Fractional distillation separates by boiling point

Steps:

• Fractional distillation column has decreasing temperature from bottom to top.
• Lower boiling point fractions vaporize first and rise higher to cooler regions.
• X, collected higher than Y, reaches its condensation point in cooler zone.
• Thus, X condenses at lower temperature than Y, which condenses lower in warmer zone.

Why B is correct:

• Condensation temperature equals boiling point; lower position for Y means higher condensation temperature due to hotter zone.

Why the others are wrong:

• A: Wording implies comparison of condensation temperatures, but phrasing "than at Y" is ambiguous without specifying Y's temperature directly.
• C: X has lower boiling point than Y, as it condenses higher in cooler area.
• D: Shorter chain molecules in X mean lower boiling point, opposite of longer chains.

Final answer: B