Q.13 ‘A harmful substance persists in the environment for a very long period of time’. The
UNACCEPTABLE statement for this fact is
(A) the substance degrades by second-order kinetics
(B) the substance degrades by first-order kinetics
(C) the substance is not biodegradable
(D) the substance has long half-life

The correct answer is: (A) the substance degrades by second-order kinetics. For a pollutant that “persists in the environment for a very long period of time”, saying it must follow second‑order degradation is scientifically not justified, so this statement is unacceptable, whereas the other three are consistent with high persistence.​

Question Restatement and Correct Answer

The question states: “A harmful substance persists in the environment for a very long period of time. The UNACCEPTABLE statement for this fact is…” with four options about its degradation behavior and persistence.​

• The unacceptable statement is (A) the substance degrades by second-order kinetics.​

This is because environmental “persistence” is generally linked to very slow degradation, long half-life, and often poor or no biodegradability, but it is not defined by being second‑order; many persistent chemicals are modeled by first‑order kinetics or more complex schemes.​

Option (A): Second-order kinetics (Unacceptable)

Statement: “The substance degrades by second-order kinetics.”

• Second-order kinetics means the degradation rate depends on the product of two concentrations (for example, concentration of pollutant and another reactant, or square of pollutant concentration), and does not inherently imply long persistence or slow decay.​

• Environmental persistence is usually characterized in regulations by half-life values derived assuming pseudo-first-order or first-order kinetics, not by requiring second-order behavior; saying “it degrades by second-order kinetics” is therefore not a logical consequence of the given fact that it persists for a very long period.​

Hence, this statement is not logically supported by the premise and is therefore the unacceptable option.

Option (B): First-order kinetics (Acceptable)

Statement: “The substance degrades by first-order kinetics.”

• Many environmental fate models for chemicals (including persistent pollutants) use first-order kinetics (or apparent first-order kinetics) to describe degradation, where the rate is proportional to the current concentration of the substance.​

• A substance that persists for a long time can still follow first-order kinetics; its persistence would then be reflected in a very small first-order rate constant, corresponding to a long half-life, so this statement is compatible with long persistence and thus acceptable.​

Option (C): Not biodegradable (Acceptable)

Statement: “The substance is not biodegradable.”

• A harmful substance that remains in the environment for a very long period often shows little to no biodegradation, meaning microorganisms cannot break it down efficiently; such substances are referred to as non-biodegradable and tend to accumulate.​

• Therefore, stating that a highly persistent harmful substance is not biodegradable is consistent with the premise and is an acceptable description of its behavior.​

Option (D): Long half-life (Acceptable)

Statement: “The substance has long half-life.”

• Environmental persistence is commonly quantified using half-life, defined as the time required for the concentration of a chemical to decrease to half its initial value; long persistence corresponds to a long half-life in environmental media such as water, soil, or air.​

• Saying that a substance that persists for a very long period has a long half-life directly matches the definition used in regulatory and scientific contexts, so this statement is clearly acceptable.​

By focusing on the link between persistence, biodegradability, kinetic order, and half-life, the question tests conceptual understanding that persistence is about slow degradation and long half-life, not about a specific kinetic order like second-order.​