Class X · Science · Term Assessment
Chemical Reactions & Equations
Chapter 1 — NCERT Science Grade 10
General Instructions
- All questions are compulsory. This paper consists of four sections.
- Section A (Q1–10): Multiple Choice Questions — 1 mark each (10 marks)
- Section B (Q11–18): Short Answer I — 2 marks each (16 marks)
- Section C (Q19–26): Short Answer II — 3 marks each (24 marks)
- Section D (Q27–32): Long Answer — 5 marks each (30 marks)
- Draw neat, labelled diagrams wherever required.
Section A
Multiple Choice Questions · 1 mark each · 10 × 1 = 10 marks
Q1.
Which of the following is NOT a sign that a chemical reaction has taken place?
1 mark
Q2.
In the reaction Fe₂O₃ + 2Al → Al₂O₃ + 2Fe, the type of reaction is:
1 mark
Q3.
In the reaction 2PbO(s) + C(s) → 2Pb(s) + CO₂(g), which statement is INCORRECT?
1 mark
Q4.
What happens when dilute hydrochloric acid is added to iron fillings?
1 mark
Q5.
Decomposition of silver chloride in sunlight produces:
1 mark
Q6.
Respiration is an example of:
1 mark
Q7.
In a displacement reaction, the element that displaces another must be:
1 mark
Q8.
The chemical formula of slaked lime is:
1 mark
Q9.
Which notation represents an aqueous solution in a chemical equation?
1 mark
Q10.
The green coating on copper articles is due to the formation of:
1 mark
Section B
Short Answer I · 2 marks each · 8 × 2 = 16 marks
Q11.
Why should a magnesium ribbon be cleaned with sandpaper before burning in air?
2 marks
Q12.
Define a combination reaction. Give one example with a balanced chemical equation.
2 marks
Q13.
Differentiate between exothermic and endothermic reactions with one example each.
2 marks
Q14.
What is a precipitation reaction? Write the balanced equation for the reaction between sodium sulphate and barium chloride solutions.
2 marks
Q15.
Define oxidation and reduction in terms of gain or loss of oxygen. Give one example of a redox reaction.
2 marks
Q16.
Why do chips manufacturers flush bags with nitrogen gas before sealing?
2 marks
Q17.
Balance the following equation: Fe + H₂O → Fe₃O₄ + H₂
2 marks
Q18.
What is corrosion? Name the compounds responsible for the black coating on silver and the green coating on copper.
2 marks
Section C
Short Answer II · 3 marks each · 8 × 3 = 24 marks
Q19.
Translate the following statements into balanced chemical equations:
3 marks
- Hydrogen gas combines with nitrogen to form ammonia.
- Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
- Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Q20.
Balance the following chemical equations:
3 marks
- HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
- NaOH + H₂SO₄ → Na₂SO₄ + H₂O
- BaCl₂ + H₂SO₄ → BaSO₄ + HCl
Q21.
Write balanced equations for the following and identify the type of reaction in each case:
3 marks
- Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
- Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
- Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
Q22.
In Activity 1.9, iron nails are dipped in copper sulphate solution. (a) What change is observed in the colour of the solution? (b) What is deposited on the iron nails? (c) Write the balanced chemical equation for this reaction and name the type of reaction.
3 marks
Q23.
Identify the substances oxidised and reduced in the following reactions:
3 marks
- 4Na(s) + O₂(g) → 2Na₂O(s)
- CuO(s) + H₂(g) → Cu(s) + H₂O(l)
- ZnO + C → Zn + CO
Q24.
Why are decomposition reactions considered the opposite of combination reactions? Write one equation each for a combination and a decomposition reaction.
3 marks
Q25.
A solution of substance X is used for whitewashing. (a) Name X and write its formula. (b) Write the reaction of X with water. (c) Write the reaction that causes whitewash to dry and harden on walls, naming the product formed.
3 marks
Q26.
What is rancidity? State two methods used to prevent rancidity in food. Why is keeping food in airtight containers effective?
3 marks
Section D
Long Answer · 5 marks each · 6 × 5 = 30 marks
Q27.
Explain in detail the steps involved in balancing the chemical equation Fe + H₂O → Fe₃O₄ + H₂ using the hit-and-trial method. Also write the final balanced equation with physical state symbols, given that water is used as steam.
5 marks
Q28.
Describe the electrolysis of water experiment with a neat labelled diagram. Write the balanced equation for the reaction. Why is the volume of hydrogen gas collected double that of oxygen? Name the type of reaction.
5 marks
Q29.
(a) Define a decomposition reaction. (b) Give one example each of thermal decomposition, photodecomposition, and electrolytic decomposition with balanced equations. (c) Which type of energy drives each example? (d) Are these reactions exothermic or endothermic?
5 marks
Q30.
Write balanced equations for the following reactions and identify the type of each reaction (displacement, double displacement, combination, or decomposition):
5 marks
- Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
- Zinc + Silver nitrate → Zinc nitrate + Silver
- Aluminium + Copper chloride → Aluminium chloride + Copper
- Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
- Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen
Q31.
(a) What is corrosion? Explain how iron rusts with a balanced equation. (b) State four methods to prevent corrosion. (c) Why is corrosion considered a harmful oxidation reaction? Give two economic consequences of corrosion.
5 marks
Q32.
(a) Explain oxidation-reduction reactions (redox reactions) using the example: CuO + H₂ → Cu + H₂O. Identify which substance is oxidised and which is reduced. (b) Using the same concept, analyse the reaction MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ — identify the oxidised and reduced substances. (c) State the modern (broader) definition of oxidation and reduction in terms of both oxygen and hydrogen.
5 marks
Answer Key
For Examiner's Use Only
Section A — Multiple Choice (1 mark each)
Q1 — Q10 Answers
Q1. (b) Change in shape — Physical change, not a sign of chemical reaction.
Q2. (d) Displacement reaction — Al displaces Fe from Fe₂O₃.
Q3. (b) CO₂ is getting oxidised — CO₂ is a product; C is oxidised, PbO is reduced.
Q4. (b) Hydrogen gas and iron chloride are produced — Fe + 2HCl → FeCl₂ + H₂↑
Q5. (b) Silver + Chlorine gas — 2AgCl → 2Ag + Cl₂ (sunlight)
Q6. (c) Exothermic reaction — Glucose oxidation releases energy.
Q7. (b) More reactive — A more reactive element displaces a less reactive one.
Q8. (c) Ca(OH)₂ — Calcium hydroxide = slaked lime. CaO = quick lime.
Q9. (d) (aq) — (aq) = aqueous (dissolved in water).
Q10. (c) Basic copper carbonate — Cu₂(OH)₂CO₃ forms the green patina.
Q2. (d) Displacement reaction — Al displaces Fe from Fe₂O₃.
Q3. (b) CO₂ is getting oxidised — CO₂ is a product; C is oxidised, PbO is reduced.
Q4. (b) Hydrogen gas and iron chloride are produced — Fe + 2HCl → FeCl₂ + H₂↑
Q5. (b) Silver + Chlorine gas — 2AgCl → 2Ag + Cl₂ (sunlight)
Q6. (c) Exothermic reaction — Glucose oxidation releases energy.
Q7. (b) More reactive — A more reactive element displaces a less reactive one.
Q8. (c) Ca(OH)₂ — Calcium hydroxide = slaked lime. CaO = quick lime.
Q9. (d) (aq) — (aq) = aqueous (dissolved in water).
Q10. (c) Basic copper carbonate — Cu₂(OH)₂CO₃ forms the green patina.
Section B — Short Answer I (2 marks each)
Q11 — Cleaning magnesium ribbon 2 marks
Magnesium ribbon has a layer of magnesium oxide (MgO) on its surface, formed due to slow oxidation in air. (1 mark) This oxide layer prevents the ribbon from burning properly. Cleaning with sandpaper removes this layer, exposing fresh magnesium that burns brightly. (1 mark)
Q12 — Combination Reaction 2 marks
A reaction in which two or more substances combine to form a single product is called a combination reaction. (1 mark)
Example: CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat (Quick lime reacts with water to form slaked lime.) (1 mark)
Example: CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat (Quick lime reacts with water to form slaked lime.) (1 mark)
Q13 — Exothermic vs Endothermic 2 marks
Exothermic: Reactions in which heat is released along with products. E.g., burning of natural gas: CH₄ + 2O₂ → CO₂ + 2H₂O + Heat. (1 mark)
Endothermic: Reactions in which energy is absorbed. E.g., decomposition of calcium carbonate: CaCO₃ → CaO + CO₂ (requires heat). (1 mark)
Endothermic: Reactions in which energy is absorbed. E.g., decomposition of calcium carbonate: CaCO₃ → CaO + CO₂ (requires heat). (1 mark)
Q14 — Precipitation Reaction 2 marks
A reaction that produces an insoluble solid (precipitate) when two solutions are mixed is called a precipitation reaction. (1 mark)
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq)
(White precipitate of BaSO₄ is formed.) (1 mark)
Q15 — Oxidation & Reduction 2 marks
Oxidation: Gain of oxygen (or loss of hydrogen) by a substance. Reduction: Loss of oxygen (or gain of hydrogen) by a substance. (1 mark)
Example of redox reaction: CuO + H₂ → Cu + H₂O CuO is reduced (loses O); H₂ is oxidised (gains O). (1 mark)
Example of redox reaction: CuO + H₂ → Cu + H₂O CuO is reduced (loses O); H₂ is oxidised (gains O). (1 mark)
Q16 — Nitrogen flushing 2 marks
Chips contain fats and oils which get oxidised by oxygen in air, causing rancidity — a change in smell and taste that makes them unfit for consumption. (1 mark) Nitrogen is an inert gas and does not react with fats/oils. Flushing bags with nitrogen displaces oxygen, thereby preventing oxidation and keeping chips fresh longer. (1 mark)
Q17 — Balance Fe + H₂O → Fe₃O₄ + H₂ 2 marks
Step: Balance O first (4 on RHS → 4H₂O), then H (8H → 4H₂), then Fe (3 on RHS → 3Fe). (1 mark)
3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)
Verification: Fe: 3=3 ✓, H: 8=8 ✓, O: 4=4 ✓ (1 mark)
Q18 — Corrosion 2 marks
Corrosion is the process by which metals are slowly attacked and deteriorated by substances in the environment (moisture, acids, gases). (1 mark) Black coating on silver: Silver sulphide (Ag₂S). Green coating on copper: Basic copper carbonate [Cu₂(OH)₂CO₃ / CuCO₃·Cu(OH)₂]. (1 mark)
Section C — Short Answer II (3 marks each)
Q19 — Balanced equations from statements 3 marks
(a) 3H₂(g) + N₂(g) → 2NH₃(g)
(b) 2H₂S(g) + 3O₂(g) → 2H₂O(l) + 2SO₂(g)
(c) 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)
1 mark each.
Q20 — Balance given equations 3 marks
(a) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
(b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(c) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl
1 mark each.
Q21 — Equations + type of reaction 3 marks
(a) ZnCO₃(s) → ZnO(s) + CO₂(g) — Thermal Decomposition
(b) H₂(g) + Cl₂(g) → 2HCl(g) — Combination Reaction
(c) 2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(s) — Double Displacement / Precipitation
1 mark each (½ equation + ½ type).
(b) H₂(g) + Cl₂(g) → 2HCl(g) — Combination Reaction
(c) 2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(s) — Double Displacement / Precipitation
1 mark each (½ equation + ½ type).
Q22 — Iron in copper sulphate 3 marks
(a) The blue colour of copper sulphate solution fades (becomes pale green/colourless). (1 mark)
(b) A brownish-red deposit of copper is formed on the iron nails. (1 mark)
(c) Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) Type: Displacement Reaction (iron displaces copper). (1 mark)
(b) A brownish-red deposit of copper is formed on the iron nails. (1 mark)
(c) Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) Type: Displacement Reaction (iron displaces copper). (1 mark)
Q23 — Identify oxidised/reduced substances 3 marks
(a) 4Na + O₂ → 2Na₂O: Na is oxidised (gains O); O₂ acts as oxidising agent. (1 mark)
(b) CuO + H₂ → Cu + H₂O: CuO is reduced (loses O); H₂ is oxidised (gains O). (1 mark)
(c) ZnO + C → Zn + CO: ZnO is reduced (loses O to form Zn); C is oxidised (gains O to form CO). (1 mark)
(b) CuO + H₂ → Cu + H₂O: CuO is reduced (loses O); H₂ is oxidised (gains O). (1 mark)
(c) ZnO + C → Zn + CO: ZnO is reduced (loses O to form Zn); C is oxidised (gains O to form CO). (1 mark)
Q24 — Decomposition vs Combination 3 marks
In a combination reaction, two or more reactants combine to give one product. In a decomposition reaction, one reactant breaks down into two or more products — the exact reverse process. (1 mark)
Combination example: 2H₂(g) + O₂(g) → 2H₂O(l) (1 mark)
Decomposition example: 2H₂O(l) → 2H₂(g) + O₂(g) (electrolysis) (1 mark)
Combination example: 2H₂(g) + O₂(g) → 2H₂O(l) (1 mark)
Decomposition example: 2H₂O(l) → 2H₂(g) + O₂(g) (electrolysis) (1 mark)
Q25 — Whitewashing substance X 3 marks
(a) Substance X is calcium oxide (quick lime), formula: CaO. (1 mark)
(b) Reaction with water: CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat (1 mark)
(c) Ca(OH)₂ reacts with atmospheric CO₂ to form calcium carbonate (CaCO₃), which hardens on the wall and gives a shiny finish. The chemical formula of this compound is the same as marble (CaCO₃). Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l) (1 mark)
(b) Reaction with water: CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat (1 mark)
(c) Ca(OH)₂ reacts with atmospheric CO₂ to form calcium carbonate (CaCO₃), which hardens on the wall and gives a shiny finish. The chemical formula of this compound is the same as marble (CaCO₃). Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l) (1 mark)
Q26 — Rancidity 3 marks
Rancidity is the process by which fats and oils in food are oxidised, causing a change in smell and taste, making the food unfit for consumption. (1 mark)
Two methods to prevent rancidity: (i) Adding antioxidants (e.g., BHA, BHT, vitamin E) to food. (ii) Flushing packaging with inert gases like nitrogen. (Also: refrigeration, airtight containers.) (1 mark)
Airtight containers are effective because they limit exposure to atmospheric oxygen, which is the agent that causes oxidation of fats and oils. Without oxygen, the oxidation reaction cannot proceed. (1 mark)
Two methods to prevent rancidity: (i) Adding antioxidants (e.g., BHA, BHT, vitamin E) to food. (ii) Flushing packaging with inert gases like nitrogen. (Also: refrigeration, airtight containers.) (1 mark)
Airtight containers are effective because they limit exposure to atmospheric oxygen, which is the agent that causes oxidation of fats and oils. Without oxygen, the oxidation reaction cannot proceed. (1 mark)
Section D — Long Answer (5 marks each)
Q27 — Balancing Fe + H₂O (detailed) 5 marks
Step I: Skeletal equation: Fe + H₂O → Fe₃O₄ + H₂ (½)
Step II: Count atoms — Fe: 1 vs 3; H: 2 vs 2; O: 1 vs 4. None are balanced. (½)
Step III: Balance O first (largest number). Put coefficient 4 before H₂O: Fe + 4H₂O → Fe₃O₄ + H₂ (1 mark)
Step IV: Balance H: 8H on LHS, need 4H₂ on RHS: Fe + 4H₂O → Fe₃O₄ + 4H₂ (1 mark)
Step V: Balance Fe: 3 Fe on RHS, so put 3 before Fe: 3Fe + 4H₂O → Fe₃O₄ + 4H₂ (1 mark)
Step VI (Verification): Fe: 3=3 ✓, H: 8=8 ✓, O: 4=4 ✓ (½)
Step VII (Physical states, water as steam): 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g) (½)
Step II: Count atoms — Fe: 1 vs 3; H: 2 vs 2; O: 1 vs 4. None are balanced. (½)
Step III: Balance O first (largest number). Put coefficient 4 before H₂O: Fe + 4H₂O → Fe₃O₄ + H₂ (1 mark)
Step IV: Balance H: 8H on LHS, need 4H₂ on RHS: Fe + 4H₂O → Fe₃O₄ + 4H₂ (1 mark)
Step V: Balance Fe: 3 Fe on RHS, so put 3 before Fe: 3Fe + 4H₂O → Fe₃O₄ + 4H₂ (1 mark)
Step VI (Verification): Fe: 3=3 ✓, H: 8=8 ✓, O: 4=4 ✓ (½)
Step VII (Physical states, water as steam): 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g) (½)
Q28 — Electrolysis of water 5 marks
Diagram: Neat labelled diagram showing plastic mug, two carbon electrodes (anode and cathode), inverted test tubes, 6V battery, switch, dilute H₂SO₄ solution, H₂ gas at cathode (double volume), O₂ gas at anode. (2 marks)
Balanced equation: 2H₂O(l) → Electricity → 2H₂(g) + O₂(g) (1 mark)
Reason for double volume of H₂: Water has the formula H₂O — 2 hydrogen atoms for every 1 oxygen atom. Therefore when water is decomposed, 2 moles of H₂ gas are produced for every 1 mole of O₂, i.e., hydrogen gas is collected in twice the volume of oxygen. (1 mark)
Type of reaction: Electrolytic decomposition (endothermic — electrical energy is absorbed). (1 mark)
Balanced equation: 2H₂O(l) → Electricity → 2H₂(g) + O₂(g) (1 mark)
Reason for double volume of H₂: Water has the formula H₂O — 2 hydrogen atoms for every 1 oxygen atom. Therefore when water is decomposed, 2 moles of H₂ gas are produced for every 1 mole of O₂, i.e., hydrogen gas is collected in twice the volume of oxygen. (1 mark)
Type of reaction: Electrolytic decomposition (endothermic — electrical energy is absorbed). (1 mark)
Q29 — Decomposition reactions 5 marks
(a) Definition: A reaction in which a single compound breaks down into two or more simpler substances is called a decomposition reaction. (1 mark)
(b) Examples:
Thermal decomposition (heat energy): CaCO₃(s) →Heat→ CaO(s) + CO₂(g) (1 mark)
Photodecomposition (light energy): 2AgCl(s) →Sunlight→ 2Ag(s) + Cl₂(g) (1 mark)
Electrolytic decomposition (electrical energy): 2H₂O(l) →Electricity→ 2H₂(g) + O₂(g) (1 mark)
(c) Energy source: Heat / Light / Electricity respectively.
(d) All decomposition reactions are endothermic — they require energy input to break the bonds of the reactant. (1 mark)
(b) Examples:
Thermal decomposition (heat energy): CaCO₃(s) →Heat→ CaO(s) + CO₂(g) (1 mark)
Photodecomposition (light energy): 2AgCl(s) →Sunlight→ 2Ag(s) + Cl₂(g) (1 mark)
Electrolytic decomposition (electrical energy): 2H₂O(l) →Electricity→ 2H₂(g) + O₂(g) (1 mark)
(c) Energy source: Heat / Light / Electricity respectively.
(d) All decomposition reactions are endothermic — they require energy input to break the bonds of the reactant. (1 mark)
Q30 — Five balanced equations + types 5 marks
(a) Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l) — Combination reaction (also precipitation) (1 mark)
(b) Zn(s) + 2AgNO₃(aq) → Zn(NO₃)₂(aq) + 2Ag(s) — Displacement reaction (1 mark)
(c) 2Al(s) + 3CuCl₂(aq) → 2AlCl₃(aq) + 3Cu(s) — Displacement reaction (1 mark)
(d) BaCl₂(aq) + K₂SO₄(aq) → BaSO₄(s) + 2KCl(aq) — Double displacement (precipitation) (1 mark)
(e) Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) — Displacement reaction (1 mark)
(b) Zn(s) + 2AgNO₃(aq) → Zn(NO₃)₂(aq) + 2Ag(s) — Displacement reaction (1 mark)
(c) 2Al(s) + 3CuCl₂(aq) → 2AlCl₃(aq) + 3Cu(s) — Displacement reaction (1 mark)
(d) BaCl₂(aq) + K₂SO₄(aq) → BaSO₄(s) + 2KCl(aq) — Double displacement (precipitation) (1 mark)
(e) Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) — Displacement reaction (1 mark)
Q31 — Corrosion (detailed) 5 marks
(a) Definition: Corrosion is the slow deterioration of metals by chemical attack due to moisture, oxygen, acids, or other substances in the environment. Rusting of iron:
4Fe(s) + 3O₂(g) + xH₂O → 2Fe₂O₃·xH₂O (rust)
Iron reacts with oxygen and water to form hydrated iron(III) oxide (rust). (2 marks)
(b) Four methods to prevent corrosion: (i) Painting/coating; (ii) Galvanisation (zinc coating); (iii) Alloying (e.g., stainless steel); (iv) Electroplating. (2 marks)
(c) Economic consequences: (i) Enormous annual expenditure to replace corroded iron structures (bridges, ships, railings, machinery). (ii) Structural failure of buildings, vehicles, and infrastructure, leading to safety hazards and costly repairs. (1 mark)
(b) Four methods to prevent corrosion: (i) Painting/coating; (ii) Galvanisation (zinc coating); (iii) Alloying (e.g., stainless steel); (iv) Electroplating. (2 marks)
(c) Economic consequences: (i) Enormous annual expenditure to replace corroded iron structures (bridges, ships, railings, machinery). (ii) Structural failure of buildings, vehicles, and infrastructure, leading to safety hazards and costly repairs. (1 mark)
Q32 — Redox reactions (detailed) 5 marks
(a) CuO + H₂ → Cu + H₂O analysis:
CuO + H₂ →Heat→ Cu + H₂O
CuO is reduced — it loses oxygen (Cu²⁺ gains electrons, becomes Cu). H₂ is oxidised — it gains oxygen to become H₂O. This simultaneous oxidation-reduction is a redox reaction. (2 marks)
(b) MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ analysis: MnO₂ is reduced (loses O, Mn oxidation state decreases from +4 to +2). HCl is oxidised — Cl⁻ loses electrons to form Cl₂ (oxidation state increases from -1 to 0). (2 marks)
(c) Broad definitions:
Oxidation: Gain of oxygen OR loss of hydrogen by a substance.
Reduction: Loss of oxygen OR gain of hydrogen by a substance. (1 mark)
(b) MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ analysis: MnO₂ is reduced (loses O, Mn oxidation state decreases from +4 to +2). HCl is oxidised — Cl⁻ loses electrons to form Cl₂ (oxidation state increases from -1 to 0). (2 marks)
(c) Broad definitions:
Oxidation: Gain of oxygen OR loss of hydrogen by a substance.
Reduction: Loss of oxygen OR gain of hydrogen by a substance. (1 mark)
✦ End of Question Paper and Answer Key ✦
Total Marks: 80 | Chapter 1 — Chemical Reactions and Equations
Total Marks: 80 | Chapter 1 — Chemical Reactions and Equations
NCERT · Grade X · Science
