Essential oils are made of volatile aromatic compounds. Volatile aromatic compounds are small organic molecules that tend to change from the liquid state to the gas state at room temperature. These molecules are so incredibly small that a single drop of essential oil contains around 40,000,000,000,000,000,000 (40 million trillion) of them.
The word “volatile” emphasizes their tendency to evaporate quickly at room temperature. This property is what makes them smell so potent. When you first open a bottle of essential oil, you instantly notice the aroma, and you can smell it even from a distance. The physical and chemical properties of volatile aromatic compounds allow them to quickly enter the gas state, move through the air, and directly interact with olfactory sensors in the nose. Essential oils can be made up of anywhere between 1 and 1000 different compounds with different chemical identities. For example, Birch oil is almost entirely composed of one compound: methyl salicylate. Spikenard, on the other hand, contains hundreds of compounds.
Most oils fall somewhere in between these two extremes. For instance, Frankincense essential oil contains over 65 distinct chemical compounds in various quantities. The different compounds in an essential oil are known as constituents. Each constituent has its own distinct structure, meaning that the shape, size, and arrangement of chemical bonds in that molecule is unique.
The different constituents in an essential oil determine both the oil’s aroma and the benefits it offers. The exact composition of an essential oil varies between plant species. When speaking of essential oils, the word “composition” refers to the oil’s constituent makeup or, in other words, what chemical constituents it contains and how much of those constituents are present.
For instance, Bergamot essential contains over 35 different compounds, but it has especially high levels of two constituents called limonene and linalyl acetate. Blue Tansy essential oil, on the other hand, contains over 50 compounds, with the two most abundant constituents being chamazulene and sabinene.