Inorganic material that can contribute to true color in water?

• A. Dyes.
• B. Iron.
• C. Fuels.
• D. Bacteria.

Answer: (B)

True color in water comes from substances dissolved in the water that directly absorb light, rather than from particles scattering light. Iron fits this because, when dissolved as ferrous (Fe2+) or ferric (Fe3+) iron, it is inorganic and colors the water itself. The color you might notice can range from pale to yellow, amber, or brown, depending on how much iron is present and its oxidation state. As iron oxidizes and forms solid precipitates, you can also get turbidity and staining, but the initial true color arises from dissolved iron.

Dyes are organic pigments, so they’re not the inorganic material in question. Fuels are organic compounds. Bacteria are biological and can influence color indirectly, but they aren’t inorganic dissolved substances that produce true color.