MP Board Class 10 Science Chapter 1: Chemical Reactions &…
Chapter 1: Chemical Reactions & Equations is a foundational chapter in MP Board Class 10 Science, typically carrying 6–10 marks in the annual board exam. This chapter covers the representation of chemical changes through equations, the laws of conservation of mass, types of chemical reactions, balancing of equations, and the concepts of oxidation and reduction. Questions range from 1-mark MCQs to 5-mark long-answer questions involving balancing equations and identifying reaction types. A strong grasp of this chapter is essential for all subsequent chemistry topics in Class 10 and 12.
📑 Table of Contents
🧪 Chemical Equations — Writing & Balancing
A chemical equation is a symbolic representation of a chemical reaction. It uses chemical formulas to show the reactants (starting substances) and products (substances formed). According to the Law of Conservation of Mass, matter can neither be created nor destroyed in a chemical reaction — therefore, a chemical equation must be balanced, meaning the number of atoms of each element must be the same on both sides.
✏️ How to Write a Chemical Equation
Step 1: Write the word equation (reactants → products).
Step 2: Replace names with chemical formulas.
Step 3: Count atoms on both sides.
Step 4: Add coefficients to balance atoms (never change subscripts).
Step 5: Add physical state symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution.
📝 Example: Hydrogen + Oxygen → Water
H₂ (g) + O₂ (g) → H₂O (l) (Unbalanced)
2H₂ (g) + O₂ (g) → 2H₂O (l) (Balanced ✓)
🔗 Combination Reactions
A combination reaction is a reaction in which two or more substances (elements or compounds) combine to form a single new substance.
General Form: A + B → AB
📌 Important Examples
💥 Decomposition Reactions
A decomposition reaction is a reaction in which a single compound breaks down into two or more simpler substances. It is essentially the opposite of a combination reaction. Decomposition reactions require energy in the form of heat, light, or electricity.
General Form: AB → A + B
🔬 Types of Decomposition Reactions
1. Thermal Decomposition (heat energy):
CaCO₃ (s) heat→ CaO (s) + CO₂ (g)
(Lime stone → Quick lime + Carbon dioxide — used in cement industry)
2. Photochemical Decomposition (light energy):
2AgCl (s) sunlight→ 2Ag (s) + Cl₂ (g)
(Silver chloride turns grey in sunlight — used in photography)
3. Electrolytic Decomposition (electrical energy):
2H₂O (l) electricity→ 2H₂ (g) + O₂ (g)
(Electrolysis of water — hydrogen and oxygen gases produced)
📊 Examples of Decomposition Reactions
🔄 Displacement Reactions
A displacement reaction is a reaction in which a more reactive element displaces a less reactive element from its compound. The reactivity series of metals determines whether a displacement reaction will occur.
General Form: A + BC → AC + B
(Element A displaces element B from compound BC)
📌 Classic Example: Iron with Copper Sulphate
Fe (s) + CuSO₄ (aq) → FeSO₄ (aq) + Cu (s)
Iron nail dipped in blue copper sulphate solution → greenish ferrous sulphate solution + brown copper coating on nail.
📊 More Displacement Reactions
🔁 Double Displacement Reactions
A double displacement reaction is a reaction in which two compounds exchange their ions (cations and anions) to form two new compounds. These reactions often produce a precipitate, a gas, or water.
General Form: AB + CD → AD + CB
📝 Classic Example — Precipitation:
Na₂SO₄ (aq) + BaCl₂ (aq) → BaSO₄ (s) + 2NaCl (aq)
Sodium sulphate (colourless) + Barium chloride (colourless) → White precipitate of BaSO₄ + Sodium chloride solution.
📝 Example — Neutralisation (Acid-Base):
HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)
Hydrochloric acid + Sodium hydroxide → Salt + Water (exothermic, pH becomes neutral)
📊 Precipitation Reactions
⚡ Oxidation & Reduction (Redox Reactions)
📖 Definitions
🧠 Mnemonic: OIL RIG
Oxidation Is Loss (of electrons)
Reduction Is Gain (of electrons)
🔬 Redox Reaction Examples
Example 1: Rusting of Iron
4Fe (s) + 3O₂ (g) + xH₂O (l) → 2Fe₂O₃·xH₂O (s) (Rust)
Fe is oxidised (Fe → Fe³⁺ + 3e⁻) & O₂ is reduced (O₂ + 4e⁻ → 2O²⁻)
Example 2: Burning of Magnesium
2Mg (s) + O₂ (g) → 2MgO (s)
Mg is oxidised (gains O) & O₂ is reduced (gains electrons from Mg)
Example 3: Copper Oxide + Hydrogen
CuO (s) + H₂ (g) → Cu (s) + H₂O (l)
CuO is reduced (loss of O → Cu) & H₂ is oxidised (gain of O → H₂O)
🔄 Oxidising & Reducing Agents
⚖️ Balancing Equations — 3 Solved Examples
🔹 Example 1: Burning of Methane
Unbalanced: CH₄ + O₂ → CO₂ + H₂O
Step 1: C: 1 left, 1 right ✓ (balance carbon first)
Step 2: H: 4 left, 2 right → put 2 before H₂O: CH₄ + O₂ → CO₂ + 2H₂O
Step 3: O: 2 left, 2+2=4 right → put 2 before O₂: CH₄ + 2O₂ → CO₂ + 2H₂O
Balanced ✓: CH₄ + 2O₂ → CO₂ + 2H₂O
🔹 Example 2: Burning of Ethanol
Unbalanced: C₂H₅OH + O₂ → CO₂ + H₂O
Step 1: C: 2 left → 2CO₂: C₂H₅OH + O₂ → 2CO₂ + H₂O
Step 2: H: 6 left → 3H₂O: C₂H₅OH + O₂ → 2CO₂ + 3H₂O
Step 3: O: 1+2=3 left, 4+3=7 right → need 3O₂: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
(O left: 1 from C₂H₅OH + 6 from O₂ = 7; O right: 4 + 3 = 7 ✓)
Balanced ✓: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
🔹 Example 3: Reaction of Sodium with Water
Unbalanced: Na + H₂O → NaOH + H₂
Step 1: Balance H: 2 left, 1+2=3 right → put 2 before H₂O and 2 before NaOH:
Na + 2H₂O → 2NaOH + H₂
Step 2: H: 4 left (from 2H₂O), 2+2=4 right ✓
Step 3: O: 2 left, 2 right ✓
Step 4: Na: 1 left, 2 right → put 2 before Na
Balanced ✓: 2Na + 2H₂O → 2NaOH + H₂
📝 Practice Questions
Q1. Balance the following chemical equations:
(a) Fe + H₂O → Fe₃O₄ + H₂
(b) Zn + HCl → ZnCl₂ + H₂
(c) Al + CuSO₄ → Al₂(SO₄)₃ + Cu
✅ Answer:
(a) 3Fe + 4H₂O → Fe₃O₄ + 4H₂
(b) Zn + 2HCl → ZnCl₂ + H₂
(c) 2Al + 3CuSO₄ → Al₂(SO₄)₃ + 3Cu
Q2. Identify the type of reaction in each case:
(a) 2AgBr → 2Ag + Br₂
(b) BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl
(c) 2Mg + O₂ → 2MgO
✅ Answer:
(a) Decomposition reaction (photochemical decomposition — requires sunlight)
(b) Double displacement reaction (precipitation of BaSO₄)
(c) Combination reaction (magnesium combines with oxygen)
Q3. What happens when zinc granules are added to dilute sulphuric acid? Write the balanced chemical equation.
✅ Answer: Zinc reacts with dilute sulphuric acid to produce zinc sulphate and hydrogen gas. Bubbles of hydrogen gas are observed.
Zn (s) + H₂SO₄ (aq) → ZnSO₄ (aq) + H₂ (g)
This is a displacement reaction (Zn displaces H from H₂SO₄).
Q4. In the reaction: CuO + H₂ → Cu + H₂O, identify the substance being oxidised and the substance being reduced.
✅ Answer:
Oxidised: H₂ (hydrogen gains oxygen to form H₂O)
Reduced: CuO (copper oxide loses oxygen to form Cu)
Oxidising agent: CuO (provides oxygen to H₂)
Reducing agent: H₂ (removes oxygen from CuO)
Q5. Why is respiration considered an exothermic reaction? Explain.
✅ Answer: Respiration is the oxidation of glucose (C₆H₁₂O₆) in the cells to produce carbon dioxide, water, and energy. Since energy is released in the form of heat, it is called an exothermic reaction.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
📋 Previous Year Questions (2017–2026)
PYQ 1 — MP Board 2023 (Set A)
Question: Balance the following equation and identify the type of reaction:
Fe₂O₃ + Al → Al₂O₃ + Fe
✅ Answer:
Fe₂O₃ + 2Al → Al₂O₃ + 2Fe
This is a displacement reaction (Al is more reactive than Fe, so it displaces Fe from Fe₂O₃). This reaction is also used in thermite welding and is highly exothermic.
PYQ 2 — MP Board 2025 (Set B)
Question: Write the balanced chemical equation for the reaction between lead nitrate and potassium iodide. What type of reaction is this? Name the coloured product formed.
✅ Answer:
Pb(NO₃)₂ (aq) + 2KI (aq) → PbI₂ (s) + 2KNO₃ (aq)
This is a double displacement reaction (specifically a precipitation reaction).
The yellow coloured product is lead iodide (PbI₂).
PYQ 3 — MP Board 2021 (Set C)
Question: Define oxidation and reduction. In the reaction below, identify the oxidising agent and reducing agent:
2Mg + O₂ → 2MgO
✅ Answer:
Oxidation: Addition of oxygen or removal of hydrogen (or loss of electrons).
Reduction: Addition of hydrogen or removal of oxygen (or gain of electrons).
In 2Mg + O₂ → 2MgO:
Oxidising agent: O₂ (oxygen gets reduced by gaining electrons from Mg)
Reducing agent: Mg (magnesium gets oxidised by gaining oxygen)
🎯 Key Points to Remember
- A chemical equation must always be balanced (Law of Conservation of Mass).
- Never change the subscript of a formula — only add coefficients.
- Combination = A + B → AB; Decomposition = AB → A + B
- Displacement = A + BC → AC + B; Double Displacement = AB + CD → AD + CB
- OIL RIG: Oxidation Is Loss (of e⁻), Reduction Is Gain (of e⁻)
- Oxidising agent itself gets reduced; Reducing agent itself gets oxidised.
- Practice balancing at least 10 equations to master the skill.