MP Board Class 10 Science Chapter 1: Chemical Reactions & Equations Notes 2027 — Types, Balancing, Redox & PYQs
Chapter 1: Chemical Reactions & Equations is a fundamental chapter in MP Board Class 10 Science, carrying 8–10 marks in the annual board exam. This chapter introduces the concept of chemical changes, types of chemical reactions, balancing chemical equations, and oxidation-reduction processes. Questions range from 1-mark MCQs and VSA to 3-mark short answer and 5-mark long answer questions. Mastery of writing and balancing chemical equations is essential for scoring high marks in this chapter.
📑 Table of Contents
⚗️ Chemical Equations & Balancing
A chemical equation is a symbolic representation of a chemical reaction using chemical formulas. Reactants are written on the left side and products on the right side, separated by an arrow (→).
📝 Rules for Writing Chemical Equations
- Write the correct chemical formulas of reactants and products
- Use the arrow (→) to separate reactants from products
- Indicate physical states: (s) solid, (l) liquid, (g) gas, (aq) aqueous
- Balance the equation so that the number of atoms of each element is equal on both sides
- Use coefficients (numbers before formulas) to balance — never change subscripts
📐 Solved Example 1: Burning of Magnesium
Unbalanced: Mg + O₂ → MgO
Step 1: Count atoms — Left: Mg=1, O=2 | Right: Mg=1, O=1
Step 2: Balance oxygen — put coefficient 2 before MgO: Mg + O₂ → 2MgO
Step 3: Balance magnesium — put coefficient 2 before Mg: 2Mg + O₂ → 2MgO
Balanced: 2Mg + O₂ → 2MgO
📐 Solved Example 2: Decomposition of Ferrous Sulphate
Unbalanced: FeSO₄ → Fe₂O₃ + SO₂ + SO₃
Step 1: Count atoms — Left: Fe=1, S=1, O=4 | Right: Fe=2, S=2, O=8
Step 2: Balance Fe — put coefficient 2 before FeSO₄: 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃
Step 3: Balance S — Left S=2, Right S=2 ✓ | Balance O — Left O=8, Right O=8 ✓
Balanced: 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃
📐 Solved Example 3: Reaction of Lead Nitrate with Potassium Iodide
Unbalanced: Pb(NO₃)₂ + KI → PbI₂ + KNO₃
Step 1: Count atoms — Left: Pb=1, N=2, O=6, K=1, I=1 | Right: Pb=1, I=2, K=1, N=1, O=3
Step 2: Balance I — put coefficient 2 before KI: Pb(NO₃)₂ + 2KI → PbI₂ + KNO₃
Step 3: Balance NO₃ — Left NO₃=2, put coefficient 2 before KNO₃: Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃
Step 4: Verify — Left: Pb=1, N=2, O=6, K=2, I=2 | Right: Pb=1, I=2, K=2, N=2, O=6 ✓
Balanced: Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃
➕ Combination Reactions
A combination reaction is a reaction in which two or more substances (elements or compounds) combine to form a single new substance. The general form is: A + B → AB.
🔥 Examples of Combination Reactions
- Burning of coal: C + O₂ → CO₂ (Carbon + Oxygen → Carbon dioxide)
- Formation of water: 2H₂ + O₂ → 2H₂O (Hydrogen + Oxygen → Water)
- Formation of calcium oxide: 2Ca + O₂ → 2CaO (Calcium + Oxygen → Calcium oxide)
- Burning of magnesium: 2Mg + O₂ → 2MgO (Magnesium + Oxygen → Magnesium oxide — bright white flame)
🧪 Decomposition Reactions
A decomposition reaction is a reaction in which a single compound breaks down into two or more simpler substances. The general form is: AB → A + B. Decomposition reactions are the opposite of combination reactions.
🔬 Types of Decomposition Reactions
📘 Important Experiment — Thermal Decomposition of Ferrous Sulphate
When green-coloured FeSO₄·7H₂O crystals are heated, they first lose water (turn white), then decompose to give ferric oxide (red-brown), sulphur dioxide (pungent smell), and sulphur trioxide. This is a thermal decomposition reaction.
🔄 Displacement Reactions
A displacement reaction is a reaction in which a more reactive element displaces a less reactive element from its compound. The general form is: A + BX → AX + B (where A is more reactive than B).
⚔️ Examples of Displacement Reactions
- Iron + Copper Sulphate: Fe + CuSO₄ → FeSO₄ + Cu (Iron displaces copper — blue solution turns green)
- Zinc + Copper Sulphate: Zn + CuSO₄ → ZnSO₄ + Cu (Zinc displaces copper — blue colour fades)
- Lead + Copper Chloride: Pb + CuCl₂ → PbCl₂ + Cu (Lead displaces copper)
🔄 Double Displacement Reactions
A double displacement reaction is a reaction in which two compounds exchange their ions (or radicals) to form two new compounds. The general form is: AB + CD → AD + CB.
Double displacement reactions are further classified into precipitation reactions (where an insoluble solid forms) and neutralization reactions (where an acid and base react to form salt and water).
🧪 Examples of Double Displacement Reactions
- Precipitation (Yellow Precipitate): Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃ (Yellow precipitate of PbI₂ forms)
- Precipitation (White Precipitate): NaCl + AgNO₃ → AgCl + NaNO₃ (White precipitate of AgCl forms)
- Neutralization: HCl + NaOH → NaCl + H₂O (Acid + Base → Salt + Water)
⚡ Oxidation & Reduction
📖 Definitions
- Oxidation: Addition of oxygen OR removal of hydrogen from a substance
- Reduction: Addition of hydrogen OR removal of oxygen from a substance
- Redox Reaction: A reaction where oxidation and reduction occur simultaneously
- Oxidizing Agent: A substance that provides oxygen (or removes hydrogen) — gets reduced itself
- Reducing Agent: A substance that provides hydrogen (or removes oxygen) — gets oxidized itself
📊 Common Examples of Redox Reactions
🦀 Corrosion — A Real-World Example of Oxidation
Corrosion is the slow destruction of metals due to reaction with oxygen, moisture, and other substances in the environment. The most well-known example is rusting of iron:
4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O (Hydrated iron oxide — rust)
- Rusting requires both oxygen and water
- Prevention: Painting, oiling, galvanization (coating with zinc), alloying
- Silver tarnishes (turns black) due to reaction with H₂S in air: 4Ag + 2H₂S + O₂ → 2Ag₂S + 2H₂O
🍏 Rancidity — Oxidation of Food
Rancidity is the spoilage of food containing fats and oils due to oxidation, producing unpleasant smell and taste. Prevention methods include:
- Adding antioxidants (e.g., BHA, BHT) to packaged foods
- Storing food in airtight containers
- Refrigeration to slow down oxidation
- Packing in oxygen-free environment (nitrogen flushing)
📝 Practice Questions
- Q1: Balance the following chemical equation: Fe + H₂O → Fe₃O₄ + H₂. Identify the type of reaction.
- Q2: What happens when green ferrous sulphate crystals are heated? Write the balanced chemical equation and state the type of reaction.
- Q3: Distinguish between displacement and double displacement reactions with one example each.
- Q4: In the reaction ZnO + C → Zn + CO, identify the substance that is oxidized, reduced, the oxidizing agent, and the reducing agent.
- Q5: Why do we apply paint on iron articles? Explain two methods to prevent rusting.
✅ Answer Key
- A1: 3Fe + 4H₂O → Fe₃O₄ + 4H₂. This is a displacement reaction (iron displaces hydrogen from water).
- A2: Green FeSO₄ crystals first lose water (turn white), then decompose: 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃. This is a thermal decomposition reaction.
- A3: Displacement: Fe + CuSO₄ → FeSO₄ + Cu (one element + one compound). Double displacement: Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃ (two compounds exchange ions).
- A4: Oxidized: C (Carbon gains oxygen). Reduced: ZnO (Zinc oxide loses oxygen). Reducing agent: C. Oxidizing agent: ZnO.
- A5: Paint prevents iron from coming into contact with oxygen and moisture, thus preventing rusting. Methods: (a) Galvanization — coating with zinc, (b) Oiling/greasing — creating a protective layer, (c) Alloying — making stainless steel.