Video: Order of Viscosity in Crude Oil Fractions

In which of the following are crude oil fractions arranged in order of increasing viscosity? [A] Kerosene < diesel oil < gasoline [B] Diesel oil < gasoline < kerosene [C] Gasoline < kerosene < diesel oil [D] Kerosene < gasoline < diesel oil [E] gasoline < diesel oil < kerosene


Video Transcript

In which of the following are crude oil fractions arranged in order of increasing viscosity? A) Kerosene, diesel oil, gasoline. B) Diesel oil, gasoline, kerosene. C) Gasoline, kerosene, diesel oil. D) Kerosene, gasoline, diesel oil. Or E) gasoline, diesel oil, kerosene.

Crude oil fractions come from fractional distillation. The chemicals in crude oil, which is a huge mixture of different chemicals, are separated broadly by boiling point. Crude oil fractions are actually mixtures of many different chemicals, but the fraction as a whole will have a pretty reliable boiling point. You can easily remember the fractions of crude oil in order through the mnemonic playful naughty gray kittens dance like ferocious tigers. This puts the fractions in order from the lowest boiling point to the highest boiling point. The full names of the fractions are petroleum gas, naphtha, gasoline, kerosene, diesel, lubricating oil, fuel oil, and tar.

The three we’ll need for this question are gasoline, kerosene and diesel. Our job is to put these three fractions in order of their viscosity. We already know that moving down the list, the boiling point of the fraction increases. This is because, as we go down the list, the average size of hydrocarbon in each fraction tends to increase. Another key property of a bigger hydrocarbon is that it will have a higher viscosity. This is because as the hydrocarbons get bigger, they tend to entangle around one another and stick to each other more. This makes them thicker, more like syrup, less like water.

So, gasoline is less viscous than kerosene. And kerosene is less viscous than diesel. This means our answer is C. Gasoline, kerosene, diesel oil is the correct order of increasing viscosity for these three crude oil fractions.

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