Saturated vs. Unsaturated

If you look at palmitic acid and stearic acid chains, you can see that the carbon chains are completely and evenly filled with hydrogen atoms.

In other words, the chains are saturated with hydrogen. Fats (triglycerides) that contain palmitic acid and stearic acid are therefore known as saturated fats. Fats made up of saturated fatty acids are solid at room temperature. You can also see that oleic acid is not saturated. Two of the carbons are connected by a double bond, and two of the hydrogens are missing. This fatty acid is unsaturated. Fats that have a lot of oleic acid in them are liquid at room temperature, and are therefore known to us as oils.

Oleic acid, because it contains one double bond, is also referred to as mono-unsaturated. Fatty acids that have multiple double bonds, like linoleic acid in the first figure, are called polyunsaturated. Polyunsaturated fats are also liquid at room temperature.

If you have a bottle of corn oil, what you have is a bottle of polyunsaturated oil with a high concentration of linoleic acid. Because it is polyunsaturated, it is liquid at room temperature. If you would like to solidify it and turn it into margarine, what you do is hydrogenate it. That is, you saturate it with hydrogen by breaking the carbon double bonds and attaching hydrogen. To do this, you heat the oil and add pressurized hydrogen gas and a nickel catalyst. In this way, you create "partially hydrogenated vegetable oil." PHVO is the main ingredient in things like vegetable shortening and margarine.