For acid rain, the pH of rain water should be less than

CONCEPT:

Acid Rain and pH

• Acid rain refers to rainwater that has been made acidic due to the absorption of sulfur dioxide (SO₂) and nitrogen oxides (NO_x) from the atmosphere.
• The pH scale ranges from 0 to 14, where a pH of 7.0 is neutral, values below 7.0 are acidic, and values above 7.0 are basic.
• Natural, unpolluted rainwater has a slightly acidic pH of around 5.6 due to the presence of dissolved carbon dioxide (CO₂) forming carbonic acid (H₂CO₃).
• For rainwater to be classified as acid rain, its pH must be less than 5.6, which indicates the presence of stronger acids such as sulfuric acid (H₂SO₄) and nitric acid (HNO₃).

EXPLANATION:

• A pH of 7.0 indicates neutral water, which is not acidic.
• A pH of 6.6 is slightly acidic but does not qualify as acid rain since it is higher than the threshold of 5.6.
• A pH of 5.6 is the natural pH of unpolluted rainwater and marks the upper limit for acid rain.
• A pH of 8.0 is basic, which is opposite to acidic conditions.
• To classify rainwater as acid rain, its pH must be less than 5.6.

Therefore, the correct answer is 5.6.