What is Crude Oil
Crude oil is a thick, dark liquid mixture of hydrocarbons found deep underground. It contains thousands of different molecules of various sizes, from very small to very large. Before it can be used as gasoline, these molecules must be separated and processed.
Fractional Distillation Process
The refining process begins by heating crude oil to about 350 degrees Celsius, which turns most of it into vapor. This hot vapor is then pumped into a tall column called a fractionation tower that is hotter at the bottom and cooler at the top. As the vapors rise through the tower, they cool down. Molecules with low boiling points stay as gases and rise higher, while molecules with high boiling points cool down quickly and condense back into liquids lower in the tower. Different products collect at different levels based on their molecular size and boiling point.
Gasoline Collection
Gasoline collects in the middle sections of the fractionation tower. It is made up of hydrocarbons with 5 to 12 carbon atoms in their chains. These molecules have boiling points in the range needed to make efficient fuel for car engines. Workers collect the gasoline from specific trays in the tower where the temperature is just right for these molecules.
Cracking Process
To produce more gasoline, refineries use a process called cracking. This involves breaking apart larger molecules from heavier oil fractions using heat and sometimes a catalyst. Cracking converts these larger, less useful molecules into smaller ones that can be used as gasoline. This step increases the total amount of gasoline that can be obtained from a barrel of crude oil.
Adding Additives
After the gasoline is separated and processed, refineries add various additives to improve its performance. These may include detergents to keep engines clean, antioxidants to prevent gum buildup, and corrosion inhibitors to protect fuel systems. Some regions also require ethanol to be blended into gasoline for environmental reasons. 
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