In water quality testing, brown stains that appear on metal vessels and clothes usually indicate the presence of a high concentration of which dissolved metal in the water supply?

Introduction / Context:
Staining of household vessels, bathroom fittings, and clothes is a common practical indicator of water quality problems. Different dissolved metals leave different coloured stains, so questions on this topic often appear in general science and environmental awareness exams. Brown or brownish black stains usually point towards specific metallic ions rather than ordinary hardness caused by calcium or magnesium. This question asks which dissolved metal, when present in high concentration, typically produces these characteristic brown stains.

Given Data / Assumptions:

• We are dealing with visible brown staining on vessels and clothes due to water used for washing and cleaning.
• The possible dissolved metals suggested are magnesium, calcium, manganese, and chromium.
• The question focuses on colour of staining, not just on hardness or toxicity.
• We assume a domestic water supply with no unusual industrial contamination unless specified.

Concept / Approach:
Different metal ions leave different typical stains. Calcium and magnesium are mainly responsible for hardness of water, producing white or grey scale deposits and soap scum rather than dark stains. Iron can cause reddish brown staining, while manganese tends to cause brown to black stains, especially on laundry and fixtures. Among the options given, manganese is the metal most closely associated with brown or black staining when present in elevated levels. Chromium pollution usually raises separate toxicity concerns and does not commonly present as uniform brown staining in domestic settings.

Step-by-Step Solution:
Step 1: Recall that magnesium and calcium ions cause hardness of water and usually lead to white or chalky deposits, not pronounced brown stains on fabrics. Step 2: Remember that dissolved manganese, when oxidised, forms dark brown to black manganese oxide deposits that cling to surfaces and cloth. Step 3: Compare the described brown stains from the question with the known colour effects of manganese deposits in plumbing and laundry. Step 4: Note that chromium contamination is more commonly discussed in relation to health hazards than simple household staining patterns. Step 5: Conclude that high levels of manganese ions best explain the brown stains observed and choose manganese as the correct answer.

Verification / Alternative check:
Water treatment guides for household users often classify brown or black staining of sinks, toilets, and clothing as a typical symptom of manganese rich water. They recommend specific filters or oxidation methods to remove manganese. In contrast, hardness caused by calcium and magnesium is described as scale formation on utensils, bathroom tiles, and inside kettles. Iron rich water can produce reddish or orange stains, which are visually distinct from the darker brown or black stains of manganese. This pattern of evidence supports manganese as the main cause of the stains described.

Why Other Options Are Wrong:
Magnesium ions primarily contribute to hardness and scale but do not usually produce brown stains on cloth. Calcium ions behave similarly, causing scale and soap scum rather than dark staining. Chromium ions can be present due to industrial pollution, but they are not commonly associated in basic general knowledge questions with simple brown stains on household items. Therefore, these alternatives do not match the typical visual symptom described in the question.

Common Pitfalls:
One common mistake is to assume that any water quality issue must be due to hardness and therefore select calcium or magnesium automatically. Another mistake is to confuse iron and manganese staining; while both can discolour surfaces, iron more often produces orange or reddish tones. To avoid such confusion, remember that manganese is the classic cause of brown to black staining of laundry and fixtures. Associating specific stain colours with particular metals will help in similar exam questions on water quality.

Final Answer:
Brown stains in vessels and clothes usually indicate a high concentration of manganese ions causing brown or black staining in the water supply.