Coal tar

Coal tar is a thick, dark liquid formed when coal is heated without air. Its complex mixture of compounds supplies industrial chemicals and ingredients for some medical treatments.

Destructive distillation: The chemical decomposition of a material by heating it without enough oxygen for combustion. Coal tar forms during the oxygen-limited heating of coal.
Coal-tar pitch: A viscous or solid residue left after distilling coal tar. This heavy fraction is used in binders, roofing, and carbon-product manufacturing.
William Murdoch: A Scottish engineer who pioneered practical coal-gas lighting in the late eighteenth century. His gas-making work helped establish industries that produced coal tar on a large scale.
Coal-tar contamination: Environmental pollution caused by releases of coal tar or its chemical constituents. Former gasworks and industrial sites can retain tar residues in soil and groundwater.
Coal gasification: The conversion of coal into combustible gas through reactions with air, oxygen, or steam at high temperatures. Gasification processes can produce tar alongside gas and other byproducts.
Naphthalene: A two-ring aromatic hydrocarbon used in chemical manufacturing and formerly common in moth repellents. Coal-tar fractions provided an early industrial source of naphthalene.
Friedlieb Ferdinand Runge: A German chemist who isolated several organic compounds, including aniline, from coal tar in the nineteenth century. His analyses demonstrated the chemical richness of coal-tar mixtures.
Benzo[a]pyrene: A polycyclic aromatic hydrocarbon formed during incomplete combustion and classified as a human carcinogen. It can occur in coal-tar mixtures and contributes to concerns about exposure.
Fractional distillation: A separation method that sorts a liquid mixture by differences in boiling points. Distillation separates coal tar into fractions with different boiling ranges.
Anthracene: A three-ring aromatic hydrocarbon used chiefly as a precursor to dyes and other chemicals. Its recovery from coal tar helped supply the synthetic dye industry.

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