What is Petroleum? - Definition & Uses

Instructor: Danielle Reid

Danielle has taught middle school science and has a doctorate degree in Environmental Health

Asphalt streets you walk on and fuel you use to power jets are all made from a liquid named petroleum. Continue reading to learn about petroleum, understanding what it is and how it's used.

Definition of Petroleum

Have you ever walked by a street, being paved with asphalt, taken a whiff and frowned because of the obnoxious chemical smell? The liquid material used to make asphalt (and what contributes to that offensive smell) is called petroleum. Petroleum is a liquid found deep within the Earth's surface. It's categorized as naturally occurring and is collected from drilling into the Earth. Petroleum is often described as brown or black in color.

Although it's commonly found in liquid form, underneath the Earth's surface, it can also be in the form of a gas. A natural gas is a naturally-occurring combustible mix of different hydrocarbons found below the Earth's surface. Combustible refers to the ability for this gas to be explosive or easily ignite a fire. A hydrocarbon is any type of substance that's got carbon atoms and hydrogen atoms present. The petroleum you see when you drill down will either flow out as a liquid (oil) or escape into the air as a gas. Diagram 1 illustrates this concept.

Diagram 1: Illustration Of Petroleum (Liquid and Gas) Below Earth

Petroleum Chemistry 101

The basic elements carbon, hydrogen, nitrogen, oxygen, and even metals are found in petroleum. Petroleum exists in two different forms: natural gas or liquid. When petroleum is released from the Earth as a liquid it's referred to as crude oil. Crude oil is a type of liquid that contains a very complex mixture of different hydrocarbons. The typical hydrocarbons you will find in crude oil include alkenes, cycloalkanes, and aromatic hydrocarbons.

It's not important to memorize a long list of hydrocarbons contained in crude oil. Just remember that the composition of liquid petroleum consists of a wide variety of molecules that contain carbon and hydrogen atoms. An example of what an aromatic hydrocarbon looks like is shown in diagram 2. The highlighted portion indicates why this structure is called a hydrocarbon (i.e. presence of hydrogen and carbon atoms).

Diagram 2: Structure Of The Hydrocarbon Benzene

When it's released as a natural gas, petroleum is somewhat similar to crude oil. Can you guess why the two compositions would be similar? They both contain hydrocarbons in their composition! The difference with natural gas is that it's primarily composed of alkanes. Diagram 3 shows some of the different types of alkanes found in petroleum's natural gas. An interesting fact regarding this diagram is that an estimated 80% of petroleum's natural gas contains methane. Without being too crude, that certainly gives cow pastures a run for their money in terms of smell.

Diagram 3: Structures Of Different Hydrocarbons - (a) Methane (b) Ethane and (b) Propane
natural gas

Petroleum Uses

The most common example of petroleum use (whether it's a gas or liquid) is with cars. I'm sure you know firsthand what happens when your car runs out of fuel. But I bet you didn't know that the fuel used to gas up your car originally started out as crude oil! Before that fuel made its way to your favorite gas station it had to be refined.

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