and copper ions gain them. Burning magnesium is a combination reaction, an oxidation and exothermic all at once. When a question asks "which type", read whether it wants one answer or all that apply, and justify each label in a few words.
Other slips to watch for:
Writing O instead of O₂ for oxygen gas, or H instead of H₂.
Forgetting state symbols when the question asks for them.
Calling every colour change a displacement reaction without checking which element moved.
If balancing still feels like guessing, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning, so you can compare your order of steps with a full solution.
A practice routine for this chapter
Balance ten equations from the NCERT chapter and exercises, writing the atom count table each time.
For each, write the reaction type, or types, with one line of reason.
Say three of your balanced equations aloud in words: how many molecules of what.
Two days later, redo the ones you got wrong without looking.
For a plan that fits this chapter into the rest of the subject, read our Class 10 science revision plan.
