Distillation (Crude Oil)

The question asks which physical property is exploited in fractional distillation. Distillation is a separation technique that relies on the ability of different substances to vaporise and condense at specific temperatures.

A substance vaporises at its boiling point. Because different hydrocarbon molecules in crude oil have different boiling points, they can be separated — each fraction condenses at a different height in the fractionating column.

The property exploited is differences in boiling points. Density, solubility, and colour do not determine at what temperature a substance vaporises or condenses.

We need the physical property that allows fractional distillation to separate crude oil components. The key word is distillation, which is a technique based on heating and cooling.

Differences in density would be exploited in a technique like centrifugation or filtration — not distillation. This option is incorrect.

Differences in solubility in water is used in techniques like chromatography or extraction. Differences in colour is not a separation technique at all. Both are eliminated.

Differences in boiling points is the correct answer. This is the property that determines at what temperature each fraction vaporises and later condenses in the column.

The fractionating column operates on a temperature gradient — it is not the same temperature throughout. The crude oil vapours enter at the bottom after being heated in a furnace.

Hot vapours enter at the bottom of the column at around 350°C. As they rise, they gradually cool, reaching about 20–25°C at the top. This means the bottom is hottest.

The temperature is highest at the bottom and lowest at the top. This gradient allows different fractions to condense at different heights.

We need to know where the highest temperature is found in the fractionating column.

The same throughout is incorrect — if the temperature were uniform, all fractions would condense at the same point and there would be no separation.

The top is incorrect — refinery gas (lowest boiling point) is collected at the top, meaning it is the coolest part (~20°C). The middle is also incorrect for the same reason — bitumen (highest boiling point) stays at the very bottom, confirming that is where it is hottest.

At the bottom is correct. Crude oil enters as hot vapour at approximately 350°C at the base of the column, creating the temperature gradient needed for separation.