Interpreting Crude-Oil Fractions

A fraction’s boiling range tells you about separation; its viscosity and volatility help you choose a use. Keep those jobs distinct so you can explain a result from data rather than simply recognise a name.

A fraction is not one molecule

Read Crude Oil and Fractional Distillation first if you need the column process. Crude oil contains many hydrocarbons. A fraction is a portion containing components with similar boiling behaviour; it is still a mixture and has a boiling range, rather than the single sharp boiling point of a pure substance at a stated pressure.

A sharp boiling point is not the same as a high boiling point. “Sharp” describes how narrow the temperature interval is; “high” describes its position on the temperature scale.

Explain where vapours condense

A fractionating column is hotter at the bottom and cooler at the top. Higher-boiling components can condense in warmer, lower regions; lower-boiling vapours travel farther up before reaching a region cool enough to condense. Some light gases leave the top without condensing under those conditions. Heavy residue such as bitumen remains at the bottom rather than first vaporising and condensing at the top.

The key separation property is boiling behaviour, not density. Do not use “heavy molecules sink” as the explanation for collecting different fractions.

Down the column, collected fractions generally have larger hydrocarbon molecules, higher boiling ranges and greater viscosity. They are generally less easy to ignite. These are trends for comparing fractions, not a claim that all their molecules have identical sizes or that every product must obey one exact numerical relation.

Connect properties to a use

Viscosity is resistance to flow: a viscous liquid flows less easily. Volatility is how readily a substance vaporises. They are different properties, although refinery fractions often show a related trend.

A suitable material often needs several properties. Bitumen’s use is supported by viscosity and low volatility; “it comes from the bottom” gives a location, not a full explanation of suitability. A fraction can also have competing uses as a fuel or as feedstock for chemicals.

Work from the evidence

Common misconception 1

A Crude-Oil Fraction Is Still a Mixture

Find and correct the mistake

Learner claim

“Each liquid collected from the fractionating column is one pure hydrocarbon with one exact boiling point.” Correct both parts of the claim.

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Composition of a fraction

A mixture of hydrocarbonsOne pure hydrocarbon

Boiling behaviour

A boiling rangeOne exact boiling point

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View solution step by step

1. Correct the composition

   Method

   State that each fraction is a mixture of hydrocarbons with similar chain lengths and boiling points.

   Reason

   Fractional distillation groups substances by boiling behaviour; it does not normally isolate every individual compound.

   Working

   Fraction ≠ pure compound.

2. Correct the boiling claim

   Method

   State that the mixture boils or condenses over a temperature range.

   Reason

   Its different components have different, though similar, boiling points.

   Working

   A fraction has a boiling range, not one sharp boiling point.

Examiner practice 2

Explain the Fractionating-Column Trend

4 marks

Examination question

State two properties that increase down a crude-oil fractionating column and explain why fractions are collected at different heights. [4 marks]

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State two downward trends and explain the separation

View solution step by step

1. State two trends

   2 marks

   Method

   State that boiling point and viscosity increase down the column.

   Reason

   Lower parts collect higher-boiling fractions that tend to be more viscous.

   Working

   Down the column: higher boiling point; greater viscosity.

2. Explain the separation

   2 marks

   Method

   State that components have different boiling points and condense in regions cool enough for them to become liquids.

   Reason

   The temperature gradient therefore causes different vapours to condense at different heights.

   Working

   High-boiling fractions condense lower; low-boiling fractions rise farther before condensing.

Self-mark with the mark scheme

Compare your response with each mark point. Select a point only when your response contains that evidence.

Self-mark the two stated trends and two-part boiling-point explanation.

Mark points

1. 1 mark: States that boiling point increases down the column.
2. 1 mark: States that viscosity increases down the column.
3. 1 mark: States that hydrocarbons or fractions have different boiling points/ranges.
4. 1 mark: Connects the temperature gradient to condensation at different heights.

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Guided practice 3

Deduce the Road-Surface Fraction

About 6 min

Problem

An unnamed heavy residue remains after the more volatile parts of crude oil have been separated. It has very high viscosity and low volatility. Identify the fraction most suitable for road surfaces and justify the use from the data.

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Fraction

BitumenRefinery gas

Useful evidence

Very viscousVery volatile

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Hints

Hint 1: column position

The residue contains material that does not readily vaporise during the separation.

Hint 2: use link

A road-surface material must be thick rather than easy to vaporise or ignite.

View solution step by step

1. Identify the fraction

   Method

   Choose bitumen.

   Reason

   Bitumen is the heavy fraction or residue associated with the bottom of the column.

   Working

   Fraction: bitumen.

2. Justify the use

   Method

   Use the supplied high viscosity and low volatility.

   Reason

   A thick material that does not vaporise readily can form a durable road surface.

   Working

   Very viscous bitumen → road surfacing.

Next step

Try Choosing from supplied data independently.

Try without prompts

Mind stretcher 1: Choosing from supplied dataExtension

The table gives illustrative model data for two liquid hydrocarbon mixtures compared at the same pressure. They are not standard refinery-fraction specifications.

Mixture	Boiling range	Viscosity	Ease of ignition in the stated comparison
P	80–130 °C	Low	Greater
Q	200–300 °C	High	Lower

Which would you expect to condense lower in a fractionating column? Which has evidence supporting use as a lubricant rather than an easily vaporised fuel? Explain why neither boiling range establishes that the mixture is a pure substance, and name one additional property you would want before choosing a lubricant.

Show answer

Q would condense lower because its components have higher boiling temperatures. Its greater viscosity and lower volatility support consideration as a lubricant. Both have ranges rather than one sharp boiling point, consistent with mixtures; a high temperature alone does not establish purity. Further useful evidence could include whether Q stays liquid and stable over the operating temperature range, whether its viscosity is suitable for the machine, or whether it reacts with the materials it contacts. The table supports a candidate, not guaranteed suitability for every machine.

Mind stretcher 2: Fractional Distillation MisconceptionExtension

A student claims: “Crude oil separates because lighter molecules rise to the top due to density.” Explain why this is wrong and give the correct reason.

Show answer

Fractional distillation works mainly because fractions have different boiling points. Vapours rise and condense at different temperatures/heights in the column, not because of density.

Bring the explanation together

Use the temperature gradient and different boiling behaviour to explain separation. Use material properties to justify a use. A fraction remains a mixture, and the physical separation leaves its hydrocarbon molecules intact.

Practise and check

Use the Organic Chemistry topic check to practise and check your understanding.