NCERT Class 10 Science
Chapter 1 – Chemical Reactions and Equations
NCERT Solutions (Quick and Exam-oriented)
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1. Very short answers (1 mark)
Q1. What is a chemical reaction?
Ans. A process in which one or more substances change to form new substances.
Q2. Why should chemical equations be balanced?
Ans. To satisfy the law of conservation of mass.
Q3. Name the reactants in
Zn + H₂SO₄ → ZnSO₄ + H₂
Ans. Zn and H₂SO₄.
Q4. Which gas is evolved when zinc reacts with dilute acid?
Ans. Hydrogen gas.
Q5. What is corrosion?
Ans. Slow destruction of a metal by chemical reaction with air and moisture.
Q6. Give one example of oxidation.
Ans. 2Mg + O₂ → 2MgO.
Q7. Give one example of reduction.
Ans. CuO + H₂ → Cu + H₂O.
Q8. What is a skeletal equation?
Ans. An unbalanced chemical equation.
Q9. What is rancidity?
Ans. Oxidation of oils and fats causing bad smell and taste.
Q10. Which substance is reduced in
CuO + H₂ → Cu + H₂O ?
Ans. Copper(II) oxide (CuO).
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2. Short answer questions (2–3 marks)
Q1. Write a balanced chemical equation for the reaction between iron and water (steam).
Ans.
3Fe + 4H₂O (steam) → Fe₃O₄ + 4H₂
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Q2. Why is respiration considered an exothermic reaction?
Ans.
Because energy is released when glucose is broken down in the presence of oxygen.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
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Q3. Write the word equation for the reaction of calcium oxide with water.
Ans.
Calcium oxide + Water → Calcium hydroxide
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Q4. Identify the oxidised and reduced substances in the reaction:
Zn + CuSO₄ → ZnSO₄ + Cu
Ans.
Zinc is oxidised.
Copper(II) ions are reduced.
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Q5. Why are decomposition reactions called the opposite of combination reactions?
Ans.
In combination reactions, substances combine to form one product.
In decomposition reactions, one compound breaks into two or more substances.
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Q6. Write one method to prevent rusting.
Ans.
Painting, oiling or galvanising the iron surface.
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Q7. Balance the following equation:
Al + HCl → AlCl₃ + H₂
Ans.
2Al + 6HCl → 2AlCl₃ + 3H₂
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Q8. What is meant by a precipitation reaction?
Ans.
A reaction in which an insoluble solid (precipitate) is formed.
Example:
Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl
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3. Long answer questions (5 marks)
Q1. What is a balanced chemical equation?
Explain the steps to balance a chemical equation.
Ans.
A balanced chemical equation has equal number of atoms of each element on both sides.
Steps:
1. Write the skeletal (unbalanced) equation.
2. List the atoms on both sides.
3. Balance one element at a time.
4. Adjust coefficients, not subscripts.
5. Check whether all atoms are equal.
Example:
Fe + H₂O → Fe₃O₄ + H₂
Balanced form:
3Fe + 4H₂O → Fe₃O₄ + 4H₂
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Q2. Explain oxidation and reduction with examples.
Ans.
Oxidation is the addition of oxygen or removal of hydrogen.
Reduction is the removal of oxygen or addition of hydrogen.
Example:
CuO + H₂ → Cu + H₂O
CuO loses oxygen → reduction
H₂ gains oxygen → oxidation
Thus, oxidation and reduction take place together.
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Q3. What is corrosion?
Explain rusting of iron.
Ans.
Corrosion is the gradual damage of metals due to chemical reactions with air, water or chemicals.
Rusting of iron takes place when iron reacts with oxygen and moisture to form hydrated iron oxide.
Conditions required:
Oxygen
Water (moisture)
Rust weakens iron and causes economic loss.
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Q4. What is rancidity?
How can it be prevented?
Ans.
Rancidity is the oxidation of fats and oils which results in unpleasant smell and taste.
Prevention methods:
Keeping food in air-tight containers
Refrigeration
Adding antioxidants
Packing in nitrogen gas
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4. Types of chemical reactions (as in NCERT)
(a) Combination reaction
Two or more substances combine to form a single product.
Example:
2Mg + O₂ → 2MgO
(b) Decomposition reaction
One compound breaks into simpler substances.
Example:
CaCO₃ → CaO + CO₂
(c) Displacement reaction
A more reactive element displaces a less reactive element.
Example:
Zn + CuSO₄ → ZnSO₄ + Cu
(d) Double displacement reaction
Exchange of ions between two compounds.
Example:
Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
(e) Oxidation and reduction
They always occur together.
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If you want, I can next prepare 20 exam-ready MCQs from this chapter for your students.