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The systematic exploitation of crude oil deposits, however, did not begin until the mid-19th century. “Black gold,” as the raw material is also called due to its versatile usability, consists of at least 500 components. In addition to a large range of hydrocarbons (alkanes, cycloalkanes and aromatics) and naphthenic acids, phenols, resins, aldehydes and organic sulfur compounds (e.g. thioalcohols), crude oil also contains nitrogen, oxygen, and traces of the metals vanadium and nickel. Its color varies from light yellow to black, and it darkens when exposed to light. Fresh crude oil often has a yellow to green-blue fluorescence. Depending on its sulfur content, oil is classified as “sweet” or “sour,” with “sour” crude oil having a disagreeable garlicky odor due to its high sulfur content. Its density is between 0.82 and 0.94 grams per cubic centimeter, and prolonged contact with air increases its viscosity because the volatile components gradually evaporate. It readily dissolves in ether, benzene, or carbon tetrachloride; but dissolves poorly in ethanol, and not at all in water.
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