BOARD EXAM PREP | APRIL 2026
Last Updated: April 2026
Chapter 1 of CBSE Class 10 Science — Chemical Reactions and Equations carries 10-15 marks in the board exam. This is the highest-weightage chapter in Unit 1 (Chemical Substances). Expect 1 VSA, 2 SAI, 1 SAII, and 1 LA question from this chapter in the board exam.
Chemical Reactions and Equations is the first chapter of CBSE Class 10 Science and one of the most important topics for board exams. Understanding how substances change — through combination, decomposition, displacement, and redox reactions — forms the foundation of all chemistry you will encounter in higher classes. This comprehensive guide covers every NCERT concept, with balanced equations, examples, and a 10-MCQ practice set.
Chemical Reactions vs Physical Changes
Before studying chemical reactions, it is essential to distinguish them from physical changes:
| Physical Change | Chemical Change (Reaction) |
|---|---|
| No new substance is formed | One or more new substances are formed |
| Change is reversible (usually) | Change is often irreversible |
| Chemical composition unchanged | Chemical composition changes |
| Example: melting ice, tearing paper | Example: burning paper, rusting iron |
Characteristics of Chemical Reactions
A chemical reaction is taking place when you observe one or more of the following:
- Change in State: Solid, liquid, or gas forms that was not present before (e.g., gas bubbles forming in a reaction)
- Change in Colour: Copper sulphate (blue) + iron = iron sulphate (green) + copper
- Change in Temperature: Heat released (exothermic) or absorbed (endothermic) — burning releases heat; dissolving ammonium chloride absorbs heat
- Evolution of Gas: Zinc + sulphuric acid produces hydrogen gas bubbles
- Formation of Precipitate: Mixing sodium sulphate and barium chloride forms barium sulphate as a white precipitate
Writing Chemical Equations
Word Equations
Word equations use the names of reactants and products. Example:
Magnesium + Oxygen → Magnesium oxide
Symbolic Chemical Equations
Chemical symbols and formulae replace the names:
Mg + O2 → MgO (unbalanced)
Balancing Chemical Equations
A balanced equation obeys the Law of Conservation of Mass — atoms of each element must be equal on both sides. The hit-and-trial (inspection) method is used at Class 10 level.
Example: Balancing the combustion of magnesium
Unbalanced: Mg + O2 → MgO
Balance Mg: 2Mg + O2 → 2MgO (now Mg=2 on both sides)
Check O: 2 on left, 2 on right (balanced!)
Balanced: 2Mg + O2 → 2MgO
(s) = solid | (l) = liquid | (g) = gas | (aq) = aqueous solution
Arrow pointing downward (precipitate) and arrow pointing upward (gas evolved) are used to indicate insoluble precipitate and gas evolution respectively.
Types of Chemical Reactions
1. Combination Reaction
Two or more substances combine to form a single product.
General form: A + B → AB
Examples:
- 2H2(g) + O2(g) → 2H2O(l) — Formation of water
- CaO(s) + H2O(l) → Ca(OH)2(aq) — Slaking of lime (exothermic reaction)
- C(s) + O2(g) → CO2(g) — Combustion of carbon
2. Decomposition Reaction
A single compound breaks down into two or more simpler substances.
General form: AB → A + B
Decomposition reactions require energy input — this can be:
| Type | Energy Source | Example | Balanced Equation |
|---|---|---|---|
| Thermal Decomposition | Heat | Limestone decomposition | CaCO3(s) → CaO(s) + CO2(g) |
| Photolytic Decomposition | Light | Silver chloride in sunlight | 2AgCl(s) → 2Ag(s) + Cl2(g) |
| Electrolytic Decomposition | Electricity | Electrolysis of water | 2H2O(l) → 2H2(g) + O2(g) |
3. Displacement Reaction (Single Displacement)
A more reactive element displaces a less reactive element from its compound.
General form: A + BC → AC + B
Examples:
- Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s) — Iron displaces copper (iron is more reactive)
- Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g) — Zinc displaces hydrogen from hydrochloric acid
- Mg(s) + 2AgNO3(aq) → Mg(NO3)2(aq) + 2Ag(s) — Magnesium displaces silver
4. Double Displacement Reaction
Two compounds exchange their ions to form two new compounds. Often results in a precipitate or gas.
General form: AB + CD → AD + CB
Examples:
- Na2SO4(aq) + BaCl2(aq) → BaSO4(s)↓ + 2NaCl(aq) — Barium sulphate precipitate formed
- NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) — Neutralisation (acid + base)
- AgNO3(aq) + NaCl(aq) → AgCl(s)↓ + NaNO3(aq) — White precipitate of silver chloride
5. Oxidation and Reduction Reactions (Redox)
| Process | Older Definition | Modern Definition (electrons) |
|---|---|---|
| Oxidation | Gain of oxygen OR loss of hydrogen | Loss of electrons |
| Reduction | Loss of oxygen OR gain of hydrogen | Gain of electrons |
Example of Redox reaction:
CuO(s) + H2(g) → Cu(s) + H2O(l)
CuO is reduced (loses oxygen / gains electrons) → Cu
H2 is oxidised (gains oxygen / loses electrons) → H2O
Oxidation Is Loss (of electrons) | Reduction Is Gain (of electrons)
Both oxidation and reduction always occur simultaneously in a redox reaction — hence the term “redox.”
6. Exothermic and Endothermic Reactions
- Exothermic Reactions: Release energy (heat or light) to the surroundings. The products are at lower energy than the reactants. Examples: Combustion (burning of natural gas, coal), respiration, neutralisation of acid and base, slaking of lime (CaO + H2O).
- Endothermic Reactions: Absorb energy from the surroundings. Examples: Photosynthesis (absorbs sunlight), decomposition of calcium carbonate, dissolving ammonium chloride in water (feels cold).
Comprehensive Reaction Type Summary
| Reaction Type | Definition | Key Example | Balanced Equation |
|---|---|---|---|
| Combination | A + B → AB | Burning magnesium | 2Mg + O2 → 2MgO |
| Decomposition | AB → A + B | Limestone heating | CaCO3 → CaO + CO2 |
| Displacement | A + BC → AC + B | Iron in copper sulphate | Fe + CuSO4 → FeSO4 + Cu |
| Double Displacement | AB + CD → AD + CB | Sodium sulphate + barium chloride | Na2SO4 + BaCl2 → BaSO4↓ + 2NaCl |
| Redox | Oxidation + Reduction simultaneously | CuO reduced by hydrogen | CuO + H2 → Cu + H2O |
Rancidity and Corrosion
Rancidity
When foods containing fats and oils are left exposed to air, they are oxidised and develop an unpleasant smell and taste. This process is called rancidity.
Methods to prevent rancidity:
- Flushing packets with nitrogen gas (inert atmosphere prevents oxidation)
- Adding antioxidants such as BHA (Butylated hydroxyanisole) or vitamin C
- Refrigeration — slows down oxidation rate
- Packaging in airtight containers — limits contact with oxygen
- Vacuum packing — removes oxygen from packaging
Corrosion
When a metal reacts with substances in its environment (oxygen, water, acids) and is slowly eaten away, the process is called corrosion. The corrosion of iron is specifically called rusting.
Balanced equation for rusting:
4Fe(s) + 3O2(g) + 6H2O(l) → 4Fe(OH)3(s) which dehydrates to Fe2O3.xH2O (hydrated iron oxide = rust)
Corrosion prevention: Galvanisation (zinc coating), painting, oiling/greasing, using alloys (stainless steel), electroplating.
NCERT In-Text Questions — Key Points
- Why should a magnesium ribbon be cleaned before burning? — To remove the protective oxide layer (MgO) on the surface which prevents combustion. Cleaning exposes fresh magnesium metal.
- Why is decomposition reaction opposite of combination? — In combination, two or more substances form one product. In decomposition, one compound breaks into two or more simpler substances. They are reverse processes.
- What is a precipitation reaction? — A reaction in which an insoluble product (precipitate) is formed when two solutions are mixed. The precipitate settles at the bottom of the container.
Previous Year Board Exam Questions (2022-2026)
- 2026: “Give one example each of a combination reaction and a decomposition reaction. Write the balanced chemical equations.” (3 marks)
- 2025: “What is meant by a double displacement reaction? How is it different from a displacement reaction? Give one example of each.” (3 marks)
- 2024: “Define oxidation and reduction in terms of gain or loss of oxygen. Identify the substance oxidised and the substance reduced in the reaction: CuO + H2 → Cu + H2O” (3 marks)
- 2023: “What is rancidity? State two methods to prevent rancidity.” (2 marks)
- 2022: “Identify the type of reaction: 2AgBr → 2Ag + Br2. What happens when this reaction takes place in sunlight?” (2 marks)
MCQ Practice — Chemical Reactions and Equations
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Frequently Asked Questions — Chemical Reactions and Equations
What is the law of conservation of mass and how does it apply to balancing equations?
The Law of Conservation of Mass states that mass cannot be created or destroyed in a chemical reaction. The total mass of reactants equals the total mass of products. This is why we balance chemical equations — to ensure the number of atoms of each element is equal on both sides of the equation, reflecting that no atoms are created or lost during the reaction.
What is the difference between displacement and double displacement reactions?
In a displacement reaction, one element displaces another element from its compound because it is more reactive. For example: Fe + CuSO4 → FeSO4 + Cu (iron displaces copper). In a double displacement reaction, two compounds exchange their ions with each other to form two new compounds. For example: Na2SO4 + BaCl2 → BaSO4↓ + 2NaCl. Double displacement often results in a precipitate, gas, or water.
How is rancidity different from corrosion?
Both rancidity and corrosion are oxidation processes, but they affect different substances. Rancidity is the oxidation of fats and oils in food, leading to an unpleasant smell and taste. It is prevented by antioxidants, nitrogen gas flushing, or refrigeration. Corrosion is the oxidation of metals (especially iron) when they react with oxygen and moisture in the environment, forming metal oxides. Rusting of iron is a specific example of corrosion, prevented by galvanisation, painting, or alloying.
Is photosynthesis an exothermic or endothermic reaction?
Photosynthesis is an endothermic reaction. Plants absorb solar energy (light) and use it to convert carbon dioxide and water into glucose and oxygen. Since energy is being absorbed from the surroundings (sun), it is an endothermic process. The reaction is: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2. This is also a combination reaction as simpler substances combine to form a complex molecule (glucose).
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