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Chapter 1: Chemical Reactions and Equations

Complete NCERT Solutions (Intext + Exercise) | CBSE Class 10 Science

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NCERT Class 10 Science | Ch 1: Chemical Reactions and Equations – Complete Solutions
Intext Questions (Page 6)
11 mark
Why should a magnesium ribbon be cleaned before burning in air?
Answer: Magnesium ribbon is cleaned with sandpaper to remove the layer of magnesium oxide (MgO) formed on its surface due to reaction with air. This ensures proper burning and a bright flame.
22 marks
Write the balanced equation for the following chemical reactions:
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
Answer:
(i) H₂ + Cl₂ → 2HCl
(ii) 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃
(iii) 2Na + 2H₂O → 2NaOH + H₂
32 marks
Write the balanced chemical equations for the following reactions:
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
Answer:
(a) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
(b) Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag
(c) 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu
(d) BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl
Intext Questions (Page 10)
42 marks
A solution of a substance 'X' is used for whitewashing. (i) Name the substance 'X' and write its formula. (ii) Write the reaction of 'X' with water.
Answer: (i) Substance X is quicklime (calcium oxide), formula CaO. (ii) CaO + H₂O → Ca(OH)₂ (slaked lime) + heat.
Intext Questions (Page 13)
52 marks
Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.
Answer: Water (H₂O) on electrolysis splits into hydrogen and oxygen. The volume of hydrogen collected is twice that of oxygen (2:1 ratio). The gas collected in larger amount is hydrogen (H₂).
NCERT Exercise Questions (Page 14)
Ex 11 mark
Which of the following statements about the reaction below are incorrect?
2PbO(s) + C(s) → 2Pb(s) + CO₂(g)
(a) Lead is getting reduced. (b) Carbon dioxide is getting oxidised. (c) Carbon is getting oxidised. (d) Lead oxide is getting reduced.
(i) (a) and (b) (ii) (a) and (c) (iii) (a), (b) and (c) (iv) all
Answer: (i) (a) and (b) are incorrect. Correct: (c) and (d) are correct.
Ex 21 mark
Fe₂O₃ + 2Al → Al₂O₃ + 2Fe. The above reaction is an example of:
(a) Combination reaction (b) Double displacement reaction (c) Decomposition reaction (d) Displacement reaction
Answer: (d) Displacement reaction (also a redox reaction).
Ex 31 mark
What happens when dilute hydrochloric acid is added to iron filings? Tick the correct answer.
(a) Hydrogen gas and iron chloride are produced.
(b) Chlorine gas and iron hydroxide are produced.
(c) No reaction takes place.
(d) Iron salt and water are produced.
Answer: (a) Hydrogen gas and iron chloride are produced. Fe + 2HCl → FeCl₂ + H₂.
Ex 42 marks
What is a balanced chemical equation? Why should chemical equations be balanced?
Answer: A balanced chemical equation has equal number of atoms of each element on both sides (reactants and products), obeying the law of conservation of mass. It must be balanced to show that mass is neither created nor destroyed during a reaction.
Ex 52 marks
Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Answer:
(a) 3H₂ + N₂ → 2NH₃
(b) 2H₂S + 3O₂ → 2H₂O + 2SO₂
(c) 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄↓
(d) 2K + 2H₂O → 2KOH + H₂
Ex 63 marks
Balance the following chemical equations.
(a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
(b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃
(d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl
Answer:
(a) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
(b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃ (already balanced)
(d) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl
Ex 72 marks
Write the balanced chemical equations for the following reactions.
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(b) Aluminium + Copper chloride → Aluminium chloride + Copper
(c) Calcium + Water → Calcium hydroxide + Hydrogen
(d) Sodium + Chlorine → Sodium chloride
Answer:
(a) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
(b) 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu
(c) Ca + 2H₂O → Ca(OH)₂ + H₂
(d) 2Na + Cl₂ → 2NaCl
Ex 83 marks
Write the balanced chemical equation for the following and identify the type of reaction in each case.
(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
Answer:
(a) 2KBr + BaI₂ → 2KI + BaBr₂ (Double displacement)
(b) ZnCO₃ → ZnO + CO₂ (Decomposition)
(c) H₂ + Cl₂ → 2HCl (Combination)
(d) Mg + 2HCl → MgCl₂ + H₂ (Displacement)
Ex 92 marks
What does one mean by exothermic and endothermic reactions? Give examples.
Answer: Exothermic reactions release heat energy (e.g., combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O + heat). Endothermic reactions absorb heat energy (e.g., decomposition of calcium carbonate: CaCO₃ + heat → CaO + CO₂).
Ex 102 marks
Why is respiration considered an exothermic reaction? Explain.
Answer: Respiration involves the breakdown of glucose (C₆H₁₂O₆) into CO₂ and H₂O in the presence of oxygen, releasing energy. Since energy is released, it is an exothermic reaction: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.
Ex 112 marks
Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer: Combination reactions involve two or more substances combining to form a single product (A + B → AB). Decomposition reactions involve a single compound breaking down into two or more simpler substances (AB → A + B). Example: Combination: 2Mg + O₂ → 2MgO; Decomposition: 2H₂O → 2H₂ + O₂.
Ex 123 marks
Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Answer:
Heat: CaCO₃ → CaO + CO₂ (heating limestone)
Light: 2AgCl → 2Ag + Cl₂ (photolytic decomposition)
Electricity: 2H₂O → 2H₂ + O₂ (electrolysis of water)
Ex 132 marks
What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Answer: In displacement, one element displaces another from its compound (A + BC → AC + B). In double displacement, two compounds exchange ions (AB + CD → AD + CB).
Displacement: Zn + CuSO₄ → ZnSO₄ + Cu
Double displacement: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
Ex 143 marks
In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write the reaction involved.
Answer: Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s). Copper displaces silver from its nitrate solution.
Ex 152 marks
What do you mean by a precipitation reaction? Explain by giving an example.
Answer: A reaction in which an insoluble solid (precipitate) is formed is called a precipitation reaction. Example: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq). Barium sulphate is the white precipitate.
Ex 162 marks
Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation (b) Reduction
Answer: (a) Oxidation is gain of oxygen (e.g., 2Mg + O₂ → 2MgO; 2Cu + O₂ → 2CuO).
(b) Reduction is loss of oxygen (e.g., CuO + H₂ → Cu + H₂O; ZnO + C → Zn + CO).
Ex 172 marks
A shiny brown coloured element 'X' on heating in air becomes black in colour. Name the element 'X' and the black coloured compound formed.
Answer: Element X is copper (Cu). When heated, it reacts with oxygen to form black copper(II) oxide (CuO). 2Cu + O₂ → 2CuO.
Ex 182 marks
Why do we store silver chloride in dark coloured bottles?
Answer: Silver chloride (AgCl) is light-sensitive. It decomposes in sunlight to form silver metal and chlorine gas (2AgCl → 2Ag + Cl₂). To prevent this decomposition, it is stored in dark coloured bottles that block light.
Ex 193 marks
Balance the following chemical equations and identify the type of reaction.
(a) KBr + BaI₂ → KI + BaBr₂
(b) ZnCO₃ → ZnO + CO₂
(c) H₂ + Cl₂ → HCl
(d) Mg + HCl → MgCl₂ + H₂
Answer:
(a) 2KBr + BaI₂ → 2KI + BaBr₂ (Double displacement)
(b) ZnCO₃ → ZnO + CO₂ (Decomposition)
(c) H₂ + Cl₂ → 2HCl (Combination)
(d) Mg + 2HCl → MgCl₂ + H₂ (Displacement)
Additional Important Questions
A12 marks
What is rancidity? How can it be prevented?
Answer: Rancidity is the oxidation of fats and oils in food, leading to unpleasant smell and taste. Prevention: adding antioxidants (e.g., BHA, BHT), storing food in airtight containers, refrigeration, and flushing with nitrogen gas.
A22 marks
What is corrosion? Give two examples.
Answer: Corrosion is the gradual destruction of metals by chemical reaction with their environment. Examples: rusting of iron (formation of Fe₂O₃·xH₂O), tarnishing of silver (Ag₂S), green coating on copper (basic copper carbonate).

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