Chemical Reactions and Equations Class 10 Notes – CBSE Chapter 1 Explained with Examples & PDF

CBSE Class 10 Science Notes – Chemical Reactions and Equations

Class 10 Chemistry | NCERT Chapter 1 | Board Exam Weightage: 6-8 Marks

CBSE Class 10 Notes for Chapter 1 provide complete coverage of the NCERT syllabus with exam-focused explanations. These CBSE Class 10 Science Notes include chemical equations, balancing methods, types of reactions, redox reactions, corrosion and rancidity with solved examples.

1. Introduction

A chemical reaction is a process in which one or more substances (reactants) are converted into one or more new substances (products) with different chemical properties. This involves breaking and forming of chemical bonds.

Real-Life Examples of Chemical Reactions:

• Rusting of iron: Iron reacts with oxygen and moisture to form hydrated iron oxide (rust)
• Burning of fuel: Combustion of wood, LPG, petrol produces heat and light
• Digestion of food: Complex food molecules break down into simpler substances
• Photosynthesis: Plants convert CO₂ and water into glucose using sunlight
• Baking of cake: Heat causes chemical changes in ingredients

Why This Chapter Matters:

• Board Exam Weightage: 6-8 marks (Theory + Practical)
• Foundation for entire Chemistry syllabus
• High-scoring: Balancing equations, identifying reaction types, redox concepts
• NCERT Chapter 1: Chemical Reactions and Equations (2024-25 Edition)

2. Chapter Overview

CBSE Class 10 Science Notes for this chapter cover:

• Writing and balancing chemical equations
• Law of Conservation of Mass
• Types of chemical reactions (Combination, Decomposition, Displacement, Double Displacement)
• Oxidation and Reduction (Redox reactions)
• Corrosion of metals (Rusting of iron)
• Rancidity and its prevention

3. CBSE Class 10 Science Notes – Writing & Balancing Chemical Equations

What is a Chemical Equation?

A chemical equation is a symbolic representation of a chemical reaction using chemical formulas. The reactants are written on the left side and products on the right side, with an arrow (→) pointing from reactants to products.

Types of Chemical Equations

1. Word Equation: Uses words to describe reactants and products

Zinc + Sulphuric acid → Zinc sulphate + Hydrogen

2. Skeletal (Unbalanced) Equation: Uses chemical formulas but not balanced

Fe + H₂O → Fe₃O₄ + H₂

3. Balanced Chemical Equation: Has equal number of atoms of each element on both sides

3Fe + 4H₂O → Fe₃O₄ + 4H₂

Law of Conservation of Mass

Proposed by Antoine Lavoisier (1789), this law states that mass can neither be created nor destroyed in a chemical reaction. The total mass of reactants equals the total mass of products. This is why we balance chemical equations.

Balancing Chemical Equations - Hit and Trial Method

Step-by-Step Method:

1. Write the word equation and then skeletal equation using correct formulas
2. Count atoms of each element on both sides
3. Balance one element at a time by adding coefficients (start with metals, then non-metals, then hydrogen, finally oxygen)
4. Never change subscripts in formulas - only add coefficients before formulas
5. Make coefficients simplest whole numbers
6. Verify atom count on both sides

Example: Balance the equation Fe + H₂O → Fe₃O₄ + H₂

Step 1: Count atoms

• Left side: Fe = 1, H = 2, O = 1
• Right side: Fe = 3, H = 2, O = 4

Step 2: Balance Fe atoms - multiply Fe by 3

3Fe + H₂O → Fe₃O₄ + H₂

Step 3: Balance O atoms - multiply H₂O by 4

3Fe + 4H₂O → Fe₃O₄ + H₂

Step 4: Balance H atoms - multiply H₂ by 4

3Fe + 4H₂O → Fe₃O₄ + 4H₂

Step 5: Verify - Left: Fe=3, H=8, O=4 | Right: Fe=3, H=8, O=4 ✓

State Symbols (Important for Board Exams)

Symbol	State	Example
(s)	Solid	NaCl(s), CaCO₃(s)
(l)	Liquid	H₂O(l), H₂SO₄(l)
(g)	Gas	H₂(g), O₂(g), CO₂(g)
(aq)	Aqueous (dissolved in water)	NaCl(aq), HCl(aq)

4. CBSE Class 10 Science Notes – Types of Chemical Reactions

1. Combination Reaction

Two or more reactants combine to form a single product. Usually exothermic (releases heat).

A + B → AB

Examples:

2H₂(g) + O₂(g) → 2H₂O(l) (Formation of water)

CaO(s) + H₂O(l) → Ca(OH)₂(aq) (Slaked lime formation)

2Mg(s) + O₂(g) → 2MgO(s) (Burning of magnesium)

2. Decomposition Reaction

A single compound breaks down into two or more simpler substances. Usually requires energy (endothermic).

AB → A + B

Types of Decomposition:

a) Thermal Decomposition (by heat):

2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g)

Observation: Green crystals change to reddish-brown solid

b) Electrolytic Decomposition (by electricity):

2H₂O(l) → 2H₂(g) + O₂(g)

c) Photochemical Decomposition (by light):

2AgCl(s) → 2Ag(s) + Cl₂(g) (Used in black & white photography)

3. Displacement Reaction

A more reactive element displaces a less reactive element from its compound.

A + BC → AC + B

Examples:

Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

Observation: Blue solution becomes colorless, reddish-brown copper deposits

Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)

Reactivity Series (Most to Least Reactive):

K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag > Au

A metal can displace any metal below it in the series from its salt solution.

4. Double Displacement Reaction

Two compounds exchange ions to form two new compounds.

AB + CD → AD + CB

Examples:

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq)

5. Precipitation Reaction

A type of double displacement where an insoluble solid (precipitate) is formed.

Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s)↓ + 2KNO₃(aq)

Observation: Yellow precipitate of lead iodide

Comparison Table: Types of Reactions

Type	General Form	Key Feature	Example
Combination	A + B → AB	Two or more → One	2H₂ + O₂ → 2H₂O
Decomposition	AB → A + B	One → Two or more	2H₂O₂ → 2H₂O + O₂
Displacement	A + BC → AC + B	More reactive displaces less reactive	Zn + CuSO₄ → ZnSO₄ + Cu
Double Displacement	AB + CD → AD + CB	Ion exchange between two compounds	Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

5. CBSE Class 10 Science Notes – Oxidation & Reduction (Redox)

Oxidation

A chemical process involving:

• Addition of oxygen
• Removal of hydrogen
• Loss of electrons

2Mg + O₂ → 2MgO (Magnesium gains oxygen - oxidized)

Reduction

A chemical process involving:

• Removal of oxygen
• Addition of hydrogen
• Gain of electrons

CuO + H₂ → Cu + H₂O (Copper oxide loses oxygen - reduced)

Redox Reaction

A reaction where oxidation and reduction occur simultaneously. One substance is oxidized while another is reduced.

ZnO + C → Zn + CO

• ZnO is reduced to Zn (oxygen removed)
• C is oxidized to CO (oxygen added)

Oxidizing and Reducing Agents

Agent	Action	Itself Gets	Example
Oxidizing Agent	Oxidizes other substance	Reduced	O₂, KMnO₄, CuO
Reducing Agent	Reduces other substance	Oxidized	H₂, C, CO

Memory Trick for Redox:

OIL RIG - Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons)

Or: LEO says GER - Lose Electrons Oxidation, Gain Electrons Reduction

6. CBSE Class 10 Science Notes – Corrosion & Rancidity

Corrosion

The process of deterioration of metals due to chemical attack by moisture, acids, gases, etc., forming undesirable compounds (oxides, carbonates, sulphides) on the surface.

Rusting of Iron

Conditions required: Both oxygen and moisture (water) must be present

4Fe(s) + 3O₂(g) + nH₂O(l) → 2Fe₂O₃·nH₂O(s) (Hydrated iron oxide - Rust)

Characteristics of Rust:

• Soft, porous, reddish-brown substance
• Not attached firmly to iron surface
• Weak and brittle
• Causes gradual destruction of iron articles

Prevention of Rusting

1. Painting: Creates protective barrier (gates, railings)
2. Oiling/Greasing: Prevents contact with air and moisture (machinery)
3. Galvanization: Coating with zinc (roofing sheets, buckets)
4. Electroplating: Coating with chromium or nickel
5. Alloying: Making stainless steel (iron + chromium + nickel)
6. Enamel coating: Glass-like coating (cooking utensils)

Rancidity

The oxidation of fats and oils in food materials when exposed to air, resulting in unpleasant smell and taste. This makes food unfit for consumption.

Prevention of Rancidity

1. Adding antioxidants: BHA (Butylated Hydroxyanisole), BHT
2. Refrigeration: Slows down oxidation
3. Airtight containers: Prevents exposure to air
4. Flushing with nitrogen: Inert gas prevents oxidation (chip packets)
5. Storing away from light: Light accelerates oxidation

7. Important Points (Quick Revision)

• Law of Conservation of Mass: Mass is neither created nor destroyed; atoms are rearranged
• Balancing: Only coefficients can be changed, never subscripts
• Combination: Multiple reactants → Single product (Exothermic)
• Decomposition: Single reactant → Multiple products (Endothermic)
• Displacement: More reactive metal displaces less reactive from salt solution
• Double Displacement: Ion exchange; precipitation if insoluble product forms
• Redox: Oxidation and reduction occur simultaneously
• Rusting: Requires both O₂ and H₂O; prevention by barrier methods
• Rancidity: Oxidation of fats; prevented by antioxidants and nitrogen flushing

8. Solved Examples (CBSE Pattern)

Example 1: Balancing Chemical Equation (2 Marks)

Question: Balance the following chemical equation: Al + HCl → AlCl₃ + H₂

Step-by-step Solution:

1. Count atoms: Left - Al=1, H=1, Cl=1 | Right - Al=1, H=2, Cl=3
2. Balance Cl: Multiply HCl by 3 → Al + 3HCl → AlCl₃ + H₂
3. Balance H: Multiply H₂ by 3/2 → Al + 3HCl → AlCl₃ + 3/2H₂
4. Remove fraction: Multiply entire equation by 2

Balanced Equation:

2Al(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂(g)

Example 2: Identify Reaction Type (2 Marks)

Question: Identify the type of reaction in each case:

a) 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂

b) Zn + CuSO₄ → ZnSO₄ + Cu

a) Decomposition Reaction - A single compound breaks down into multiple simpler substances upon heating.

b) Displacement Reaction - Zinc (more reactive) displaces copper (less reactive) from copper sulphate solution.

Example 3: Redox Reaction Identification (3 Marks)

Question: In the reaction CuO + H₂ → Cu + H₂O, identify:

a) Substance oxidized

b) Substance reduced

c) Oxidizing agent

d) Reducing agent

Analysis:

• CuO loses oxygen → becomes Cu (Reduction)
• H₂ gains oxygen → becomes H₂O (Oxidation)

Answers:

• a) H₂ is oxidized (gains oxygen)
• b) CuO is reduced (loses oxygen)
• c) CuO is oxidizing agent (oxidizes H₂, itself gets reduced)
• d) H₂ is reducing agent (reduces CuO, itself gets oxidized)

Example 4: Corrosion Reasoning (3 Marks)

Question: Why do iron nails rust faster in coastal areas than in deserts? Suggest two methods to prevent rusting.

Explanation: Rusting requires both oxygen and moisture. Coastal areas have high humidity (moisture in air) and salty air accelerates corrosion. Deserts have dry air with very low moisture.

Prevention Methods:

1. Painting - creates protective barrier
2. Galvanization - coating with zinc
3. Oiling/Greasing - prevents air contact
4. Electroplating - with chromium/nickel

Example 5: 5-Mark Conceptual Question

Question: a) What is a balanced chemical equation? Why should it be balanced?

b) Write balanced equations for:

i) Phosphorus burns in chlorine to form phosphorus pentachloride

ii) Sodium hydroxide reacts with hydrochloric acid

iii) Zinc carbonate is heated

a) Balanced Chemical Equation: An equation with equal number of atoms of each element on both sides. It must be balanced according to Law of Conservation of Mass - mass can neither be created nor destroyed.

b) Balanced Equations:

i) 2P(s) + 5Cl₂(g) → 2PCl₅(s)

ii) NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

iii) ZnCO₃(s) → ZnO(s) + CO₂(g)

Example 6: Case-Study Based Question

Passage: A student observed that a shiny brown element 'X' when heated in air turned black. When hydrogen gas was passed over the heated black substance, it again turned shiny brown.

Questions: a) Name element 'X' and black compound. b) Write chemical equations. c) Identify types of reactions.

a) Element 'X' is Copper (Cu). Black compound is Copper(II) oxide (CuO).

b) Equations:

2Cu(s) + O₂(g) → 2CuO(s) (Black)

CuO(s) + H₂(g) → Cu(s) + H₂O(l)

c) Reaction Types:

• First: Combination reaction
• Second: Redox reaction (Displacement + Reduction)

9. Smart Tricks & Memory Aids

Balancing Shortcut Trick:

Remember order: Metals → Non-metals → Hydrogen → Oxygen

Mnemonic: "Mama Never Hits Oliver"

Always balance oxygen last as it appears in multiple compounds.

Reaction Type Identification:

• Combination: Many → One (Marriage - two become one)
• Decomposition: One → Many (Divorce - one becomes many)
• Displacement: One kicks out another (Bullying - strong displaces weak)
• Double Displacement: Partners swap (Exchange dance)

Redox Quick Check:

OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons)

Or: LEO the lion says GER

Lose Electrons = Oxidation, Gain Electrons = Reduction

State Symbol Memory:

• (s) - Solid
• (l) - Liquid
• (g) - Gas
• (aq) - Aqueous (aqua = water)

Board Answer-Writing Strategy:

1. Always write word equation first, then balanced chemical equation
2. Include state symbols for full marks
3. For reaction type questions, write definition + equation
4. In redox questions, clearly mention what gains/loses oxygen
5. For corrosion: Must mention both O₂ and H₂O required
6. Write conditions (heat, light, electricity) above the arrow

10. Visual Learning – Diagrams

Figure 1: Flowchart showing steps to balance a chemical equation using Hit and Trial method

ALT: CBSE Class 10 Science Notes Chemical Reactions and Equations - Balancing Equations Flowchart

Figure 2: Four Main Types of Chemical Reactions - Combination, Decomposition, Single Displacement, Double Displacement

ALT: CBSE Class 10 Science Notes Chemical Reactions and Equations - Types of Reactions Diagram

Figure 3: Redox Reaction showing electron transfer - Oxidation (loss of electrons) and Reduction (gain of electrons)

ALT: CBSE Class 10 Science Notes Chemical Reactions and Equations - Redox Reaction Diagram

Figure 4: Rusting of Iron - Showing anodic and cathodic sites, electron flow, and formation of hydrated iron oxide (rust)

ALT: CBSE Class 10 Science Notes Chemical Reactions and Equations - Rusting of Iron Diagram

Figure 5: Laboratory illustration showing different types of chemical reactions with apparatus

ALT: CBSE Class 10 Science Notes Chemical Reactions and Equations - Chemical Reactions Laboratory Diagram

11. Most Important Board Questions

1 Mark Questions ★

What is a balanced chemical equation?

An equation with equal number of atoms of each element on both sides of the equation.

Name the law that requires balancing of chemical equations.

Law of Conservation of Mass (Antoine Lavoisier)

What is the chemical formula of rust?

Fe₂O₃·nH₂O (Hydrated iron(III) oxide)

What type of reaction is: 2H₂ + O₂ → 2H₂O?

Combination reaction

2-3 Mark Questions ★★

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

Magnesium forms a layer of magnesium oxide (MgO) when exposed to air. This layer prevents further reaction, so it must be cleaned to expose fresh magnesium for burning.

Differentiate between displacement and double displacement reactions with examples.

Displacement: More reactive element displaces less reactive from compound (Zn + CuSO₄ → ZnSO₄ + Cu). Double displacement: Two compounds exchange ions (Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl).

Why are decomposition reactions called the opposite of combination reactions?

In combination, two or more substances combine to form one product. In decomposition, one substance breaks into two or more products. They are reverse processes.

4-5 Mark Questions ★★★

Explain redox reaction with example. Identify oxidizing and reducing agents.

Redox reaction involves simultaneous oxidation and reduction. Example: CuO + H₂ → Cu + H₂O. CuO is reduced (oxidizing agent), H₂ is oxidized (reducing agent).

What is corrosion? Explain rusting of iron and methods to prevent it.

Corrosion is deterioration of metals by chemical attack. Rusting: 4Fe + 3O₂ + nH₂O → Fe₂O₃·nH₂O. Prevention: Painting, galvanization, oiling, electroplating, alloying.

12. Common Mistakes to Avoid

Mistake	Correction
Changing subscripts while balancing	Only change coefficients, never subscripts in formulas
Wrong reaction type identification	Count reactants and products carefully
Confusing oxidation and reduction	Oxidation = gain of O/loss of H; Reduction = loss of O/gain of H
Ignoring state symbols	Always include (s), (l), (g), (aq) for full marks
Incomplete rusting explanation	Must mention both oxygen AND moisture required
Wrong reactivity series order	Remember: K Na Ca Mg Al Zn Fe Pb H Cu Ag Au

13. Practice Section

Multiple Choice Questions (MCQs)

Q1. Which of the following is a displacement reaction?

1. 2H₂ + O₂ → 2H₂O
2. Zn + CuSO₄ → ZnSO₄ + Cu
3. CaCO₃ → CaO + CO₂
4. NaOH + HCl → NaCl + H₂O

Q2. The reaction 2AgCl → 2Ag + Cl₂ is an example of:

1. Thermal decomposition
2. Photochemical decomposition
3. Electrolytic decomposition
4. Displacement reaction

Q3. In the reaction CuO + H₂ → Cu + H₂O, CuO acts as:

1. Reducing agent
2. Oxidizing agent
3. Catalyst
4. Inert substance

Assertion-Reason Question

Assertion (A): White silver chloride turns grey in sunlight.

Reason (R): Silver chloride undergoes decomposition reaction in presence of sunlight.

Answer: Both A and R are true and R is the correct explanation of A.

Explanation: 2AgCl → 2Ag + Cl₂. Silver formed is grey, chlorine escapes.

14. Frequently Asked Questions (FAQ)

What is a balanced chemical equation?

A balanced chemical equation is a symbolic representation of a chemical reaction where the number of atoms of each element is equal on both sides of the equation. It follows the Law of Conservation of Mass.

What is redox reaction?

A redox (reduction-oxidation) reaction is a chemical reaction where both oxidation and reduction occur simultaneously. One substance loses electrons (oxidation) while another gains electrons (reduction).

How to prevent rusting?

Rusting can be prevented by painting, oiling/greasing, galvanization (zinc coating), electroplating (chromium/nickel), enamel coating, or making alloys like stainless steel. All methods create a barrier between iron and oxygen/moisture.

Are these CBSE Class 10 Science Notes based on NCERT?

Yes, these CBSE Class 10 Science Notes are 100% aligned with the latest NCERT textbook (2024-25 edition) for Chapter 1 "Chemical Reactions and Equations" and follow the current CBSE syllabus and examination pattern.

What is rancidity and how can it be prevented?

Rancidity is the oxidation of fats and oils in food, causing unpleasant smell and taste. It can be prevented by adding antioxidants (BHA, BHT), refrigeration, storing in airtight containers, flushing with nitrogen gas, and keeping away from light.

15. Conclusion

Master Chemical Reactions for Board Success!

These comprehensive CBSE Class 10 Notes for "Chemical Reactions and Equations" cover all essential concepts, balanced equations, and solved examples to help you excel in your board examinations.

Key Takeaways:

• Practice balancing equations using Hit and Trial method
• Remember state symbols (s, l, g, aq) for full marks
• Identify reaction types by counting reactants and products
• Understand redox: OIL RIG (Oxidation Is Loss, Reduction Is Gain)
• Corrosion needs O₂ + H₂O; Rancidity is oxidation of fats

Next Chapter: Acids, Bases and Salts – CBSE Class 10 Notes

Best of luck for your CBSE Class 10 Board Examinations!

These CBSE Class 10 Science Notes are prepared by experienced Chemistry faculty following the latest NCERT curriculum.