O Levels Chemistry (5070)•5070/11/O/N/23
Explanation
Limewater's basic properties and indicator reactions
Steps:
- Identify limewater as Ca(OH)₂(aq), a weak base with pH > 7.
- Evaluate each option against base characteristics, flame tests, and indicator behaviors.
- Test D: Methyl orange turns yellow in basic solutions (pH > 4.4).
- Confirm D matches while others contradict known chemistry.
Why D is correct:
- Methyl orange indicator is yellow in basic solutions (pH > 4.4), and limewater's Ca(OH)₂ provides OH⁻ ions for basic pH.
Why the others are wrong:
- A: Bases like Ca(OH)₂ have pH above 7, not below.
- B: Blue-green flame test indicates copper ions, absent in Ca(OH)₂.
- C: Ammonia (NH₃) is a base; it does not react with Ca(OH)₂ to form an ammonium salt.
Final answer: D
Topic: The characteristic properties of acids and bases
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