The distinction between fat and oil is mainly a physical one. If the substance is solid at ambient temperature, it is called fat. If it is liquid, it is called oil. This distinction is merely for convenience since all oils solidify when the temperature is made low enough and all fats melt if the temperature is made high enough. This temperature-based distinction between oils and fats is not precise since ambient temperatures vary depending on location. Countries in the northern hemisphere have lower ambient temperatures while countries near the equator have higher. Thus what may be usually called “fat” in a cold country may be called “oil” in a warm tropical country. In addition, it is more precise to use a melting range for plant oils rather than a melting point since the fat melts into oil within a certain range of temperatures. Oils and fats have a number of common physical and chemical properties. Their common physical properties are: (a) they float on water and are not soluble; (b) they have lubricating properties and are greasy to the touch; and (c) they leave little or no residue when burned. Their common chemical characteristic is that they may be converted into glycerin and one or more fatty Acids.

When oils and fats are split into glycerin and fatty acids, the resulting mixture contain three molecules of fatty acid for each molecule of glycerin. Historically, it is because of this proportion of acid to glycerin that the chemical compounds found in the oil or fat are called triglycerides (tri = three). Fats and oils are basically mixtures of varying amounts of triglycerides. Triglyceride plant oils include not only edible, but also inedible plant fats and oils such as linseed oil, tung oil, and castor oil, which are generally used in lubricants, paints, cosmetics, pharmaceuticals and other industrial products. Although plant oils are generally thought of as esters of glycerin and a varying blend of fatty acids, they in fact contain free fatty acids and diglycerides as well.

While there are a large number of triglycerides in nature, the triglycerides of seven acids, namely, lauric, myristic, palmitic, stearic, oleic, linoleic, and linolenic, constitute the majority of natural fats and oils. In certain fats and oils, one triglyceride usually predominates, and another triglyceride in other fats and oils. But in some fats and oils, several triglycerides may be present in significant proportions. The properties of fats and oils, therefore, depend on the characteristics of the triglycerides of which they are mixtures and the relative proportion of the triglycerides present. Since fats and oils are merely physical mixtures of triglycerides, it is possible in most cases to separate them more or less completely into their component triglycerides by simple mechanical means such as chilling and application of pressure.