Which of the following processes occur during fractional distillation of petroleum?

Step 1: Analyze the question We are asked which physical processes take place during fractional distillation of petroleum (crude oil).

Step 2: The concept Fractional distillation separates a mixture of liquids according to their different boiling points. It involves two main changes of state:

• Evaporation (vaporisation): Crude oil is heated in a furnace until most of it turns into vapour.
• Condensation: The vapour rises up the fractionating column, where the temperature falls with height. Each fraction condenses back into a liquid at the level where the temperature is just below its boiling point.

Step 3: Step-by-step solution

1. Crude oil is heated to about 400 °C, so it evaporates.
2. The hot vapours enter the fractionating column near the bottom.
3. As they rise, they cool. Fractions with high boiling points (bitumen, lubricating oil, fuel oil) condense low down. Those with low boiling points (petrol, refinery gases) condense higher up or remain as gas.
4. The liquids are collected at different levels.

Step 4: Eliminate the wrong options

• Sublimation (solid → gas directly) does not occur here, so C is out.
• Diffusion is not the main process that separates the fractions, so A, B and C are out.
• Only evaporation and condensation describe the process.

Answer: The correct option is D. Evaporation and condensation.

Exam tip: Any distillation, whether simple or fractional, is "evaporate, then condense". Remember it as "Boil up, cool down". If an option includes sublimation, it is usually wrong, since sublimation is for substances like iodine, ammonium chloride and dry ice.