Petrol (gasoline) is not typically black. Crude oil, however, is black. The confusion likely stems from misunderstanding the refining process. While crude oil itself is dark, petrol, after refining, is generally a light yellow or amber color. The dark color of crude oil comes from complex, multi-cyclic naphthenes and other heavy hydrocarbons similar to those found in coal.
Here's a breakdown:
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Crude Oil: As mentioned, crude oil's dark color is due to the presence of heavy, complex hydrocarbon molecules.
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Refining Process: Crude oil is refined through a process called fractional distillation. This process separates the various components of crude oil based on their boiling points.
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Petrol as a Fraction: Petrol is a lighter fraction that boils off at lower temperatures. When properly refined, this fraction is typically clear or lightly colored.
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Residue: The heavier, dark components, including the naphthenes, remain at the bottom of the distillation column and are further processed into things like asphalt and lubricating oils. If these heavier components are not completely removed during the refining process, they could potentially darken the petrol, but this is undesirable and typically corrected.
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Additives: Sometimes, dyes are added to petrol for identification or branding purposes. These dyes can alter the color, but they do not cause petrol to naturally be black. For example, leaded gasoline used to be dyed red.
In summary, petrol should not be black. The black color is characteristic of crude oil and heavier hydrocarbon fractions which are separated from petrol during the refining process.
