A Levels Chemistry (9701)•9701/12/M/J/21
Explanation
Using a basic solid to neutralize acidic soil
Steps:
- Identify the goal: raise soil pH from 5.5 (acidic) to above 6.0 by adding a base.
- Select a solid compound, as specified for practical application.
- Evaluate options for basic properties that increase pH via hydroxide ions or alkalinity.
- Confirm the compound reacts with soil acids to form water and salts, elevating pH.
Why C is correct:
- Ca(OH)₂ (calcium hydroxide) is a strong base that dissociates to release OH⁻ ions, neutralizing H⁺ in acidic soil per the reaction Ca(OH)₂ + 2H⁺ → Ca²⁺ + 2H₂O.
Why the others are wrong:
- A. (NH₄)₂SO₄ is an acidic salt; ammonium ions hydrolyze to produce H⁺, lowering pH.
- B. NH₃ is ammonia gas (not solid) and forms a weak base solution, but doesn't meet the solid requirement.
- D. Ca(NO₃)₂ is a neutral salt with no significant effect on pH.
Final answer: C
Topic: Group 2
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