Chemical Reactions and Equations — Class 10 Science
"Chemistry is the music of matter — every reaction is a story."
1. About the Chapter
This is the opening chapter of Class 10 Science. Introduces:
- Chemical reactions and their characteristics
- Writing and balancing chemical equations
- Types of reactions (5 main types)
- Oxidation and reduction (redox)
- Corrosion and rancidity (real-world applications)
Why Important
- Foundation for all chemistry
- Used in industry, medicine, daily life
- Topic appears across science studies
2. What is a Chemical Reaction?
Definition
A chemical reaction is a process in which one or more substances (reactants) are transformed into new substances (products) with different properties.
Characteristics (Signs of Chemical Reaction)
- Change in colour (e.g., iron rusting, leaves turning yellow)
- Change in state (e.g., wax melting then re-solidifying differently)
- Evolution of gas (e.g., bubbling soda)
- Formation of precipitate (solid in liquid)
- Change in temperature (heating or cooling)
- Emission of light (e.g., burning magnesium)
Reactants and Products
Reactants → Products
Example: Hydrogen + Oxygen → Water
- Reactants: H₂, O₂
- Product: H₂O
3. Chemical Equations
Word Equation
Magnesium + Oxygen → Magnesium oxide
Symbol Equation
Mg + O₂ → MgO
Balancing Equations
A balanced equation has equal atoms of each element on both sides (Law of Conservation of Mass).
Unbalanced: Mg + O₂ → MgO
- Mg: 1 on each side ✓
- O: 2 on left, 1 on right ✗
Balanced: 2Mg + O₂ → 2MgO
- Mg: 2 on each side ✓
- O: 2 on each side ✓
Why Balance?
Mass cannot be created or destroyed (Lavoisier's Law of Conservation of Mass, 1789).
Symbols Used
- (s): solid
- (l): liquid
- (g): gas
- (aq): aqueous (dissolved in water)
- ↑: gas evolves
- ↓: precipitate forms
- Δ: heated
4. Types of Chemical Reactions
Type 1: Combination Reaction
Two or more substances combine to form a SINGLE product.
A + B → AB
Examples:
- 2Mg + O₂ → 2MgO (magnesium burns)
- CaO + H₂O → Ca(OH)₂ (calcium oxide → slaked lime; exothermic)
- 2H₂ + O₂ → 2H₂O
Type 2: Decomposition Reaction
A SINGLE substance breaks into two or more products.
AB → A + B
Types:
- Thermal decomposition (by heat): CaCO₃ → CaO + CO₂ (limestone heated)
- Electrolytic (by electricity): 2H₂O → 2H₂ + O₂
- Photochemical (by light): 2AgCl → 2Ag + Cl₂ (silver chloride in photographic film)
Type 3: Displacement Reaction
A more reactive element displaces a less reactive one.
A + BC → AC + B
Example: Iron + Copper sulphate → Iron sulphate + Copper Fe + CuSO₄ → FeSO₄ + Cu
Activity Series (most reactive first): K, Na, Ca, Mg, Al, Zn, Fe, Pb, Cu, Hg, Ag, Au
A metal higher in series displaces metal lower.
Type 4: Double Displacement Reaction
Two compounds exchange ions.
AB + CD → AD + CB
Example: Sodium sulphate + Barium chloride → Barium sulphate + Sodium chloride Na₂SO₄ + BaCl₂ → BaSO₄ ↓ + 2NaCl
(BaSO₄ is precipitate.)
This is also called precipitation reaction.
Type 5: Redox Reaction (Oxidation-Reduction)
Oxidation = gain of oxygen OR loss of hydrogen OR loss of electrons. Reduction = loss of oxygen OR gain of hydrogen OR gain of electrons.
These occur TOGETHER (one substance oxidised, another reduced).
Example: CuO + H₂ → Cu + H₂O
- CuO loses O → reduced to Cu
- H₂ gains O → oxidised to H₂O
5. Oxidation and Reduction (Detailed)
Definition (Modern, Electronic)
Oxidation: loss of electrons Reduction: gain of electrons
Memory: OIL RIG (Oxidation Is Loss; Reduction Is Gain — of electrons).
Oxidising Agent vs Reducing Agent
- Oxidising agent: causes oxidation; itself reduced
- Reducing agent: causes reduction; itself oxidised
Example: 2Mg + O₂ → 2MgO
- Mg loses electrons → oxidised; Mg is reducing agent
- O₂ gains electrons → reduced; O₂ is oxidising agent
6. Effects of Oxidation in Daily Life
Corrosion
Slow oxidation of metals.
Rusting of iron: 4Fe + 3O₂ + xH₂O → 2Fe₂O₃.xH₂O (hydrated iron oxide)
- Requires both oxygen and water
- Forms reddish-brown rust
- Major economic loss globally
Prevention:
- Painting (barrier)
- Greasing/oiling
- Galvanisation (zinc coating)
- Electroplating (chromium)
- Alloying (stainless steel: iron + chromium + nickel)
Rancidity
Oxidation of oils/fats in food.
- Develops bad smell and taste
- Common in fried foods, butter
Prevention:
- Sealed packaging (no oxygen)
- Refrigeration (slows oxidation)
- Antioxidants (BHA, BHT)
- Vacuum packing
- Nitrogen flushing (replace oxygen with N₂)
7. Worked Examples
Example 1: Balance the equation
Balance: Al + HCl → AlCl₃ + H₂
- Al: 1 → ?
- H: 1 → 2
- Cl: 1 → 3
Try: 2Al + 6HCl → 2AlCl₃ + 3H₂
- Al: 2 = 2 ✓
- H: 6 = 6 ✓
- Cl: 6 = 6 ✓
Example 2: Identify Type
Identify: 2KClO₃ →(heat) 2KCl + 3O₂
- One substance breaks into more → DECOMPOSITION
Example 3: Identify Type
Identify: Zn + 2HCl → ZnCl₂ + H₂
- Zn displaces H from HCl → DISPLACEMENT
Example 4: Identify Type
Identify: AgNO₃ + NaCl → AgCl↓ + NaNO₃
- Ion exchange between two compounds → DOUBLE DISPLACEMENT (also precipitation)
Example 5: Find Oxidising Agent
In CuO + H₂ → Cu + H₂O:
- CuO loses O (reduced) → CuO is OXIDISING AGENT
- H₂ gains O (oxidised) → H₂ is REDUCING AGENT
8. Common Mistakes
-
Unbalanced equations
- Always balance by checking atoms of each element.
-
Wrong subscripts
- You can CHANGE coefficients (in front), NOT subscripts (in formula). H₂O is always H₂O, not H₃O.
-
Sign of arrow
- Forward arrow → for irreversible. Double arrow ⇌ for reversible.
-
Confusing types
- Decomposition: 1 → many. Combination: many → 1.
-
Oxidation/reduction confusion
- OIL RIG: Oxidation = lose electrons; Reduction = gain electrons.
9. Indian Heritage
Ancient Indian Chemistry
- Rasaśastra (alchemy) — medieval Indian chemistry texts
- Charaka (~600 BCE) — described various chemical processes
- Indian metallurgy: zinc extraction discovered in Zawar (Rajasthan) — earliest in world
Modern Indian Chemistry
- C.V. Raman — Nobel 1930 (Raman Effect in chemistry)
- Har Gobind Khorana — Nobel 1968 (biochemistry)
- C.N.R. Rao — Bharat Ratna 2014 (materials chemistry)
10. Conclusion
Chemical Reactions and Equations is THE foundation of Class 10 Chemistry:
- Master writing and balancing equations
- Memorise 5 types of reactions
- Understand oxidation, reduction, corrosion, rancidity
- Connect to real-world applications
Practice:
- 15+ balancing problems
- Identify each reaction type
- Daily life examples (rusting, food spoilage, fireworks)
This chapter prepares you for Chapters 2-4 (Acids/Bases, Metals/Non-metals, Carbon).
Every reaction tells a story — learn to read it.
