Potassium permanganate (KMnO₄) field check: The disappearance of the pink colour in well water after adding KMnO₄ indicates the presence of what contaminant class?

Introduction / Context:
KMnO₄ is a strong oxidant. A quick field cue is the persistence or disappearance of its pink tint when dosed into water. This qualitative observation helps infer the presence of oxidizable constituents in the sample.

Given Data / Assumptions:

• A small dose of KMnO₄ is added to the well water sample.
• Pink colour disappears quickly.
• No sophisticated instrumentation is assumed.

Concept / Approach:

Organic matter and certain reduced inorganic species consume KMnO₄, decolourizing the solution. If oxidizable substances are absent or minimal, a faint pink persists for some time. Thus, colour loss indicates a demand for oxidant due to organics or other reducing agents.

Step-by-Step Solution:

Verification / Alternative check:

Permanganate demand tests in laboratories quantify this effect. Utilities sometimes apply pre-oxidation with permanganate to address taste/odour and oxidizable contaminants.

Why Other Options Are Wrong:

• Acidity/alkalinity do not directly and rapidly consume KMnO₄ in the same indicative way.
• Turbidity refers to particulates; it does not necessarily decolourize an oxidant.
• “All the above” overgeneralizes beyond the chemical behavior involved.

Common Pitfalls:

• Assuming colour loss always means only organics; some reduced inorganics can also consume oxidant.
• Using this as a quantitative measure—it's qualitative without a controlled test.

Final Answer:

Organic matter (oxidizable substances).