Doc Brown's GCSE/IGCSE/O Level KS4 science–CHEMISTRY Revision Notes

Oil, useful products, environmental problems, introduction to organic chemistry

2. Fractional distillation of crude oil in an oil refinery AND

the uses of fractions (related to their molecular properties)

–This page describes the separation of useful products from crude oil by the process of fractional distillation, part of the oil refining process in the petrochemical industry. Crude oil provides the starting raw material for making lots of different chemicals for a variety of uses. The uses of the fractions from fractional distillation fuel gas, LPG, refinery gas, gasoline, petrol, naphtha, paraffin, kerosene, diesel oil, gas oil, fuel oil, lubricating oils, wax and bitumen fractions are tabulated and many are non–renewable fuels. The uses of a fraction is related to its physical properties e.g. ease of vaporisation & boiling point or its viscosity ('stickiness') and the dangers of flammability are pointed out too. There is also a discussion on what makes a good fuel and reference to alternative fuels. These notes on fractional distillation of oil and the uses of oil fractions are designed to meet the highest standards of knowledge and understanding required for students/pupils doing GCSE chemistry, IGCSE chemistry, O Level chemistry and KS4 science courses.

Index of KS4 Science GCSE/IGCSE/O Level Chemistry Oil & Organic Chemistry revision notes pages: 1. Fossil Fuels & carbon Cycle : 2. Fractional distillation of crude oil & physical properties and uses of fractions, what makes a good fuel? : 3. ALKANES - saturated hydrocarbons, structure, uses, combustion : 4. Pollution, carbon monoxide, sulfur/nitrogen oxides, climate change-global warming : 5. Alkenes - unsaturated hydrocarbons, structure and chemistry : 6. Cracking - a problem of supply and demand, other products : 7. Polymers, plastics, uses and problems : 8. Introduction to Organic Chemistry - Why so many series of organic compounds? : 9a. Alcohols, Ethanol, manufacture, physical properties & chemical reactions : 9b. Biofuels & alternative fuels, hydrogen, biogas, biodiesel : 10a. Carboxylic acids - chemistry and uses : 10b. Esters, chemistry and uses including perfumes : 11. Condensation polymers, Nylon & Terylene, comparing thermoplastics, fibres and thermosets : 12. Natural Molecules - carbohydrates - sugars - starch : 13. Amino acids, proteins, enzymes & chromatography : 14. Oils, fats, margarine and soaps : 15. Vitamins, drugs-analgesic medicines & food additives and aspects of cooking chemistry! : 16. Ozone destruction, CFC's and free radicals : 17. Extra notes, ideas and links on Global Warming and Climate Change : Multiple Choice and Gap-Fill Quizzes: m/c QUIZ on Oil Products (GCSE/IGCSE easier-foundation-level) : m/c QUIZ on Oil Products (GCSE/IGCSE harder-higher-level) : IGCSE/GCSE m/c QUIZ on other Aspects of Organic Chemistry : and 3 Easy linked GCSE/IGCSE Oil Products word-fill worksheets

ALL my Advanced Level Organic Chemistry revision notes

2a. The SEPARATION of the crude oil mixture into fractions by fractional distillation

AND the USES of these fractions

topCrude oil is formed from the organic remains of plants and animals buried and heated under pressure over millions of years (See Fossil Fuels).

The complex mixture of hydrocarbons can be separated into fractions by the technique of fractional distillation.

Crude oil cannot be used directly but must be refined before commercially useful products are produced by the petrochemical industry (collectively called petrochemicals).

The oil refining process principally involves fractional distillation into useful fractions i.e. products with specific uses, but further processing may needed to diversify both the quantity and nature of particular oil based products.

A fraction is a mixture of liquids (in this case hydrocarbons) with a relatively narrow (restricted) boiling point range of molecules.

Within each fraction obtained from crude oil the hydrocarbon molecules have a similar number of carbon atoms and similar physical properties.

The uses of the fractions very much depends on their physical properties, which in turn are dependant on the length of the molecule i.e. the carbon atom chain in a hydrocarbon molecule.

doc b oil notes

  • Hydrocarbon molecules are only  made of a chemical combination of carbon and hydrogen atoms.

  • They are compounds because they consist of atoms of at least two different elements.

  • All the bonding is covalent C–C or C–H bonds

doc b oil notes

  • What goes on in an oil refinery?
  • Crude oil is a complex mixture of many compounds, but mainly hydrocarbon compound molecules.
  • This means crude oil can be separated by physical methods, in this case by fractional distillation, because they have different boiling and condensation points.
    • The liquids must also be completely soluble in each other, that is they must all be miscible liquids.
    • When the temperature is high enough, the kinetic energy of a particular hydrocarbon molecule will be sufficient for it to escape the intermolecular forces in the liquid and become a gas.
    • The intermolecular forces are much weaker than the strong carbon - carbon bonds in the hydrocarbon molecule, so it vaporises without decomposes.
  • At the bottom of the fractionating column the crude oil is heated to vapourise it (evaporated or boiled) and the vapour passed into the fractionating column – a large construction of many levels and pipes, see the 'simple' diagram below!
    • A fractionating column acts in the same way as a fractional distillation apparatus in the school/college laboratory but on an industrial scale!
    • In an oil refinery the fractionating columns are very tall with huge surface area to give the best chance of separating the dozens of hydrocarbons in the crude oil (see diagram on the left.
  • This is a continuous process (not a batch process). The fractionating column works continuously with heated–vapourised crude oil piped in at the bottom and the various fractions condensed and constantly tapped off from various levels, each with a different condensation temperature range.
  • Up the fractioning column the temperature gradually decreases (temperature gradient), so the highest boiling (least volatile) molecules tend to be at the bottom and the lowest boiling (most volatile) hydrocarbons go to the top. The rest of the hydrocarbon molecules then condense out in narrow temperature range i.e. the different fractions condense out in a gradual way from top to bottom depending on their boiling point.
  • In other words the most volatile fraction, i.e. the molecules with the lowest boiling points (shortest hydrocarbon molecules), boil or evaporate off first and go higher up the column and condense out at the higher levels in the fractionating column at the lowest temperature.
  • The rest of the hydrocarbon molecules separate out according to their boiling/condensation point so that the highest boiling fraction, i.e. the less volatile molecules with higher boiling points (longest hydrocarbon molecules), tend to condense more easily lower down the column, albeit at the higher temperatures.
  • The process is perhaps more correctly called fractional condensation but it is still referred to as fractional distillation.
  • The bigger the molecule, the greater the intermolecular attractive forces between the molecules, so the higher the boiling point or condensation point (see physical property trends).
    • This is an important rule to know since the intermolecular forces (intermolecular bonding) affect the physical properties including melting point and viscosity too, and this has a bearing on how each fraction is used, see below.
  • Note: Covalent chemical bonds like C–C or C–H are not broken in the process, only the intermolecular force of attraction is weakened to allow the initial evaporation or boiling and this .
THE FRACTIONAL DISTILLATION OF CRUDE OIL names  of fractions at the different condensation levels

(% in crude oil)

C atoms in the hydrocarbon molecule fraction approximate boiling range in oC of the fraction USES of the fraction which mainly depends on its physical properties

Many fuels are hydrocarbons

A simplified diagram of a fractionating column used in the fractional distillation of crude oil

doc b oil notes

The decrease and increase trends for the hydrocarbon molecules are given on the left of fractionating column

Fuel Gas, LPG, Refinery Gas


1 to 4

mainly propane and butane gases which can be compressed or liquified

< 25oC methane doc b oil notesgas fuel, C3–4 easily liquefied, portable energy source e.g. bottled gas for cooking (butane), higher pressure cylinders (propane), feedstock for other organic chemicals
Gasoline – Petrol


5 to 8 25 to 75oC easily vaporised, highly flammable, easily ignited, car fuel – petrol molecules


6 to 10 75 to 190oC no good as a fuel, but valuable raw material source of organic molecules to make other things, cracked to make more petrol and alkenes
Paraffin, Kerosene


10 to 16 190 to 250oC less volatile, less flammable than petrol, domestic heating fuel, (paraffin) aircraft jet fuel (kerosene)
Diesel oil, Gas oil


14 to 20 250 to 350oC less volatile than petrol, car and larger vehicle fuel (diesel), central heating fuel, cracked to make more petrol and alkenes
RESIDUE – fuel oil, lubricating oils, waxes

AND bitumen


20 to 70


over 70

high boiling liquids or low melting solids, all boil over 350oC not so easily evaporated, not as flammable, safe to store, liquid fuel oil for power stations and ships, quite viscous (sticky) and can also be used for lubricating oils (lubricants, 'mineral oils'), low melting solids used as candle wax, clear waxes and polishes (can be dyed)


bitumen/asphalt – low melting solid used on roads as it forms a thick, black, tough and resistant adhesive surface on cooling, used as a roofing waterproofing material (it sticks rock chips on roofs or road surfaces)
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For section 2b. a mental picture of the increasing length of hydrocarbon molecules will help you understand more about how, and why, the physical properties of hydrocarbon molecules changes with increasing length AND how their physical properties affect how each fraction is used commercially after the fractional distillation of crude oil.

Note that the longer the hydrocarbon molecule, the more flexible or wiggly it gets!

doc b oil notesdoc b oil notes

2b. More on relating the physical properties of the hydrocarbon fractions to their uses and dangers

The important physical properties of hydrocarbons like alkanes all depend on the forces between the molecules - the intermolecular forces (intermolecular bonding). This is NOT covalent bonding, covalent bonds are the much stronger C-C and C-H bonds between the atoms in the molecule itself. Quite simply, with important consequences (e.g. how they are used), the bigger the molecule the bigger these weak electrical attractive forces are, which I've done my best to illustrate in the two diagrams (right and below). As the hydrocarbon molecule gets bigger, the surface to surface contact area increases allowing more points for the intermolecular bonding attractive forces to happen. Hence the increase in viscosity, melting points and boiling points with increase in molecular mass. The intermolecular bonds are much weaker than the covalent bonds, so when the hydrocarbon molecules have enough kinetic energy, it is the intermolecular forces which are overcome on vapourisation and the molecules stays intact.

The different fractions are a range of physical properties which vary with molecular size. Down the list above (and below) the longer the carbon chain, the bigger the molecule gets ...

  1. ... the more viscous the molecule (stickiness! less runny, more sticky) as the intermolecular attractive forces between molecules increases the bigger the molecule in a series of molecules of similar structure.

    • Note that intermolecular forces are non–polar weak electrical attractive forces, often described as Van der Waals forces, and correctly described as instantaneous dipole – induced dipole forces (by advanced level students only!).

      • You need to distinguish this intermolecular attractive force from the much stronger force of the covalent bonds between the carbon atoms (C-C) of the chain of the hydrocarbon.

  2. ... the molecule has a higher melting point as more vibrational kinetic energy is needed to overcome the increasing intermolecular attractive forces holding the molecules together to form the crystals which increases with increase in size of molecule.

  3. ... the molecule has a higher boiling point as more particle kinetic energy is needed to overcome the increasing intermolecular forces between the liquid molecules. 3. follows from 2. ie the intermolecular forces increase between the hydrocarbon molecules increases as they get bigger (longer carbon chain).

  4. topdoc b oil notes... the molecule is less flammable as they become less volatile, again due to increasing intermolecular forces with increasing size of molecule so for example, petrol (small molecules) is much more flammable than lubricating oil (much bigger molecules, much longer carbon chain).

    • doc b oil notesThis raises health and safety issues about handling, distributing and storing flammable hydrocarbons. The smallest molecules (natural gas to petrol) are the most volatile and therefore the most easily ignited. Any naked flame or spark could set off a fire and explosion and even

  5. Further comments on the use of the fractions related to the use of the hydrocarbons from crude oil
    • The examples are discussed in order of increasing molecular size – increase in carbon chain length.
    • All fuels are processed at the oil refinery to reduce the concentration of sulfur/sulphur compounds (desulfurisation/desulphurisation).
    • It should be noted that liquid fuels like petrol, diesel, central heating oil etc. are east to store and distribute to wherever they are need in homes or factories and they are so readily available, that change may be necessary, but it will be slow.
      • See notes on pollution, global warming
  6. Methane natural gas, either from gas fields (eg under North Sea) or from an oil refinery can be piped to power electricity generation or domestic heating in the home.
  7. doc b oil notesThe refinery gas fractions, can be stored under pressure as bottled gas, and because the gas readily flows under the control of a simple valve, so can be conveniently pumped to burner systems, but it is easily ignited and explosive.
  8. Vehicle fuels like petrol must be liquid at room temperature for compact and convenient storage but they must be easily vapourised to mix with air in the engine prior to ignition. The ease of vaporisation does however make them flammable!
  9. Paraffin and kerosine are bigger molecules, less flammable and safer, but not as easily ignited.
  10. Diesel is not as volatile or flammable as petrol and doesn't have to be vapourised first, the diesel fuel is sprayed into the engine cylinder and mixed with air and ignites under compression.
  11. Fuel oil molecules are getting quite big, but not too viscous to pump to a central heating burner for domestic use. Fuel oil is not very volatile and so not as flammable and dangerous to use as petrol or diesel etc.
  12. Lubricating oil must be quite viscous to stick onto surfaces. Smaller molecules might be more runny but they would evaporate away! It is also water repellent and helps reduce corrosion on moving metal parts from factory machines to cars and bicycles.
  13. Candle wax is very convenient as a solid for a humble lamp (especially in power cuts!), but via a wick, the heat from the flame is sufficient to vaporise the hydrocarbons to burn them and give a big enough luminous yellow flame to act as a source of light.
  14. Bitumen is a water repellent solid at room temperature but is readily melted (sometimes too easily in hot weather). Used as base for a road chipping top surface or sometimes directly. It is also used to waterproof roofing felt.
  15. For more products derived from crude oil other than fuels see ...

2c. Energy resource evaluation - What makes a good fossil fuel?

It may seem a curious question these days, but its only fair to consider what is the best fuel?

but some fossil fuels pollute more than others, and some leave a bigger 'carbon footprint'!

Factors that should be taken into consideration - often factors overlap

  • Combustion - burning characteristics (see also toxicity, pollution and climate change):

    • Ease of ignition - how easily does it burn, gas most easy to ignite and control, less so for petrol/diesel and coal the most difficult to ignite and control.

    • Energy value: e.g. kJ of heat energy released per kg, sometimes referred to as the 'energy density', the bigger the better.

    • Waste material: Burning hydrocarbons doesn't produce must waste other than the gaseous products, but burning coal produce smoke and large quantities of ash.

  • Availability: Geographical convenience - is it imported?, fluctuations in oil production levels and the market price.

    • The price of oil can vary with market forces determined by the World's economy AND political instability and wars, particularly in the major oil producing Middle-East Arab Gulf states affect the price too.

    • Therefore, without alternative energy resources, we are at the mercy of forces beyond our control.

    • If stocks or production rates fall, the price of crude oil rises and richer countries can afford more costly oil and can stockpile it, developing countries will struggle to compete.

    • In order to preserve our crude oil and gas supplies we sometimes compromise our ideals when dealing with the politics of countries we may think unsuitable.

  • Storage and distribution: Important health and safety issues to consider here e.g.

    • Coal is very dirty but safe to store, difficult to ignite,

    • Natural gas (explosive flammable gas) much more dangerous to store in large tanks under pressure, but the gas is easy and more convenient to distribute via pipes, but gas leaks are potentially VERY dangerous, as you see sometimes on the news!

    • Petrol is quite volatile and the flammable vapour easily ignited.

    • Diesel is not as volatile and central heating oil even less so, but spills of any of these fuels is potentially harmful to the environment or a fire hazard.

  • Production Costs:

    • Costs of exploration and extraction can be high for oil

    • Coal mines are dangerous to operate, good health and safety policies don't come cheaply, and its the same for operating oilfields and petrochemical complexes - oil refineries.

    • Costs of transporting the fuel.

    • AND even after considering all of these factors, as already mentioned ...

      • ... the market price can fluctuate with market forces and may also depend on political situations too!

      • eg the politics of the Middle East or European countries importing Russian natural gas etc.!

  • Toxicity, Pollution and climate change:

    • Greenhouse effect - which fuel produces the least or most carbon dioxide for the energy released?, methane is one of the best fossil fuels in this respect.

    • The sulphur content of fuel (most removed before fuel used to minimise sulphur dioxide and acid rain formation), some coals are very high in sulfur, but can't be removed from it, though you can clean the smoke by removing the sulfur dioxide using an alkali like calcium hydroxide (limewater).

    • The efficiency of combustion e.g. minimum carbon monoxide and soot levels, again methane is one of the most clean burning fuels.

    • Coal produces a lot of smoke and larger hydrocarbon fuels like oil, tend to burn with a sooty flame.

  • Ease of use: Transferred easily e.g. oil and gas readily piped around and readily ignited for a quick start in power station. Coal is more trouble to transport and does not ignite as easily.

  • See also

2d. Alternative fuels to fossil fuels - more on other energy resources


    • Apart from the obvious value of crude oil as an non-renewable energy source, should we using this very valuable source of organic chemicals by merely burning most of it?

    • AND how long will oil reserves last?

    • AND what happens if the oil runs out?

    • Therefore isn't it in our own interest to manage the finite oil reserves remaining and conserve them?

    • AND seek other sources of energy to power our lives?

    • AND perhaps influence the course of global warming?

    • BUT the trouble is, oil is very convenient, readily available, and new reserves are still being found and the oil and gas trapped in deep layers of shale are being exploited,

    • AND it will take time to develop new technologies.

    • The world's population is steadily increasing and countries like China and India have huge energy demands for home and industry, and are pressured into building many fossil fuel power stations, though China is a leading player in large scale wind power projects.

  • It should be noted that liquid fuels like petrol, diesel, central heating oil etc. are east to store and distribute to wherever they are need in homes or factories and they are so readily available, that change may be necessary, but progress will be slow.


    • Hydrogen gas can be used as fuel and a long-term possible alternative to fossil fuels.

    • It burns with a pale blue flame in air reacting with oxygen to be oxidised to form water.

      • hydrogen + oxygen ==> water

      • 2H2(g) + O2(g) ==> 2H2O(l) 

    • It is a non-polluting clean fuel since the only combustion product is water and so its use would not lead to all environmental problems associated with burning fossil fuels.

      • It is easily distributed in pipes like natural gas, but there are health and safety issues to do with storage and distribution since it is, like natural gas, highly flammable and explosive.

    • It would be ideal if it could be manufactured by electrolysis of water e.g. using solar voltaic-cells or some kind of but the technology is in its infancy.

    • Hydrogen can be used to power fuel cells see the "Extra Electrochemistry" page

  • Other alternative renewable fuels are discussed on other pages ...




Multiple Choice Quizzes and Worksheets

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (easier–foundation–level)

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (harder–higher–level)

KS4 Science GCSE/IGCSE m/c QUIZ on other aspects of Organic Chemistry

and (c) doc b 3 linked easy Oil Products gap–fill quiz worksheets

ALSO gap–fill ('word–fill') exercises originally written for ...

... AQA GCSE Science (c) doc b Useful products from crude oil AND (c) doc b Oil, Hydrocarbons & Cracking etc.

... OCR 21st C GCSE Science (c) doc b Worksheet gap–fill C1.1c Air pollutants etc ...

... Edexcel 360 GCSE Science Crude Oil and its Fractional distillation etc ...

... each set are interlinked, so clicking on one of the above leads to a sequence of several quizzes

ALL my Advanced Level Organic Chemistry revision notes


Revision notes on fractional distillation of crude oil uses of fractions KS4 Science GCSE/IGCSE/O level Chemistry Information on fractional distillation of crude oil uses of fractions for revising fractional distillation of crude oil uses of fractions for AQA GCSE Science, Edexcel Science chemistry IGCSE Chemistry notes on fractional distillation of crude oil uses of fractions OCR 21st Century Science, OCR Gateway Science notes on fractional distillation of crude oil uses of fractions WJEC gcse science chemistry notes on fractional distillation of crude oil uses of fractions CIE O Level chemistry CIE IGCSE chemistry notes on fractional distillation of crude oil uses of fractions CCEA/CEA gcse science chemistry (help for courses equal to US grade 8, grade 9 grade 10) science chemistry courses revision guides explanation chemical equations for fractional distillation of crude oil uses of fractions educational videos on fractional distillation of crude oil uses of fractions guidebooks for revising fractional distillation of crude oil uses of fractions textbooks on fractional distillation of crude oil uses of fractions explanations of fractional distillation of crude oil uses of fractions