Uploaded October 2020 | Updated September 2026, 2 weeks ago
Here is an example of crude oil, probably from under the North Sea. Crude oil can be obtained directly by drilling under certain areas of the Earth’s surface. It is a thick, black liquid. In Chemistry, a thick, honey-like liquid is described as being ‘viscous’. Crude oil is a mixture of hydrocarbons of various carbon chain lengths. It is described as a fossil fuel. This is because it is formed when large quantities of dead organisms, mostly zooplankton and algae, are buried underneath sedimentary rock and subjected to both intense heat and pressure. This process takes millions of years.
As an example of how much crude oil can change, here is a different sample. This is called light crude oil. This is much runnier and is characteristic of crude oil from Texas, in the USA. Because crude oil is a mixture, it can be separated using a technique called fractional distillation. Unlike what you may have seen in the past, the mixture is not separated into pure substances: it is separated into smaller mixtures of hydrocarbons with similar carbon chain lengths. These smaller mixtures are called ‘fractions’.
On the industrial scale, the process of fractional distillation occurs in an oil refinery. For example, there is a large oil refinery in Grangemouth, near Edinburgh. An oil refinery contains a special building called a fractionating tower. This is represented in the diagram.
The crude oil is heated at the bottom of the tower to around 350 0C. This turns it into a gas. This mixture of gases then moves up the tower. The tower is hotter at the bottom and cooler at the top. Parts of the mixture condense when the temperature falls below their boiling point. These structures are called ‘bubble caps’: they allow gases to travel up the tower, but prevent liquids running back down.
The fraction with the lowest boiling point is called refinery gas. This is a mixture of hydrocarbons with 1-3 carbon atoms per molecule. This can be used to make the natural gas, used in Bunsen burners and oven hobs. It does not naturally smell. The smell is added later by added a gas called methanethiol, also known as mercaptan. This smells strongly of rotten eggs and is intended to alert us to potentially explosive gas leaks.
The next fraction is called gasoline, also known as motor spirit or petrol. This is widely used as the fuel in car engines. This is a mixture of hydrocarbons with approximately 5-10 carbon atoms per molecule. It condenses at around 120 0C.
The next fraction is called kerosene, also known as paraffin, is widely used as a fuel in jet engines, and some rocket engines. This is a mixture of hydrocarbons with approximately 10-16 carbon atoms per molecule. It condenses at around 170 0C.
The next fraction is called diesel, also known as gas oil. This is widely used in diesel car engines. These are very different to petrol engines because they do not contain a spark plug: the fuel is ignited by pressure alone. For tax reasons, any diesel not used for road vehicles in the UK has a red dye added. Diesel is a mixture of hydrocarbons with approximately 14-20 carbon atoms per molecule. It condenses at around 270 0C.
The next fraction is called fuel oil, also known as furnace oil. This is commonly burned in the engine boilers of large ships. This is a mixture of hydrocarbons with approximately 20-70 carbon atoms per molecule. It condenses at around 600 0C.
The next fraction is called bitumen, also known as asphalt. This is a sticky, black and highly viscous liquid. The primary use (70%) of bitumen is in road construction, where it is used as the glue, or binder that holds gravel together. It is also used in the production of roofing felt and for sealing flat roofs. Bitumen is a mixture of hydrocarbons with over 70 carbon atoms per molecule. It has a very high boiling point.
The contents of these fractions can vary, depending on the source of the crude oil and the set-up of the fractionating tower. Intermediate fractions can also be produced. For example, naphtha is a common fraction that is normally produced between refinery gas and gasoline. Naphtha is also known as mineral turpentine or white spirit. It is used primarily to clean paint brushes and as the fuel in cigarette lighters and barbeque starting fluid. It is also an important feedstock in the synthesis of plastics. Naphtha is a mixture of hydrocarbons with approximately 5-9 carbon atoms per molecule. It condenses at around 70 0C.
The fractions are lined up here with gasoline on the left to bitumen on the right. You can clearly see that as the carbon chain length increases, the colours become darker. The fractions also become more viscous and their boiling points increase.
Here is an example of crude oil, probably from under the North Sea. Crude oil can be obtained directly by drilling under certain areas of the Earth’s surface. It is a thick, black liquid. In Chemistry, a thick, honey-like liquid is described as being ‘viscous’. Crude oil is a mixture of hydrocarbons of various carbon chain lengths. It is described as a fossil fuel. This is because it is formed when large quantities of dead organisms, mostly zooplankton and algae, are buried underneath sedimentary rock and subjected to both intense heat and pressure. This process takes millions of years.
As an example of how much crude oil can change, here is a different sample. This is called light crude oil. This is much runnier and is characteristic of crude oil from Texas, in the USA. Because crude oil is a mixture, it can be separated using a technique called fractional distillation. Unlike what you may have seen in the past, the mixture is not separated into pure substances: it is separated into smaller mixtures of hydrocarbons with similar carbon chain lengths. These smaller mixtures are called ‘fractions’.
On the industrial scale, the process of fractional distillation occurs in an oil refinery. For example, there is a large oil refinery in Grangemouth, near Edinburgh. An oil refinery contains a special building called a fractionating tower. This is represented in the diagram.
The crude oil is heated at the bottom of the tower to around 350 0C. This turns it into a gas. This mixture of gases then moves up the tower. The tower is hotter at the bottom and cooler at the top. Parts of the mixture condense when the temperature falls below their boiling point. These structures are called ‘bubble caps’: they allow gases to travel up the tower, but prevent liquids running back down.
The fraction with the lowest boiling point is called refinery gas. This is a mixture of hydrocarbons with 1-3 carbon atoms per molecule. This can be used to make the natural gas, used in Bunsen burners and oven hobs. It does not naturally smell. The smell is added later by added a gas called methanethiol, also known as mercaptan. This smells strongly of rotten eggs and is intended to alert us to potentially explosive gas leaks.
The next fraction is called gasoline, also known as motor spirit or petrol. This is widely used as the fuel in car engines. This is a mixture of hydrocarbons with approximately 5-10 carbon atoms per molecule. It condenses at around 120 0C.
The next fraction is called kerosene, also known as paraffin, is widely used as a fuel in jet engines, and some rocket engines. This is a mixture of hydrocarbons with approximately 10-16 carbon atoms per molecule. It condenses at around 170 0C.
The next fraction is called diesel, also known as gas oil. This is widely used in diesel car engines. These are very different to petrol engines because they do not contain a spark plug: the fuel is ignited by pressure alone. For tax reasons, any diesel not used for road vehicles in the UK has a red dye added. Diesel is a mixture of hydrocarbons with approximately 14-20 carbon atoms per molecule. It condenses at around 270 0C.
The next fraction is called fuel oil, also known as furnace oil. This is commonly burned in the engine boilers of large ships. This is a mixture of hydrocarbons with approximately 20-70 carbon atoms per molecule. It condenses at around 600 0C.
The next fraction is called bitumen, also known as asphalt. This is a sticky, black and highly viscous liquid. The primary use (70%) of bitumen is in road construction, where it is used as the glue, or binder that holds gravel together. It is also used in the production of roofing felt and for sealing flat roofs. Bitumen is a mixture of hydrocarbons with over 70 carbon atoms per molecule. It has a very high boiling point.
The contents of these fractions can vary, depending on the source of the crude oil and the set-up of the fractionating tower. Intermediate fractions can also be produced. For example, naphtha is a common fraction that is normally produced between refinery gas and gasoline. Naphtha is also known as mineral turpentine or white spirit. It is used primarily to clean paint brushes and as the fuel in cigarette lighters and barbeque starting fluid. It is also an important feedstock in the synthesis of plastics. Naphtha is a mixture of hydrocarbons with approximately 5-9 carbon atoms per molecule. It condenses at around 70 0C.
The fractions are lined up here with gasoline on the left to bitumen on the right. You can clearly see that as the carbon chain length increases, the colours become darker. The fractions also become more viscous and their boiling points increase.
