Revision Notes: Class 10 Science Chapter 1 Chemical Reactions and Equations

A fast, one-glance recap of Class 10 Science Chapter 1 (Chemical Reactions and Equations) — for the full worked explanations, see the Solutions. These Class 10 Science Chapter 1 notes are ideal for quick revision just before exams.

Last Updated: September 23, 2026

Common Mistakes Students Make in Chemical Reactions and Equations

Why This Chapter Matters

Chemical Reactions and Equations opens the Class 10 Science syllabus and is one of the most heavily-tested chapters in board exams — equation balancing, reaction-type identification, and everyday examples (corrosion, rancidity) appear across MCQ, short-answer and long-answer questions almost every year. It also lays the foundation for later chapters like Acids, Bases and Salts and Metals and Non-metals, which build directly on the reaction types introduced here.

Revision Notes: Chemical Reactions and Equations

• Chemical reaction: a process where reactants are converted into new substances (products) with different properties — signs include colour change, gas evolution, precipitate formation, temperature change, or a change of state
• Chemical equation: a short-hand representation of a chemical reaction using symbols/formulae of reactants and products, e.g. Mg + O2 → MgO
• Balanced equation: has equal numbers of atoms of each element on both sides, in line with the Law of Conservation of Mass
• Combination reaction: two or more reactants combine to form a single product, e.g. CaO + H2O → Ca(OH)2 (usually exothermic)
• Decomposition reaction: a single compound breaks down into two or more simpler products — thermal (heat, e.g. CaCO3 → CaO + CO2), electrolytic (electricity, e.g. 2H2O → 2H2 + O2), photolytic (light, e.g. 2AgCl → 2Ag + Cl2)
• Displacement reaction: a more reactive element displaces a less reactive one from its compound, e.g. Fe + CuSO4 → FeSO4 + Cu
• Double displacement reaction: two compounds exchange ions, often forming an insoluble precipitate, e.g. Na2SO4 + BaCl2 → BaSO4↓ + 2NaCl
• Oxidation: gain of oxygen or loss of hydrogen by a substance; Reduction: loss of oxygen or gain of hydrogen; a redox reaction involves both happening together
• Exothermic reaction: releases heat to the surroundings (e.g. respiration, burning fuels); Endothermic reaction: absorbs heat from the surroundings (e.g. decomposition of calcium carbonate on strong heating)
• Corrosion: a metal is gradually eaten away by reaction with substances in its environment (moisture, acids, gases) — e.g. rusting of iron (formation of hydrated iron(III) oxide); prevented by painting, oiling, galvanising, or alloying
• Rancidity: oxidation of fats and oils in food over time, producing an unpleasant smell and taste; slowed by adding antioxidants, storing in airtight containers, or refrigeration
• Key test: a burning splinter gives a pop sound in the presence of hydrogen gas; a glowing splinter re-lights in the presence of oxygen gas; carbon dioxide turns lime water milky

See also: Extra Questions (HOTS) | Solutions

How to Use These Notes

These revision notes are meant for quick recall in the last few days before an exam or unit test — they are not a substitute for first reading the full chapter and working through examples in the NCERT textbook. A good routine is: read the chapter once, solve the in-text and end-of-chapter questions using the linked Solutions, then come back to this page a day or two before the exam to refresh the key definitions and formulas without re-reading the whole chapter.

Frequently Asked Questions

What are the main types of chemical reactions covered in this chapter?
Combination, decomposition, displacement, double displacement, and oxidation-reduction (redox) reactions — learning to identify which type a given reaction belongs to is the core skill tested from this chapter.

What is the difference between a balanced and an unbalanced chemical equation?
A balanced equation has equal numbers of atoms of each element on both the reactant and product sides, satisfying the law of conservation of mass; an unbalanced equation does not, and must be balanced before it correctly represents the actual reaction.

What to Revise First (and Last) in This Chapter

Written by Satish

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