CBSE Class 10 Science Chapter 1 PDF Notes – Chemical Reactions and Equations 2026-27

If you need Class 10 Science Chapter 1 PDF Notes that explain Chemical Reactions and Equations clearly, this guide covers it all: how to spot a chemical change, how to write and balance equations, the five main types of reactions, oxidation and reduction, and the everyday effects of corrosion and rancidity.

This is the first chapter of the NCERT Class 10 Science textbook and the base for the whole chemistry unit. Board papers ask balancing, type-identification and activity-based questions from it every year, so a strong grip here pays off in Chapters 2 to 4 as well.

Use the table of contents below to jump to the part you need, or read top to bottom for a complete revision of the chapter.

Table of Contents

• Chapter Overview
• Important Topics and Concepts
• Key Equations to Learn
• Types of Reactions at a Glance
• Important Definitions and Terms
• Short Answer Questions
• Long Answer Questions
• MCQs with Answers
• Quick Revision Points
• Chapter Summary
• Exam Tips and PYQ Pattern
• Frequently Asked Questions
• Conclusion

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Chapter Overview

A chemical reaction is a process in which one or more substances (reactants) change into new substances (products) with different properties. Everyday examples include milk turning sour, iron rusting, food being digested and cooking gas burning.

The chapter teaches you to recognise a reaction from observable signs, write it as a word equation and then as a balanced chemical equation, and classify it as combination, decomposition, displacement, double displacement or redox. It ends with two harmful oxidation effects you see at home: corrosion and rancidity.

Important Topics and Concepts

How to tell a chemical reaction has happened

• Change in state (for example, a gas forms or a solid appears)
• Change in colour
• Evolution of a gas (bubbles)
• Change in temperature (heat released or absorbed)
• Formation of a precipitate (an insoluble solid)

Chemical equations

A chemical equation shows a reaction using symbols and formulae. Reactants go on the left, products on the right, joined by an arrow. A word equation such as “magnesium + oxygen → magnesium oxide” becomes the skeletal equation Mg + O₂ → MgO.

Balancing chemical equations

By the law of conservation of mass, mass is neither created nor destroyed, so the number of atoms of each element must be equal on both sides. Balance by changing coefficients only, never the subscripts in a formula. Example: Fe + H₂O → Fe₃O₄ + H₂ balances to 3Fe + 4H₂O → Fe₃O₄ + 4H₂.

Writing state symbols and conditions

Add (s) solid, (l) liquid, (g) gas and (aq) aqueous solution after each formula. Write conditions such as heat, pressure or a catalyst above or below the arrow, for example CO(g) + 2H₂(g) → CH₃OH(l) at 340 atm.

Combination reaction

Two or more reactants form a single product. Example: CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat. Quick lime reacts vigorously with water to give slaked lime, which is used for whitewashing. Burning of coal (C + O₂ → CO₂) and formation of water (2H₂ + O₂ → 2H₂O) are also combination reactions.

Exothermic and endothermic reactions

Exothermic reactions release heat (burning of natural gas, respiration, decomposition of vegetable matter into compost). Endothermic reactions absorb heat (decomposition of calcium carbonate on heating).

Decomposition reaction

• Thermal decomposition: 2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g) on heating; CaCO₃ → CaO + CO₂; 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ (brown fumes of NO₂).
• Electrolytic decomposition: 2H₂O(l) → 2H₂(g) + O₂(g) using electricity; hydrogen collects at the cathode with double the volume of oxygen.
• Photolytic decomposition: 2AgCl(s) → 2Ag(s) + Cl₂(g) in sunlight (white silver chloride turns grey). Silver bromide behaves the same way, which is used in black-and-white photography.

Displacement reaction

A more reactive element displaces a less reactive element from its compound. Example: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). The iron nail turns brownish and the blue solution fades to light green. Zinc and lead also displace copper from copper sulphate.

Double displacement reaction

Two compounds exchange ions. Example: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq). A white precipitate of barium sulphate forms, so this is also a precipitation reaction.

Oxidation and reduction (redox)

Oxidation is gain of oxygen or loss of hydrogen; reduction is loss of oxygen or gain of hydrogen. Example: CuO + H₂ → Cu + H₂O (heated). Copper oxide is reduced and hydrogen is oxidised. Heating copper powder in air turns it black as CuO forms (2Cu + O₂ → 2CuO). The substance that gets oxidised is the reducing agent and the one that gets reduced is the oxidising agent.

Corrosion

When a metal is attacked by moisture, air, acids and so on, it corrodes. Iron forms reddish-brown rust, copper gets a green coating and silver turns black. Corrosion damages car bodies, bridges, railings and ships. It can be prevented by painting, oiling, galvanising and alloying.

Rancidity

When fats and oils are oxidised, their smell and taste change; this is rancidity. It is slowed by adding antioxidants, flushing chip packets with nitrogen, keeping food in airtight containers and refrigeration.

Key Equations to Learn

Types of Reactions at a Glance

Important Definitions and Terms

Short Answer Questions

Q1. Why should a magnesium ribbon be cleaned before burning in air?

Magnesium ribbon gets a layer of magnesium oxide on its surface when kept in air. This layer stops the ribbon from burning properly, so it is rubbed with sandpaper to expose fresh metal.

Q2. Why is respiration considered an exothermic reaction?

During respiration, glucose combines with oxygen in our cells to form carbon dioxide and water, and energy is released. Since energy is given out, it is an exothermic reaction.

Q3. Why are decomposition reactions called the opposite of combination reactions?

In a combination reaction, two or more substances combine to form one product. In a decomposition reaction, one substance breaks down into two or more products. So the two processes work in opposite directions.

Q4. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Iron is more reactive than copper, so it displaces copper from copper sulphate. Ferrous sulphate forms, so the blue solution turns light green, and reddish-brown copper deposits on the nail.

Q5. Why do we apply paint on iron articles?

Paint forms a protective layer that keeps air and moisture away from the iron surface. Without air and water, iron cannot rust.

Q6. Oil and fat containing food items are flushed with nitrogen. Why?

Nitrogen is an unreactive gas. Flushing packets with nitrogen removes oxygen, so the fats and oils in the food cannot get oxidised and turn rancid.

Q7. Why is silver chloride kept in dark-coloured bottles?

In sunlight, silver chloride decomposes into silver and chlorine. Dark bottles block light and prevent this photolytic decomposition.

Q8. In the electrolysis of water, why is the volume of gas at one electrode double that at the other?

Water contains hydrogen and oxygen in the ratio 2 : 1 by number of molecules formed (2H₂O → 2H₂ + O₂). So the volume of hydrogen collected is double the volume of oxygen.

Long Answer Questions

Q1. What is a balanced chemical equation? Why should chemical equations be balanced? Balance: Fe + H₂O → Fe₃O₄ + H₂.

A balanced chemical equation has the same number of atoms of each element on the reactant and product sides.

Equations must be balanced because of the law of conservation of mass: mass can neither be created nor destroyed in a chemical reaction, so the total mass of products equals the total mass of reactants.

Balancing step by step: start with the compound having the most atoms (Fe₃O₄). Oxygen: 4 on the right, so write 4H₂O on the left. Hydrogen: now 8 on the left, so write 4H₂ on the right. Iron: 3 on the right, so write 3Fe on the left.

Balanced equation: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)

Q2. Explain the following types of reactions with one example each: (a) combination (b) decomposition (c) displacement (d) double displacement.

(a) Combination: two or more substances combine to form one product, e.g. CaO + H₂O → Ca(OH)₂.

(b) Decomposition: one substance breaks into simpler substances, e.g. CaCO₃ → CaO + CO₂ on heating.

(c) Displacement: a more reactive element pushes out a less reactive one from its compound, e.g. Zn + CuSO₄ → ZnSO₄ + Cu.

(d) Double displacement: two compounds exchange ions, often forming a precipitate, e.g. AgNO₃ + NaCl → AgCl + NaNO₃ (white precipitate).

Q3. What is corrosion? Explain how rusting of iron can be prevented.

Corrosion is the slow destruction of a metal when it reacts with air, moisture or chemicals around it. Rusting of iron is the most common example: iron reacts with oxygen and water to form reddish-brown hydrated iron oxide (rust).

Methods to prevent rusting: painting the surface; applying oil or grease; galvanising (coating with zinc); chrome plating; making alloys such as stainless steel; and keeping iron objects dry.

Rusting weakens iron structures such as bridges, ships and railings, so preventing it saves large amounts of money every year.

MCQs with Answers

1. Which of the following is a physical change?
(a) Rusting of iron (b) Melting of ice (c) Burning of paper (d) Digestion of food
Answer: (b) Melting of ice – No new substance forms.

2. In 2Mg + O₂ → 2MgO, the type of reaction is
(a) Decomposition (b) Displacement (c) Combination (d) Double displacement
Answer: (c) Combination

3. The colour of the precipitate formed when lead nitrate reacts with potassium iodide is
(a) White (b) Yellow (c) Blue (d) Green
Answer: (b) Yellow – Lead iodide is yellow.

4. Which gas is evolved when zinc reacts with dilute sulphuric acid?
(a) Oxygen (b) Carbon dioxide (c) Hydrogen (d) Sulphur dioxide
Answer: (c) Hydrogen

5. Heating ferrous sulphate crystals gives off a smell of
(a) Burning sulphur (b) Rotten eggs (c) Ammonia (d) Chlorine
Answer: (a) Burning sulphur – SO₂ and SO₃ are released.

6. In CuO + H₂ → Cu + H₂O, the substance oxidised is
(a) CuO (b) H₂ (c) Cu (d) H₂O
Answer: (b) H₂ – Hydrogen gains oxygen.

7. Which of these is an endothermic process?
(a) Respiration (b) Burning of coal (c) Decomposition of calcium carbonate (d) Formation of slaked lime
Answer: (c) Decomposition of calcium carbonate

8. White silver chloride turns grey in sunlight because it
(a) Melts (b) Decomposes to silver (c) Absorbs water (d) Reacts with oxygen
Answer: (b) Decomposes to silver

9. Rancidity can be prevented by
(a) Adding salt (b) Flushing with nitrogen (c) Heating the food (d) Adding water
Answer: (b) Flushing with nitrogen

10. Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl is an example of
(a) Combination (b) Decomposition (c) Displacement (d) Double displacement and precipitation
Answer: (d) Double displacement and precipitation

Quick Revision Points

• Signs of a reaction: change in state, colour or temperature, gas evolution, precipitate.
• Balance equations by changing coefficients, never subscripts.
• Combination: many → one. Decomposition: one → many (heat, light or electricity).
• Displacement needs a more reactive element; double displacement swaps ions.
• Oxidation = gain of O or loss of H; reduction is the reverse.
• Electrolysis of water gives H₂ and O₂ in a 2 : 1 volume ratio.
• Corrosion and rancidity are both oxidation effects.
• Prevent rust by painting, oiling, galvanising or alloying.

Chapter Summary

A chemical reaction changes reactants into new products, which we detect from changes in state, colour, temperature, gas evolution or precipitation. We represent reactions using chemical equations and balance them to obey the law of conservation of mass, adding state symbols and conditions for clarity.

Reactions are classified as combination, decomposition, displacement, double displacement and redox. Some release heat (exothermic) and some absorb it (endothermic). Oxidation in daily life shows up as corrosion of metals and rancidity of fats, both of which can be slowed with simple protective methods.

Exam Tips and PYQ Pattern

• Balancing an equation (with state symbols) is asked almost every year, usually for 1 to 3 marks.
• Activity-based questions on lead nitrate, ferrous sulphate, the iron nail in copper sulphate and electrolysis of water are common.
• Assertion-reason questions often test redox and exothermic/endothermic reactions.
• In “identify the type” questions, name the reaction and justify it in one line.

Frequently Asked Questions

What are the five types of chemical reactions in Class 10 Chapter 1?

Combination, decomposition, displacement, double displacement and oxidation-reduction (redox) reactions.

Is rusting a combination reaction?

Rusting is an oxidation reaction in which iron combines with oxygen and water to form hydrated iron oxide. It is commonly discussed as corrosion.

Why is decomposition of calcium carbonate endothermic?

It needs a continuous supply of heat to break calcium carbonate into calcium oxide and carbon dioxide, so heat is absorbed.

Conclusion

Chemical Reactions and Equations gives you the language of chemistry. Once you can balance an equation, name the reaction type and spot oxidation and reduction, the next chapters on acids, metals and carbon become much easier. Practise five balancing questions daily before your exam and revise the key equations table.

More Notes: Class 10 Science Chapter 2: Acids, Bases and Salts notes | All Class 10 Science notes

Author Bio: Written and reviewed by the Nextoper Editorial Team, a group of CBSE science educators who build chapter notes directly from the NCERT textbook and past board paper patterns.

Freshness Note: These notes follow the NCERT Class 10 Science textbook (Reprint 2026-27) and the CBSE syllabus for the 2026-27 session. Last checked on 7 November 2026.