Unlike other oils and fats used in soapmaking, essential oils are not fatty acids. They evaporate readily, leaving no mark on a piece of paper. If you buy an “essential oil” and wonder if it is pure or not, drip a drop onto a piece of white paper. If it leaves a greasy mark after it has evaporated, it is probably impure.

Essential oils contain a number of chemical compounds with names that sound like something you wouldn't want anywhere near you. Primarily, essential oils contain a kind of chemical called a “terpinoid.” There are many kinds of these, including ketones, aldehydes, and esters, all of which are variously arranged hydrocarbons.

If you decide to study the chemistry of essential oils, you'll find that there are often similar scents and therapeutic properties to oils that have a similar chemical makeup. For example, melissa, lemongrass, and lemon eucalyptus, although unrelated plants, all contain certain aldehydes that smell lemony. Roman chamomile, lavender, clary sage, and bergamot, some of the most commonly used essential oils, are all related through esters. Although they do not smell the same, these oils have similar effects on the body, and they blend well with each other.