CBSE Class 10 Science Chemical Reactions and Equations Assignment Set 06

This is Assignment Set 06 for CBSE Class 10 Science Chapter 1, Chemical Reactions and Equations, with 18 questions carrying 42 marks. It covers types of reactions, oxidation and reduction, electrolysis of water, precipitation reactions and the method of balancing equations, with stepwise answers.

Very Short Answer Questions (1 Mark)

Question 1. Name the type of the following reaction: Fe + S → FeS.
Answer:
Iron and sulphur join to form a single product, so it is a combination reaction.
In simple words: Two elements form one compound.

Teacher's Note:
1. One product from two reactants means combination.
2. Heating is needed to start it.

Question 2. Name the reaction in which oxygen is removed from a substance.
Answer:
The removal of oxygen from a substance is called reduction.
In simple words: Loss of oxygen is reduction.

Teacher's Note:
1. Gain of oxygen is oxidation.
2. Both occur together.

Question 3. White silver chloride turns grey in sunlight. Why?
Answer:
Sunlight decomposes silver chloride into grey silver metal and chlorine gas: 2AgCl →^sunlight 2Ag + Cl₂
In simple words: Light splits AgCl into silver and chlorine.

Teacher's Note:
1. It is a photochemical decomposition.
2. Silver chloride is stored in dark bottles.

Question 4. Balance the chemical equation: Pb(NO₃)₂(s) →^heat PbO(s) + NO₂(g) + O₂(g)
Answer:
2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g) + O₂(g)
In simple words: Put 2, 2, 4 and 1 as the coefficients.

Teacher's Note:
1. Count N and O on both sides.
2. Balance oxygen last.

Question 5. Identify the component oxidised in the following reaction: CuO + H₂ → Cu + H₂O
Answer:
Hydrogen gains oxygen to form water, so H₂ is oxidised (and CuO is reduced).
In simple words: Hydrogen gains oxygen.

Teacher's Note:
1. Gain of oxygen is oxidation.
2. CuO is reduced to Cu.

Question 6. Write a balanced chemical equation to represent the following reaction: Carbon monoxide reacts with hydrogen gas at 340 atm. to form methyl alcohol.
Answer:
CO(g) + 2H₂(g) →^{340 atm} CH₃OH(l)
In simple words: One CO and two H₂ make one methanol.

Teacher's Note:
1. Write the condition over the arrow.
2. Check H: 4 = 4.

Short Answer Questions (2 Marks)

Question 7. A magnesium ribbon is burnt in oxygen to give a white compound ‘X’ accompanied by emission of light. (a) Write the chemical formula of ‘X’. (b) Write a balanced chemical equation, when ‘X’ is dissolved in water.
Answer:
(a) 2Mg + O₂ → 2MgO, so X is MgO.
(b) MgO + H₂O → Mg(OH)₂
In simple words: Burning gives magnesium oxide, which forms magnesium hydroxide with water.

Teacher's Note:
1. Show the burning equation first.
2. The product dissolving is a combination reaction.

Question 8. What is meant by thermal decomposition reaction? Explain with an example.
Answer:
A decomposition reaction carried out by heating is called a thermal decomposition reaction.
Example: CaCO₃(s) →^heat CaO(s) + CO₂(g)
In simple words: A compound breaks down on heating.

Teacher's Note:
1. Name the form of energy used.
2. Give a balanced equation.

Question 9. What is observed when carbon dioxide gas is passed through lime water: (i) For a short duration. (ii) For a long duration.
Answer:
(i) Lime water turns milky: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
(ii) The milkiness disappears as soluble calcium hydrogencarbonate forms: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂
In simple words: First milky, then clear again.

Teacher's Note:
1. Write both equations.
2. Milky = CaCO₃.

Question 10. Write balanced chemical equations to represent the following statements: (i) Carbon (coke) burns in air to form carbon dioxide gas. (ii) A piece of sodium metal on putting in water forms caustic soda and hydrogen gas.
Answer:
(i) C(s) + O₂(g) → CO₂(g)
(ii) 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
In simple words: Write the formulae and balance them.

Teacher's Note:
1. Sodium hydroxide is caustic soda.
2. Check Na, H and O.

Question 11. Name the reducing agent in the following reaction: 3MnO₂ + 4Al → 3Mn + 2Al₂O₃. State which is more reactive, Mn or Al and why?
Answer:
Aluminium takes oxygen from MnO₂, so Al is oxidised and is the reducing agent.
Al is more reactive than Mn because it displaces Mn from its oxide.
In simple words: The metal that removes oxygen is the more reactive one.

Teacher's Note:
1. Reducing agent is the substance oxidised.
2. Use displacement to compare reactivity.

Short Answer Questions (3 Marks)

Question 12. (i) Define a decomposition reaction. Write chemical equation for the reaction that occurs when lead nitrate is heated strongly in a boiling tube. (ii) In electrolytic decomposition of water two gases are liberated at the electrodes. Give the mass ratio of the gas liberated at the cathode and at the anode.
Answer:
(i) A reaction in which a single reactant breaks down into two or more products. 2Pb(NO₃)₂(s) →^heat 2PbO(s) + 4NO₂(g) + O₂(g)
(ii) 2H₂O → 2H₂ + O₂. Mass of H₂ = 2 × 2 = 4 g; mass of O₂ = 32 g. Ratio = 4 : 32 = 1 : 8.
In simple words: Hydrogen to oxygen by mass is 1 : 8.

Teacher's Note:
1. H = 1, O = 16.
2. Volume ratio is 2 : 1 but mass ratio is 1 : 8.

Question 13. When hydrogen sulphide gas is passed through a blue solution of copper sulphate, the colour of the solution fades and a black precipitate is obtained. (a) Name the type of reaction mentioned above. (b) Why does the colour of the solution fade away? (c) Write the chemical name of the black precipitate formed. (d) Give the balanced chemical equation for the reaction involved.
Answer:
(a) Double displacement (precipitation).
(b) Blue Cu²⁺ ions are used up as CuS forms; the acid left is colourless.
(c) Copper sulphide.
(d) H₂S + CuSO₄ → CuS↓ + H₂SO₄
In simple words: Black copper sulphide settles and the blue colour goes.

Teacher's Note:
1. Name the precipitate with its colour.
2. Check S and H balance.

Question 14. Justify with the help of an example that displacement reaction is also a redox reaction.
Answer:
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Iron loses electrons to form Fe²⁺ (oxidised); Cu²⁺ gains electrons to form Cu (reduced).
Since oxidation and reduction occur together, this displacement reaction is a redox reaction.
In simple words: Iron is oxidised while copper ions are reduced at the same time.

Teacher's Note:
1. Name what is oxidised and what is reduced.
2. Mention the colour change as evidence.

Question 15. Lead nitrate solution is added to a test tube containing potassium iodide solution. (a) Write the name and colour of the compound precipitated. (b) Write the balanced chemical equation for the reaction involved. (c) Name the type of this reaction justifying your answer.
Answer:
(a) A yellow precipitate of lead iodide, PbI₂.
(b) Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)
(c) Double displacement (precipitation): Pb²⁺ and I⁻ combine, K⁺ and NO₃⁻ combine.
In simple words: Ions swap partners and yellow PbI₂ settles.

Teacher's Note:
1. Colour of the precipitate is the key clue.
2. Source key wrongly said lead oxide.

Long Answer Questions (5 Marks)

Question 16. What is observed after about 1 hour of adding the strips of copper and aluminium separately to ferrous sulphate solution filled in two beakers? Name the reaction if any change in colour is noticed. Also, write chemical equation for the reaction.
Answer:
Copper strip: no change, because copper is less reactive than iron.
Aluminium strip: the green colour of FeSO₄ fades to colourless and iron is deposited, as aluminium is more reactive than iron.
The reaction is a displacement reaction.
2Al + 3FeSO₄ → Al₂(SO₄)₃ + 3Fe
In simple words: Only the more reactive metal, aluminium, displaces iron.

Teacher's Note:
1. Say what happens in both beakers.
2. Link observation to reactivity.

Question 17. Describe the method of balancing a chemical equation.
Answer:
Step 1: Write the skeleton equation, for example H₂ + O₂ → H₂O.
Step 2: Count the atoms of each element on both sides. Here H: 2 = 2 but O: 2 on the left and 1 on the right.
Step 3: Balance oxygen by putting 2 before H₂O: H₂ + O₂ → 2H₂O. Now H: 2 on the left, 4 on the right.
Step 4: Put 2 before H₂: 2H₂ + O₂ → 2H₂O. Now H: 4 = 4 and O: 2 = 2.
Start with the compound having the most atoms, change only the numbers (coefficients) and never change a formula. This is the hit and trial method.
In simple words: Change only the numbers in front of the formulae until each element is equal on both sides.

Teacher's Note:
1. Count atoms before and after.
2. Write the final check.

Case Based Question (4 Marks)

Question 18. Oxidation is the process of gaining of oxygen or losing of hydrogen. Reduction is the process of losing of oxygen or gaining of hydrogen. The substance which undergoes oxidation is the reducing agent while the substance which undergoes reduction is known as the oxidising agent. Oxidation and reduction always take place together and these type of reactions are known as redox reactions. Some of the examples of redox reactions are given below: a. Pb₃O₄ + 8HCl → 3PbCl₂ + Cl₂ + 4H₂O b. 2Mg + O₂ → 2MgO c. CuSO₄ + Zn → Cu + ZnSO₄ d. V₂O₅ + 5Ca → 2V + 5CaO e. 3Fe + 4H₂O → Fe₃O₄ + 4H₂ f. CuO + H₂ → Cu + H₂O
(i) Give two examples of oxidation reaction from your everyday life. (ii) Write the oxidising agent in the reaction a and b.
Answer:
(i) Corrosion (rusting) and rancidity.
(ii) In reaction a, Pb₃O₄ gives oxygen to HCl (HCl loses hydrogen), so the oxidising agent is Pb₃O₄. In reaction b, O₂ is the oxidising agent because Mg gains oxygen.
In simple words: The oxidising agent is the substance that is itself reduced.

Teacher's Note:
1. The oxidising agent is the substance that is itself reduced.
2. Part (iii) of the source (about the anode) is outside the syllabus and is left out.