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Question

Which of the following statements is INCORRECT?

This question was previously asked in
RRB NTPC 2024 Undergraduate CBT 1 Question Paper (29-Aug-2025) (Shift 1)
The correct answer is

When pH of rain water is more than 5.6, it is called acid rain.

In evaluating the given statements, we determine which is incorrect by assessing each statement's accuracy regarding pH and its environmental implications.

  1. When pH of rain water is more than 5.6, it is called acid rain.

    This statement is incorrect. Regular rainwater has a natural pH of around 5.6 due to the presence of dissolved carbon dioxide forming carbonic acid. "Acid rain" occurs when the pH is less than 5.6, not more.

  2. When pH of rain water is less than 5.6, it is called acid rain.

    This statement is correct. Acid rain typically has a pH lower than 5.6 due to the presence of sulfuric and nitric acids resulting from industrial pollution.

  3. Tooth decay starts when the pH of the mouth is lower than 5.5.

    This statement is accurate. Tooth enamel demineralizes at a pH below 5.5, which leads to decay.

  4. Living organisms can survive only in a narrow range of pH change.

    This statement is valid. Most biological systems function optimally within a narrow pH range, and significant deviations can lead to dysfunction or death.

Considering the explanations above, the statement "When pH of rain water is more than 5.6, it is called acid rain." is incorrect, as acid rain has a pH less than 5.6.

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