HPAS Previous Year Question Paper 2021–Science & Technology

Let’s evaluate each statement to identify the incorrect one:

1. Calcium oxide reacts vigorously with water to produce lime.

• Evaluation: Calcium oxide (quicklime) reacts vigorously with water to form calcium hydroxide (slaked lime). This reaction is highly exothermic and is represented by the equation:
  CaO (s) + H₂O (l) → Ca(OH)₂ (aq)
• Conclusion: This statement is correct.

1. Slaked lime slowly creates a layer of calcium nitrate on the walls during whitewashing.

• Evaluation: Slaked lime (calcium hydroxide) is used in whitewashing. When applied to walls, it reacts slowly with carbon dioxide in the air to form calcium carbonate, which gives a shiny finish to the walls. The reaction is:
  Ca(OH)₂ (aq) + CO₂ (g) → CaCO₃ (s) + H₂O (l)
  Calcium nitrate is not formed in this process.
• Conclusion: This statement is incorrect.

1. Silver chloride is sensitive to sunlight.

• Evaluation: Silver chloride decomposes when exposed to sunlight, breaking down into silver and chlorine gas. This photolytic decomposition is represented by:
  2AgCl (s) → 2Ag (s) + Cl₂ (g)
• Conclusion: This statement is correct.

1. Decomposition reactions require energy for breaking down reactants.

• Evaluation:Decomposition reactions involve breaking down a compound into simpler substances, often requiring energy input in the form of heat, light, or electricity. For example, the decomposition of calcium carbonate into calcium oxide and carbon dioxide requires heat:
  CaCO₃ (s) → CaO (s) + CO₂ (g)
• Conclusion: This statement is correct.

Summary:

Correct Statements: A, C, D

Incorrect Statement: B

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