I. Choose the Correct Answer
1. The gas released at the cathode in the electrolysis of water is
(A) Oxygen
(B) Hydrogen ✓
(C) Chlorine
(D) Nitrogen
Solution:
During electrolysis of water:
Hydrogen gas is liberated at the cathode and oxygen gas is liberated at the anode.
Therefore correct answer is:
Hydrogen
2. Identify the chemical equation in which the chemical reaction takes place.
(A) FeSO₄ + Pb → PbSO₄ + Fe
(B) ZnSO₄ + Fe → FeSO₄ + Zn
(C) 2AgNO₃ + Cu → Cu(NO₃)₂ + Ag ✓
(D) PbCl₂ + Cu → CuCl₂ + Pb
Solution:
Copper is more reactive than silver.
Hence copper displaces silver from silver nitrate solution.
2AgNO₃ + Cu → Cu(NO₃)₂ + 2Ag
Therefore option (C) is correct.
3. Ferrous sulphate crystals lose their green colour when heated because this compound
(A) Breaks down into simpler compounds
(B) Loses water molecules ✓
(C) Releases sulphur dioxide gas
(D) Produces brown smoke
Solution:
Ferrous sulphate crystals contain water molecules called water of crystallization.
When heated, these water molecules are removed and the crystals lose their green colour.
4. The chemical reaction in which iron undergoes corrosion is
(A) Combination and redox reaction ✓
(B) Displacement and redox reaction
(C) Redox and oxidation reaction
(D) Displacement and oxidation reaction
Solution:
Rusting of iron involves:
• Combination of iron with oxygen
• Oxidation of iron
Hence it is a combination and redox reaction.
5. Silver nitrate turns into grey colour in the presence of sunlight because
(A) Silver chloride decomposes to form silver ✓
(B) Silver chloride decomposes to form chlorine
(C) Silver chloride undergoes oxidation
(D) Silver chloride undergoes reduction
Solution:
Silver chloride decomposes in sunlight and forms silver metal.
Silver metal is grey in colour.
6. The metal that displaces copper from copper sulphate
(A) Gold
(B) Silver
(C) Copper
(D) Iron ✓
Solution:
Iron is more reactive than copper.
Hence iron displaces copper from copper sulphate solution.
Fe + CuSO₄ → FeSO₄ + Cu
7. The ratio of hydrogen and oxygen gases released in the electrolysis of water is
(A) Hydrogen : Oxygen :: 1 : 2
(B) Oxygen : Hydrogen :: 2 : 3
(C) Hydrogen : Oxygen :: 2 : 1 ✓
(D) Oxygen : Hydrogen :: 3 : 2
Solution:
Water contains hydrogen and oxygen in the ratio:
H₂O = 2 : 1
Hence hydrogen and oxygen gases are released in the ratio 2 : 1.
II. Fill in the Blanks
8. During electrolysis of water, oxygen is liberated on ___________.
Answer: Anode
During electrolysis:
• Oxygen is released at anode.
• Hydrogen is released at cathode.
9. ___________ gas is used to avoid rancidity of fried food or processed food.
Answer: Nitrogen gas (N₂)
Nitrogen gas prevents oxidation of food and helps avoid rancidity.
III. Answer the Following Questions
10. The magnesium ribbon burns with a dazzling white flame in air and changes into magnesium oxide. Identify the type of reaction.
Solution:
Magnesium combines with oxygen to form magnesium oxide.
2Mg + O₂ → 2MgO
This is a combination reaction.
11. Suggest any two methods to prevent rancidity.
Solution:
1. Store food in airtight containers.
2. Keep food in refrigerator.
12. What is precipitation reaction? Give an example.
Solution:
A reaction in which an insoluble solid called precipitate is formed is known as precipitation reaction.
Example:
AgNO₃ + NaCl → AgCl↓ + NaNO₃
Here silver chloride is the precipitate.
IV. Answer the Following Questions
13. What is chemical reaction? Give any two changes occurs during chemical reaction.
Solution:
A chemical reaction is a process in which one or more substances change into new substances with different properties.
Any two changes during chemical reaction:
1. Change in colour
2. Evolution of gas
14. Sodium sulphate and barium chloride are mixed and a precipitate is formed.
(a) Chemical Equation:
Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl
(b) Name of Reaction:
Double displacement reaction.
(c) Name of Precipitate:
Barium sulphate.
15. Define redox reaction with a chemical equation.
Solution:
A reaction in which oxidation and reduction occur simultaneously is called redox reaction.
Example:
CuO + H₂ → Cu + H₂O
16. Complete the reactions.
(a)
CH₄ + 2O₂ → CO₂ + 2H₂O
(b)
CaCO₃ ⎯Δ⎯→ CaO + CO₂
(c)
2Pb(NO₃)₂ ⎯Δ⎯→ 2PbO + 4NO₂ + O₂
17. Iron reacts with copper sulphate producing iron sulphate and copper.
(a) Balanced Chemical Equation:
Fe + CuSO₄ → FeSO₄ + Cu
(b) Name of Reaction:
Displacement reaction.
(c) Element Replacement:
Iron replaces copper from copper sulphate solution.
18. Mention any three changes occurs during chemical reaction.
Solution:
1. Change in colour
2. Change in temperature
3. Formation of precipitate