International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

What do larger hydrocarbons tend to do during fractional distillation?

1. Condense at high temperatures

2. Condense at the middle of the tower

3. Evaporate at low temperatures

4. Condense at low heights in the tower

The correct answer is: Condense at low heights in the tower

During fractional distillation, larger hydrocarbons tend to condense at low heights in the distillation column due to their higher boiling points. As the mixture is heated, the hydrocarbons vaporize, and as they rise up the column, they reach areas of different temperatures. Larger hydrocarbons have more complex structures and generally require higher temperatures to remain in the gaseous state because they have stronger intermolecular forces (such as van der Waals forces) due to their larger size and greater surface area. As the vapors rise, they encounter cooler regions of the column, leading to condensation once the temperature drops below their boiling point. Since larger hydrocarbons condense at lower heights in the column, they can be effectively separated from smaller hydrocarbons, which have lower boiling points and will rise higher before condensing. This principle allows for the separation of components based on their boiling point differences, a crucial aspect of fractional distillation in refining processes.