Despite the distillation and purification processes, petroleum products contain traces of metals such as nickel, vanadium, and copper, along with small amounts of sulphur. Lead used to be added to petrol to improve engine efficiency. The practice, which began in the 1920s, was eventually phased out in the 1980s, and leaded petrol was banned completely in the UK in 1999 amid concerns over toxic emissions.
This all means that petrol is a homogenous mixture of hydrocarbons and trace impurities. Distilled and processed from crude oil, it consists of around 150 different natural components including butane, pentane, and benzene.
Read on to learn more about the chemicals and other components that are found in petrol
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Chemicals found in petrol
Petrol, also known as gas or gasoline, is a complex mixture of various hydrocarbons. There are approximately 150 types of hydrocarbons in petrol. These include butane, pentane, isopentane, benzene, ethylbenzene, toluene, and xylene.
Key Takeaways
Petrol is mainly a mixture of hundreds of hydrocarbons
Petrol also has some traces of impurities such as metals (e.g. lead, antimony, cobalt, and mercury)
Lead is no longer added to petrol during the distillation process, but natural traces can still be found
Small amounts of nitrogen can also be found in petrol, but these are mainly from the ambient air
Isoalkanes and aromatics primarily compose petrol, along with other types of hydrocarbons
Hydrocarbons
Hydrocarbons are naturally occurring components of petrol. Their similar boiling points are a sign that they evaporated and condensed together at a certain level of the fractional distillation process.
While hydrocarbons are naturally present in petrol, other chemicals such as anti-rust and anti-icing agents are added to the mixture to improve the engine’s performance.
Some of the main types of chemicals found in petrol are outlined below.
1. Hydrogen
As the name suggests, hydrogen is an integral part of hydrocarbons. The type of hydrocarbon changes depending on the proportion of hydrogen and carbon. For example, alkanes are classified as saturated hydrocarbons.
Each carbon of an alkane has the maximum number of hydrogens bonded to it, meaning it contains only single bonds. Alkenes, however, have at least one pair of carbon that have double bonds.
2. Carbon
Carbon atoms form a chain or rings of hydrocarbons. They can either be arranged in a simple chain or they could be branched isomers. In some cases, carbons may form double bonds and triple bonds with other carbons. Ring or cyclic hydrocarbons like benzene are sometimes called aromatic compounds because of their distinct smell.
3. Oxygen
As petrol has already undergone distillation, it only contains a trace amount of oxygen (usually between 0.1 – 1.0% w/w). When petrol is ignited, it combines with oxygen in a highly exothermic reaction known as combustion. If combustion is complete, the byproducts are carbon dioxide and water. However, if the combustion process is not complete or efficient, carbon monoxide can also be produced.
4. Nitrogen
Trace amounts of nitrogen can be found in petrol too. The internal combustion engines in cars and other vehicles also produce nitrogen oxide, which forms smog. However, most of the nitrogen comes from the ambient air and not from the fuel itself. The reaction is facilitated by the heat produced by fuel combustion. 
5. Metals
Although lead is no longer added to petrol during the distillation process, some traces of it can still be found in petrol. Other traces of metals include the following:
- Antimony
- Arsenic
- Beryllium
- Cadmium
- Chromium
- Cobalt
- Manganese
- Mercury
- Molybdenum
- Nickel
- Selenium
- Vanadium
What is the chemical formula for petrol?
As petrol is a complex mixture of hydrocarbons there is no single chemical formula. Nonetheless, you can use the generic formula CnH2n+2 for alkanes only.
Petrol is primarily composed of isoalkanes and aromatics, as shown in the percentage breakdown below:
- 4-8% alkanes
- 2-5% alkenes
- 25-40% isoalkanes
- 3-7% cycloalkanes
- 1-4% cycloalkenes
- 20-50% total aromatics (0.5-2.5% benzene)
What is the chemical name for petrol?
Just like the chemical compositions of petrol as a mixture, there is no single chemical name for petrol. However, it can be generically called a hydrocarbon mixture.
The difference in chemicals between petrol and diesel
The proportions of hydrocarbons in petrol and diesel are different; diesel is made up of approximately 75% aliphatic hydrocarbons and 25% aromatic hydrocarbons. It’s more viscous and less volatile than petrol, making it harder to ignite under normal pressure. Diesel also contains more energy than petrol. 
Conclusion
Petrol is a complex homogenous mixture of chemicals. It primarily contains hydrocarbons, but some traces of metals, oxygen, and nitrogen are also found in petrol. The hydrocarbons in petrol have similar boiling points, allowing them to be separated during fractional distillation process. There is a wide variety of hydrocarbons in petrol, but they are primarily aromatics and isoalkanes. The aromatics are cyclic hydrocarbons, while isoalkanes are the branched-chain alkanes.
