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 6, 2026

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.