Utah tar sands

The words, tar, asphalt, bitumen, pitch, and heavy oil often have been used to refer to highly viscous liquid, semi-solid, or solid, naturally occurring hydrocarbon substances. The word bitumen originated with the Romans and included all the various types of natural hydrocarbons. In its modern, common, usage it is understood to be synonymous with petroleum in both its liquid and solid forms. Commercially, the word bitumen refers only to the solid and semisolid hydrocarbon compounds. The definition of asphalt varies but, according to the American Society of Testing Materials, asphalts are black to dark-brown solid or semisolid materials which liquefy when heated, in which the predominating constituents are bitumens which occur in nature or are derived by the partial evaporation and distillation of petroleum. Most definitions of tar tend to relate it to the residue that results the natural sense it has come to mean asphalt of bitumen but generally, the word tar is used when the substance has less of a solid consistency and is more of a heavy, viscous oil.