Which of the following causes black stain on the plumbing fixtures?

• A. Iron
• B. Zinc
• C. Manganese
• D. Copper

Answer: (C)

Black staining on plumbing fixtures is usually caused by manganese in the water. When manganese is present and the water is exposed to oxidizing conditions (like air or added oxidants), manganese ions oxidize and form manganese dioxide, a dark precipitate that clings to surfaces. That solid manganese dioxide appears as a black or dark brown stain on sinks, tubs, and fixtures.

Iron can cause staining as well, but its color is typically reddish-brown rather than black. Copper staining tends to be blue-green, especially on chrome or aluminum surfaces, due to copper ions. Zinc deposits are usually white or dull gray and not the dark staining associated with manganese. Understanding this helps diagnose water quality and informs treatment options aimed at removing manganese, such as oxidation followed by filtration.