Laundry and water quality: brown spotting on washed fabrics is most commonly caused by which dissolved constituent present in high concentration?

Introduction / Context:
Household water quality directly affects laundering outcomes. Elevated concentrations of certain dissolved metals can stain fabrics during washing or drying. This question tests practical understanding of iron-related staining phenomena in domestic water supplies.

Given Data / Assumptions:

• Observation: brown spots on fabrics after washing.
• Source: dissolved constituents in supply water.
• No specialty chemicals or dyes are assumed.

Concept / Approach:
Iron in water can exist as ferrous (Fe2+) or ferric (Fe3+) forms. Upon aeration or oxidation (e.g., exposure to oxygen, chlorine), ferrous iron converts to ferric hydroxide particulates, which deposit on fabrics, creating yellow-brown to rust-colored stains.

Step-by-Step Solution:

Verification / Alternative check:
Practical mitigation includes installing iron removal units, using appropriate oxidants and filtration, or using sequestration agents. Consistency with known domestic water complaints supports the identification of iron as the culprit.

Why Other Options Are Wrong:
Zinc: Typically colorless at normal levels; not a brown stain former.
Iodine: Can cause yellowish tints at high levels but is uncommon in this context.
Bromine: Rare in domestic supplies and would not produce rust-brown spots.

Common Pitfalls:
Confusing tannins or manganese staining with iron; however, manganese tends to produce darker blackish stains, while tannins may cause yellow tea-like discoloration rather than distinct rust spots.

Final Answer:
Iron