Toxicity ranking in water pollution Among the following, which is considered the most lethal class of water pollutants to aquatic life at low concentrations?

Introduction / Context:
Different pollutants harm aquatic ecosystems through various mechanisms: toxicity, oxygen demand, pH alteration, turbidity, or eutrophication. Acute lethality at low concentrations is characteristic of certain organic and inorganic poisons that directly disrupt respiration or cellular metabolism in fish and invertebrates.

Given Data / Assumptions:

• Comparison is about acute lethality at relatively low concentrations.
• Typical receiving waters without extraordinary buffering or dilution.
• Focus on broad pollutant categories listed in the options.

Concept / Approach:
Phenols and cyanides are notorious for high aquatic toxicity. Cyanide forms undissociated hydrogen cyanide (HCN), a fast-acting respiratory poison. Phenolic compounds disrupt membranes and can be lethal in the mg/L range. Chlorine is also toxic, but it dissipates/reacts and is typically dosed deliberately at low levels in water treatment; in receiving waters it is regulated to very low residuals. Alkalis primarily shift pH, and suspended solids exert physical rather than strong toxic chemical effects. Therefore, the most lethal class listed is phenol and cyanide.

Step-by-Step Solution:

Verification / Alternative check:
Ecotoxicology references consistently rank cyanide among the most acutely toxic common industrial pollutants; many phenols also have low LC50 values for fish.

Why Other Options Are Wrong:

• Chlorine: Toxic but typically controlled; its persistence is limited by reactions.
• Alkalis: Harm via pH changes; toxicity depends on buffering; not inherently as lethal at low concentrations.
• Suspended solids: Cause turbidity/smothering rather than strong acute toxicity at very low levels.
• Phosphates: Drive eutrophication, not acute lethality.

Common Pitfalls:
Equating regulatory stringency (e.g., very low chlorine residuals) with inherent lethality ranking; the mechanisms and contexts differ.

Final Answer:
Phenol and cyanide