Question

What is the smallest straight-chain alkane that is a liquid at room temperature (which is about 25 °C)?

Short answer

The smallest chain alkane is pentane which can be at liquid form at room temperature due to high boiling point.

Step-by-step solution

Phases of a substance

There are 3 phases of a compound, that are solid liquid and gas. The transition of solid to liquid can be estimated at a particular point through melting point and same for the transition of Liquid to gas needs boiling point.

Boiling point of a compound depends on the strength of the attractive forces between the individual molecules. If there are strong force of attraction between the molecules, a greater amount of force is needed to separate them which increases its boiling temperature.

As the addition of carbon in a straight chain increases, the surface area also increases which increases its boiling point. So, more branching means higher boiling temperature.

Reason of the answer

Alkanes with lower chain remains as a gas in room temperature as their melting boiling point is very low. From carbon no 5, the alkanes are stable as a liquid in room temperature due to higher boiling point. So, pentane is the smallest straight chain alkane that is a liquid at room temperature.

In pentane, the forces that are generally seen are London forces, Van der Waals forces, Induced dipole -Induced Dipole forces etc.