Chemical Reactions and Equations – Class 10 Science

📖 Chapter 1 | Class 10 Science

🧪 Chemical Reactions & Equations

NCERT Science — Understanding reactions, equations, types & everyday effects

📋 Topics Covered

1. Chemical Equations — Writing & Balancing
2. Types of Chemical Reactions — Combination, Decomposition, Displacement, Double Displacement, Redox
3. Exothermic & Endothermic Reactions
4. Oxidation & Reduction (Redox)
5. Corrosion & Rancidity
6. In-text Questions & Answers
7. Exercise Questions & Answers

1. 📝 Chemical Equations — Writing & Balancing

Signs of a Chemical Reaction

🌡 Change in Temperature 🟢 Change in State 🔵 Change in Colour 💨 Evolution of Gas 🧱 Formation of Precipitate

Word Equation → Chemical Equation

📐 Steps to Write a Chemical Equation

1.Write Word Equation: Reactants → Products

2.Replace words with Chemical Formulae (skeletal equation)

3.Balance the equation (hit-and-trial method)

4.Add physical state symbols: (s), (l), (g), (aq)

5.Add conditions (heat, light, pressure, catalyst) above/below arrow if needed

⚖ Balancing Example: Fe + H₂O → Fe₃O₄ + H₂

Step	Equation	Status
Skeletal	Fe + H₂O → Fe₃O₄ + H₂	❌ Unbalanced
Balance O	Fe + 4H₂O → Fe₃O₄ + H₂	⏳ Partial
Balance H	Fe + 4H₂O → Fe₃O₄ + 4H₂	⏳ Partial
Balance Fe	3Fe + 4H₂O → Fe₃O₄ + 4H₂	✅ Balanced

✅ Final: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)

🔑 Law of Conservation of Mass: Mass can neither be created nor destroyed. The number of atoms of each element must be equal on both sides of a balanced equation.

Physical State Symbols

■(s) — Solid

💧(l) — Liquid

💨(g) — Gas

💦(aq) — Aqueous (dissolved in water)

↑Conditions written above/below arrow: Heat, Light, Pressure, Catalyst

2. 🔁 Types of Chemical Reactions

🗺 Reaction Types at a Glance

➕

Combination
A + B → AB

➖

Decomposition
AB → A + B

⇄

Displacement
A + BC → AC + B

🔀

Double Disp.
AB+CD→AD+CB

⚡

Redox
Oxidation+Reduction

2.1 Combination Reaction — A + B → AB

➕ Two or more substances combine → ONE product

CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat (Quick lime + Water → Slaked lime)
C(s) + O₂(g) → CO₂(g) (Burning coal)
2H₂(g) + O₂(g) → 2H₂O(l) (Formation of water)

💡Combination reactions are usually exothermic (release heat)

📐 Combination Reaction Diagram

Reactant 1

Reactant 2

➡

Single Product

Example: CaO + H₂O → Ca(OH)₂ 🔥 Releases Heat (Exothermic)

2.2 Decomposition Reaction — AB → A + B

➖ ONE reactant breaks down → two or more products

2FeSO₄(s) ^Heat→ Fe₂O₃(s) + SO₂(g) + SO₃(g) (Thermal)
2Pb(NO₃)₂(s) ^Heat→ 2PbO(s) + 4NO₂(g) + O₂(g) (Thermal)
2H₂O(l) ^Electric→ 2H₂(g) + O₂(g) (Electrolytic)
2AgCl(s) ^Sunlight→ 2Ag(s) + Cl₂(g) (Photo decomposition)
CaCO₃(s) ^Heat→ CaO(s) + CO₂(g) (Thermal — used in cement industry)

⚡ Energy Sources in Decomposition

🔥

Thermal
Heat energy

CaCO₃ → CaO + CO₂

☀

Photo
Light energy

2AgCl → 2Ag + Cl₂

⚡

Electrolytic
Electric energy

2H₂O → 2H₂ + O₂

2.3 Displacement Reaction — A + BC → AC + B

⇄ More reactive element displaces less reactive element

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) (Iron displaces copper)
Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s) (Zinc displaces copper)
Pb(s) + CuCl₂(aq) → PbCl₂(aq) + Cu(s) (Lead displaces copper)

💡Reactivity order: Zn > Fe > Pb > Cu (more reactive displaces less reactive)

2.4 Double Displacement Reaction — AB + CD → AD + CB

🔀 Exchange of ions between two compounds

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq) (White precipitate)
Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s)↓ + 2KNO₃(aq) (Yellow precipitate)

💡If an insoluble product (precipitate) forms, it is also called a precipitation reaction

2.5 Oxidation–Reduction (Redox)

💥 Oxidation

→Gain of oxygen

→Loss of hydrogen

Example: 2Cu + O₂ → 2CuO
(Cu gains oxygen → oxidised)

🧊 Reduction

→Loss of oxygen

→Gain of hydrogen

Example: CuO + H₂ → Cu + H₂O
(CuO loses oxygen → reduced)

🔑 Redox Rule: In any redox reaction, one substance is oxidised and the other is simultaneously reduced. They always occur together.

🧪 Redox Diagram: CuO + H₂ → Cu + H₂O

3. 🌡 Exothermic & Endothermic Reactions

🔥 Exothermic Reactions

•Heat is released during reaction

•Products have less energy than reactants

📌 CaO + H₂O → Ca(OH)₂ + Heat
📌 CH₄ + 2O₂ → CO₂ + 2H₂O (burning gas)
📌 Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
📌 Decomposition of vegetable matter into compost

❄ Endothermic Reactions

•Energy is absorbed during reaction

•Products have more energy than reactants

📌 Electrolysis of water: 2H₂O → 2H₂ + O₂
📌 Ba(OH)₂ + NH₄Cl → BaCl₂ + NH₃ + H₂O
📌 Photosynthesis: CO₂ + H₂O → C₆H₁₂O₆ + O₂
📌 All decomposition reactions need energy

📊 Energy Profile Diagrams

Exothermic

Endothermic

4. ⚡ Oxidation & Reduction (Extended)

Term	Gain	Loss	Example
Oxidation	Oxygen ⬆	Hydrogen ⬇	2Cu + O₂ → 2CuO
Reduction	Hydrogen ⬆	Oxygen ⬇	CuO + H₂ → Cu + H₂O
Oxidising Agent	Provides oxygen / accepts electrons		O₂, CuO, MnO₂
Reducing Agent	Provides hydrogen / donates electrons		H₂, C, CO

ZnO + C → Zn + CO (C is oxidised to CO; ZnO is reduced to Zn)
MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ (HCl oxidised; MnO₂ reduced)

5. 🛡 Corrosion & Rancidity — Effects of Oxidation

🔴 Corrosion

•Metal attacked by moisture, acids, gases

•Iron → Rust (reddish-brown, Fe₂O₃·xH₂O)

•Silver → Black coating (Ag₂S)

•Copper → Green coating (basic copper carbonate)

🛡Prevention: Paint, galvanising, alloying

🧈 Rancidity

•Oxidation of fats & oils

•Changes smell and taste of food

•Food becomes unfit to eat

🛡Prevention: Antioxidants, airtight containers, flushing with N₂ gas (chips)

6. ❓ In-Text Questions & Answers

📌 Set 1 — After Chemical Equations

1Why should a magnesium ribbon be cleaned before burning in air?

Magnesium ribbon is cleaned with sandpaper to remove the layer of magnesium oxide (MgO) from its surface. This oxide layer prevents the ribbon from burning properly. Cleaning exposes fresh magnesium metal, allowing it to burn with a dazzling white flame.

2Write balanced equations: (i) H₂ + Cl₂ → HCl (ii) BaCl₂ + Al₂(SO₄)₃ → BaSO₄ + AlCl₃ (iii) Na + H₂O → NaOH + H₂

(i) H₂ + Cl₂ → 2HCl
(ii) 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃
(iii) 2Na + 2H₂O → 2NaOH + H₂↑

3Write balanced equations with state symbols: (i) BaCl₂ + Na₂SO₄ → BaSO₄ + NaCl (ii) NaOH + HCl → NaCl + H₂O

(i) BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s)↓ + 2NaCl(aq)
(ii) NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

📌 Set 2 — After Decomposition

1Name substance ‘X’ used for whitewashing and write its reaction with water.

Substance X is Calcium oxide (CaO) — Quick lime.
CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat
Ca(OH)₂ (slaked lime) is used for whitewashing. It reacts with CO₂ in air to form CaCO₃, giving a shiny finish.

2In electrolysis of water, why is one test tube gas double the other? Name this gas.

In electrolysis: 2H₂O → 2H₂ + O₂. The molar ratio of H₂ to O₂ is 2:1. Therefore, the volume of hydrogen gas collected at the cathode is double the volume of oxygen at the anode.

📌 Set 3 — After Displacement & Redox

1Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Iron is more reactive than copper. Iron displaces copper from copper sulphate solution:
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
The blue colour of CuSO₄ fades as Cu²⁺ ions are replaced by Fe²⁺ ions (FeSO₄ is light green). Copper gets deposited on the iron nail, turning it brownish.

2Give an example of a double displacement reaction other than Na₂SO₄ + BaCl₂.

AgNO₃(aq) + NaCl(aq) → AgCl(s)↓ + NaNO₃(aq)
Silver nitrate and sodium chloride exchange ions. AgCl forms a white precipitate — this is also a precipitation reaction.

3Identify oxidised and reduced substances: (i) 4Na + O₂ → 2Na₂O (ii) CuO + H₂ → Cu + H₂O

(i) Na is oxidised (gains oxygen to form Na₂O); O₂ is reduced (gains electrons)
(ii) H₂ is oxidised (gains oxygen → H₂O); CuO is reduced (loses oxygen → Cu)

7. 📚 Exercise Questions & Answers

🟢 MCQs

1Which statements about 2PbO(s) + C(s) → 2Pb(s) + CO₂(g) are incorrect? (a) Lead is getting reduced (b) CO₂ is getting oxidised (c) Carbon is getting oxidised (d) PbO is getting reduced

Answer: (i) (a) and (b)
Lead (Pb) IS getting reduced (loses oxygen from PbO). CO₂ is a product — it is NOT getting oxidised. Carbon IS getting oxidised (C → CO₂). PbO IS getting reduced. So (a) is incorrect (lead IS reduced) and (b) is incorrect (CO₂ is not being oxidised, it is being formed).

2Fe₂O₃ + 2Al → Al₂O₃ + 2Fe — This is an example of which reaction?

Answer: (d) Displacement reaction
Aluminium (more reactive) displaces iron from iron oxide. This is the Thermite reaction. Iron gets reduced; Aluminium gets oxidised — so it is also a redox reaction.

3What happens when dilute HCl is added to iron fillings?

Answer: (a) Hydrogen gas and iron chloride are produced.
Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)↑

🟡 Short Answer Questions

4What is a balanced chemical equation? Why should equations be balanced?

A balanced chemical equation has equal number of atoms of each element on both sides (LHS = RHS). It obeys the Law of Conservation of Mass — mass cannot be created or destroyed. Balancing ensures the equation correctly represents the actual chemical change.

5Translate and balance: (a) H₂ + N₂ → NH₃ (b) H₂S + O₂ → SO₂ + H₂O (c) BaCl₂ + Al₂(SO₄)₃ → AlCl₃ + BaSO₄ (d) K + H₂O → KOH + H₂

(a) 3H₂(g) + N₂(g) → 2NH₃(g)
(b) 2H₂S(g) + 3O₂(g) → 2SO₂(g) + 2H₂O(l)
(c) 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 2AlCl₃(aq) + 3BaSO₄(s)↓
(d) 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)↑

6Balance: (a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O (b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O (c) NaCl + AgNO₃ → AgCl + NaNO₃ (d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl

(a) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
(b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃ (already balanced)
(d) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

7Write balanced equations: (a) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O (b) Zn + AgNO₃ → Zn(NO₃)₂ + Ag (c) Al + CuCl₂ → AlCl₃ + Cu (d) BaCl₂ + K₂SO₄ → BaSO₄ + KCl

(a) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O (balanced)
(b) Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag
(c) 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu
(d) BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl

8Write balanced equations and identify reaction type: (a) KBr + BaI₂ → KI + BaBr₂ (b) ZnCO₃ → ZnO + CO₂ (c) H₂ + Cl₂ → HCl (d) Mg + HCl → MgCl₂ + H₂

(a) 2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(s) — Double Displacement
(b) ZnCO₃(s) → ZnO(s) + CO₂(g) — Decomposition
(c) H₂(g) + Cl₂(g) → 2HCl(g) — Combination
(d) Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)↑ — Displacement

9What are exothermic and endothermic reactions? Give examples.

Exothermic: Reactions that release heat energy. E.g., burning of natural gas: CH₄ + 2O₂ → CO₂ + 2H₂O + heat.
Endothermic: Reactions that absorb energy. E.g., electrolysis of water: 2H₂O → 2H₂ + O₂ (requires electrical energy).

10Why is respiration considered an exothermic reaction?

During respiration, glucose combines with oxygen in our body cells to produce CO₂, water, and energy:
C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + energy
Since heat (energy) is released during the process, it is an exothermic reaction.

11Why are decomposition reactions called the opposite of combination reactions?

In combination: two or more substances combine → one product (A + B → AB).
In decomposition: one substance breaks down → two or more products (AB → A + B).
They are exact opposites in terms of reactant-product relationship.
Example — Combination: 2H₂ + O₂ → 2H₂O | Decomposition: 2H₂O → 2H₂ + O₂

12Write one equation each for decomposition by heat, light, and electricity.

Heat: CaCO₃(s) → CaO(s) + CO₂(g)
Light: 2AgCl(s) ^Sunlight→ 2Ag(s) + Cl₂(g)
Electricity: 2H₂O(l) ^Electric→ 2H₂(g) + O₂(g)

13Difference between displacement and double displacement? Write equations.

Displacement: One element displaces another from its compound.
Fe + CuSO₄ → FeSO₄ + Cu (Fe displaces Cu)

Double Displacement: Two compounds exchange ions to form two new compounds.
Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl (ions exchange)

14In refining of silver, silver is recovered from silver nitrate by copper. Write the reaction.

Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)
Copper is more reactive than silver, so it displaces silver from silver nitrate solution.

15What is a precipitation reaction? Explain with examples.

A precipitation reaction is a double displacement reaction that produces an insoluble product (precipitate) when two aqueous solutions are mixed.
Example 1: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ (white ppt) + 2NaCl(aq)
Example 2: AgNO₃(aq) + NaCl(aq) → AgCl(s)↓ (white ppt) + NaNO₃(aq)

16Explain oxidation and reduction in terms of oxygen gain/loss. Give 2 examples each.

Oxidation (gain of oxygen):
(i) 2Cu + O₂ → 2CuO (Cu gains O — oxidised)
(ii) C + O₂ → CO₂ (C gains O — oxidised)

Reduction (loss of oxygen):
(i) CuO + H₂ → Cu + H₂O (CuO loses O — reduced)
(ii) ZnO + C → Zn + CO (ZnO loses O — reduced)

17A shiny brown element ‘X’ turns black on heating. Name X and the black compound.

Element X is Copper (Cu) — it is shiny brown.
On heating: 2Cu + O₂ → 2CuO
The black compound formed is Copper(II) oxide (CuO).

18Why do we apply paint on iron articles?

Paint is applied on iron articles to prevent corrosion (rusting). Paint forms a protective layer between the iron surface and the atmosphere (oxygen and moisture), which prevents the oxidation reaction that forms rust (Fe₂O₃·xH₂O).

19Oil and fat containing food items are flushed with nitrogen. Why?

Nitrogen is an inert gas that does not react with food. Flushing with nitrogen displaces oxygen from the packaging, preventing the oxidation of fats and oils. This stops rancidity and increases the shelf life of food items like chips.

20Explain corrosion and rancidity with one example each.

Corrosion: When a metal is attacked by substances in its environment (moisture, acids, gases) causing it to deteriorate.
Example: Rusting of iron — 4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O (reddish-brown rust)

Rancidity: Oxidation of fats and oils in food, causing unpleasant smell and taste.
Example: Butter left uncovered turns rancid due to oxidation by atmospheric oxygen.