Fractional Distillation by Volatility

Steps:

• Petroleum is heated to vaporize components in the fractionating column.
• Vapors rise, encountering decreasing temperatures from bottom to top.
• Higher-volatility fractions remain gaseous longer and condense higher up.
• Lower-volatility fractions condense lower down, separating by vaporization ease.

Why D is correct:

• Volatility is the tendency to form vapor; it increases upward as top fractions have lower boiling points and vaporize more readily, per Raoult's law relating vapor pressure to molecular interactions.

Why the others are wrong:

• A: Boiling point decreases from bottom to top, as heavier fractions require higher temperatures to vaporize.
• B: Chain length decreases upward, with shorter hydrocarbon chains being more volatile.
• C: Viscosity decreases from bottom to top, as longer-chain fractions are thicker and flow less easily.

Final answer: D