Question

Rain is called acid rain when its :

• A. pH falls below 7
• B. pH falls below 6
• C. pH falls below 5.6
• D. pH is above 7

Hint:

Do not confuse neutral water (pH = 7) with normal rainwater (pH = 5.6).
Normal rain is slightly acidic due to atmospheric carbon dioxide.
Thus, the boundary for acid rain is 5.6, not 7.0.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks for the specific threshold value of pH below which rain is classified as "acid rain."

Step 2: Key Formula or Approach:
Normal rain is naturally slightly acidic due to dissolved carbon dioxide, which forms weak carbonic acid:
\[ \text{CO}_2\text{(g)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{H}_2\text{CO}_3\text{(aq)} \]

Step 3: Detailed Explanation:

• Because of the natural presence of carbon dioxide in the atmosphere, clean unpolluted rain typically has a pH of about 5.6.

• When pollutants like sulfur dioxide (\(\text{SO}_2\)) and nitrogen oxides (\(\text{NO}_x\)) are released into the air from industrial emissions or combustion of fossil fuels, they react with water, oxygen, and other chemicals to form sulfuric acid and nitric acid.

• These strong acids dissolve in rainwater, causing its pH to drop significantly.

• By environmental and scientific definition, precipitation is classified as acid rain when its pH value falls below the natural threshold of 5.6.


Step 4: Final Answer:
Rain is called acid rain when its pH falls below 5.6.