Which statement about boiling points of alkanes is correct?

• A. Boiling point increases as the carbon chain length increases.
• B. Boiling point increases with more branching in the molecule.
• C. All alkanes have boiling points below room temperature.
• D. Boiling point decreases as the carbon chain length increases.

Answer: (A)

Boiling points of alkanes are governed mainly by London dispersion forces between molecules. These temporary dipole attractions grow with molecular size and surface area, so longer carbon chains have more electrons and a larger area to interact with neighboring molecules. More contact means stronger intermolecular forces, which take more energy to overcome, leading to higher boiling points as the chain length increases.

Branching changes things a bit: branched isomers have less surface area in contact with others than straight-chain ones, so their dispersion forces are weaker and their boiling points are lower for the same number of carbons. It’s not true that all alkanes boil below room temperature—some small ones are gases at room temperature (like methane and ethane), while pentane, hexane, and longer ones are liquids or even solids at room temperature.

So the statement that best fits the data is that boiling point increases as the carbon chain length increases.