Among the listed fractions, which has the lowest boiling point?

• A. Refinery gases
• B. Gasoline
• C. Kerosene
• D. Bitumen

Answer: (A)

Boiling point is determined by molecular size and the strength of intermolecular forces: smaller molecules have weaker forces between them and boil at lower temperatures.

Refinery gases are the lightest fraction from crude oil, made up of very small hydrocarbon molecules such as methane, ethane, propane, and butane. Because these molecules are so small, the attractive forces between them are weak, so they vaporize and boil at very low temperatures. That means their boiling point is lower than the other listed fractions.

Gasoline, kerosene, and bitumen all contain larger molecules with more atoms, which increase the surface area over which dispersion forces act. The stronger these forces, the more heat is needed to separate the molecules into a gas, so their boiling points are higher. Bitumen is the heaviest and has the strongest forces among them, giving it a very high boiling point (often not even reached under ordinary conditions).

So refinery gases have the lowest boiling point because their small molecules require the least energy to vaporize compared with the larger molecules in gasoline, kerosene, and bitumen.