Fractional distillation: temperature and petroleum trends
Explain how the column separates a mixture physically and compare the four required bottom-to-top property changes.
Core and Extended · Chemistry 0620, 2026–2028.
Before you begin
- Explain vaporisation/condensation and distinguish a physical separation from cracking.
- Read a temperature gradient and compare boiling behaviour.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does condensation do in this separation?
What you will learn
- Describe fractional distillation of petroleum into useful fractions.
- Explain decreasing chain length, greater volatility, lower boiling points and lower viscosity towards the top.
A hot bottom and cooler top separate boiling ranges
Petroleum is heated so suitable components vaporise and enter a fractionating column that is hotter towards the bottom and cooler towards the top. Higher-boiling components condense lower down; lower-boiling ones can travel higher before condensing, and the lightest gases leave near the top. The separated fractions contain molecules with similar boiling ranges rather than necessarily one chemically pure compound.
Fractional distillation separates the existing mixture by vaporisation and condensation. It does not break carbon–carbon bonds to shorten a molecule: cracking is a separate chemical process in the alkene lessons. Very heavy residue remains at the lower end in the school column comparison. This schematic gives relative positions and purposes rather than one universal set of industrial temperatures.
Keep all four upward trends in the correct direction
From the bottom towards the top of the stated petroleum column, average chain length decreases, volatility increases, boiling points decrease and viscosity decreases. Higher volatility means easier vaporisation, while viscosity concerns resistance to flow. A shorter-chain fraction can therefore be more volatile and less viscous without those terms meaning the same property.
The trend concerns these fractions and their molecular-size comparison. It does not prove every molecule at an outlet is identical or give an exact boiling point from its use name. If supplied data give viscosity values or a boiling range, read those values directly and retain the distinction between a range and one pure-substance boiling point.
Pause and explain
Which comparison belongs to a higher outlet in the stated column?
Worked example
Compare a lower-column and upper-column fraction using the four required trends, then distinguish this separation from cracking.
Show the worked solution
- The upper fraction has generally shorter chains and lower boiling points.
- It is more volatile and less viscous than the lower fraction in this model.
- Fractional distillation separates molecules physically; cracking breaks larger molecules chemically.
Answer Upwards: shorter chains, higher volatility, lower boiling points and lower viscosity; physical separation rather than bond-breaking cracking.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the heating and column process that separates petroleum into fractions.
Give me a hint
Follow vaporisation, movement in a temperature gradient and condensation.
Compare my reasoning
- Heating vaporises suitable components of the mixture.
- The vapour meets a column hotter at the bottom and cooler at the top.
- Components condense at suitable levels by boiling behaviour and are collected in fractions, with lighter gases near the top.
Vaporise the mixture, use the hot-bottom/cool-top gradient, condense and collect fractions by boiling ranges.
Look for these in your work
- The physical sequence is complete.
- The temperature gradient and fraction identity are correct.
Compare a higher and lower fraction for chain length, volatility, boiling point and viscosity.
Give me a hint
Write every direction, not only light versus heavy.
Compare my reasoning
- Higher fraction: shorter chains and lower boiling points in the stated comparison.
- Higher fraction: greater volatility and lower viscosity.
- The lower fraction has the opposite relative tendencies; fractions remain mixtures within their ranges.
Higher: shorter chains, more volatile, lower boiling and less viscous. Lower: the opposite relative tendencies.
Look for these in your work
- All four properties are included.
- Volatility and viscosity are not confused.
A report says the column makes short-chain molecules by breaking long-chain bonds, then gives one exact boiling point for every molecule in a fraction. Assess both claims.
Give me a hint
Separate physical sorting from chemical cracking and a boiling range from one pure substance.
Compare my reasoning
- Fractional distillation sorts existing molecules by vaporisation/condensation.
- Bond-breaking to create smaller molecules belongs to cracking.
- A fraction generally contains a mixture with a boiling range, so one exact point for every molecule is unsupported.
Distillation separates physically; cracking breaks bonds. A fraction's boiling range is not one exact value for every molecule.
Look for these in your work
- The two process types are distinguished.
- The fraction's mixture nature limits the temperature claim.
Common mistakes
- Making the top of the column hotter than the bottom in this model.
- Calling greater volatility a higher boiling point.
- Treating every fraction as a single pure molecule.
- Describing fractional distillation as carbon–carbon bond breaking.
What can you explain now?
State the four required bottom-to-top trends.
Compare with the explanation
Decreasing chain length, increasing volatility, lower boiling points and lower viscosity.
The cooler upper outlets collect the lighter/lower-boiling fractions in the school model.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Sketch the column temperature gradient and label all four upward trends, then explain why the process is separation rather than cracking.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.