Lighter Fractions Rise in Fractional Distillation

Steps:

• Crude oil contains hydrocarbons of varying chain lengths, with shorter chains having lower boiling points.
• In the fractionating column, heat vaporizes components, and those with lower boiling points rise to the cooler top.
• Top fractions condense first, consisting of shorter-chain molecules like gases and petrol.
• Bottom fractions are heavier, longer-chain residues like bitumen with higher boiling points.

Why C is correct:

• Shorter chains at the top have weaker intermolecular forces, leading to lower boiling points and shorter lengths, as defined by hydrocarbon boiling trends.

Why the others are wrong:

• A: Top fractions have lower viscosity due to smaller molecules, not higher.
• B: Top fractions have higher volatility from easier vaporization, not lower.
• D: Top fractions have higher volatility despite shorter chains.

Final answer: C