O Levels Chemistry (5070)•5070/11/M/J/25
Explanation
Purification increases melting point of solids via crystallisation Steps:
- Impure sample melts at 120°C, indicating depression by impurities.
- For solid compounds, crystallisation removes impurities effectively.
- Pure substances have sharp, higher melting points than impure ones.
- Thus, purified X via crystallisation melts at 125°C.
Why A is correct:
- Impurities lower melting point by disrupting crystal lattice; removal via crystallisation raises it to pure value.
Why the others are wrong:
- B: Distillation separates liquids by boiling point, not suitable for purifying solids like X.
- C: Crystallisation purifies solids by increasing melting point, not decreasing it.
- D: Distillation inapplicable to solids; purification raises, not lowers, melting point.
Final answer: A
Topic: Separation and purification
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