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: Recall why ordinary rain is not perfectly neutral.
Rainwater always dissolves a little atmospheric $\text{CO}_2$ as it falls, forming a weak carbonic acid solution: \[ \text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \]
Step 2: Fix the natural baseline pH.
This natural dissolving of carbon dioxide gives clean, unpolluted rain a pH of about $5.6$, which is already mildly acidic even without any pollution.
Step 3: See what pollution adds on top.
When industrial gases like $\text{SO}_2$ and $\text{NO}_x$ enter the atmosphere, they form much stronger acids, sulfuric and nitric acid, and drag the pH of rain down further below this natural baseline.
Step 4: State the accepted cutoff.
Scientists label rain as acid rain only once it drops below its own natural, unpolluted value, which is the benchmark $5.6$. \[ \boxed{\text{pH falls below } 5.6} \]