why does oil stain pavement?

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why does oil stain pavement?

In: Chemistry
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Staining is a process where color chemicals bind to the substrate with a chemical reaction.

Oil doesn’t truly stain pavement, it sort of gets it very dirty.

Concrete is a pourous material, so it will absorb anything you put on it (especially liquids). Especially if they are left there for a good amount of time, like an oil drip. The problem with concrete getting “oily and dirty” is two fold 1) oil has a color, so you see it and 2) to dissolve and take away oil, you need an organic solvent or soap and water. In the case of the latter, rain from the sky won’t clean it. So it stays around forever, and we call it a stain.

A secondary effect of oil left on concrete for a long time is that the impurities in motor oil may be solid, dark particles. Once a solid particle diffuses into a porous surface, it is very difficult to get them back out. This would all depend on the age of the oil, how dirty the engine is, etc.

The key lesson here is that if you wait too long to clean that oil, it might never be possible to clean it, it is “irreversibly dirty” and thus we say it is “stained.”