Oils, any of several chemicals that form the odoriferous essences of a number of plants. The term essential oil is also applied to similar synthetic substances prepared from coal tar chemicals, and semi-synthetic substances prepared from natural essential oils (see Chemistry, Organic; Petroleum).

SOURCES  
Essential oils come from the flowers, fruits, leaves, roots, seeds, and bark of many plants. Oil of lavender, for example, is derived from a flower, oil of patchouli from a leaf, and oil of orange from a fruit. The oils are formed in the green (chlorophyll bearing) parts of the plant, and with plant maturity are transported to other tissues, particularly to the flowering shoots. The exact function of an essential oil in a plant is unknown; it may be to attract insects for pollination, or to repel harmful insects, or it may be simply a metabolic intermediate.

The essential oils are volatile liquids, mostly insoluble in water, but freely soluble in alcohol, ether, and vegetable and mineral oils. They are usually not oily to the touch. They may be grouped into five classes, according to their chemical structure: alcohols, esters, aldehydes, ketones, and lactones and oxides.

 PRODUCTION AND USES  
Essential oils are obtained by one of four methods: steam distillation, extraction by volatile solvents, expression by hand or machine, and enfleurage, a process in which fat is used as a solvent. In modern practice, essential oils, synthesized or obtained from natural sources by any one of the above four methods, are often purified by vacuum distillation.

Essential oils are used to impart flavour and delicate aroma to coffee, tea, wines, and distilled liquors. They are the basic ingredients in the manufacture of perfumes, and are also used in soaps, disinfectants, and similar products. Essential oils have an important use in medicine, either for their flavouring qualities or for their pain-relieving and physiological value. See articles on many of the specific essential oils mentioned.