Question

Dispersion forces are the only intermolecular forces present in motor oil, yet it has a high boiling point. Explain.

Short answer

The motor oil has many straight chain hydrocarbons which provides more surface area for the bonding due to weak dispersion forces. Thus, due to high surface area the oil has higher boiling point.

Step-by-step solution

London dispersion forces

The motor oil is composed of different hydrocarbons of different chain length. The intermolecular forces present in these hydrocarbons are the weak London dispersion forces. Due to this weak bonding, these compounds have comparatively low boiling point.

Effect of surface area on the boiling point

Yet, compared to their counterparts, they have higher boiling point. This is because of the higher surface area provided by these compounds, which thus increases the intermolecular interactions in number. Thus, the motor oil has high boiling point though the forces involved are weak dispersion forces.