Question

pH of acid rain water is generally in the range of

• A. $1.0 - 3.0$
• B. $3.5 - 5.6$
• C. $5.9 - 6.9$
• D. $7.1 - 7.5$

Hint:

Remember that the pH scale is logarithmic. A drop from pH 6 to pH 5 means the rain is 10 times more acidic. A drop from 6 to 4 means it is 100 times more acidic!

Answer 1

The Correct Option is B

To understand the pH range of acid rain, we must first define what is considered "normal" rain and how atmospheric pollutants alter its chemistry. 1. Normal Rain vs. Acid Rain: Normal, unpolluted rainwater is slightly acidic with a pH of approximately $5.6$. This acidity is natural and occurs because carbon dioxide ($CO_2$) from the atmosphere dissolves in water to form weak carbonic acid ($H_2CO_3$): $$CO_2(g) + H_2O(l) \rightleftharpoons H_2CO_3(aq)$$

2. Formation of Acid Rain: Acid rain occurs when fossil fuels are burned, releasing sulfur dioxide ($SO_2$) and nitrogen oxides ($NO_x$) into the atmosphere. These gases react with water, oxygen, and other chemicals to form sulfuric acid ($H_2SO_4$) and nitric acid ($HNO_3$), which are much stronger than carbonic acid.

3. Typical pH Range: When these strong acids mix with precipitation, the pH of the rainwater drops below the natural threshold of $5.6$. The generally accepted range for acid rain is between

3.5 and 5.6. In highly polluted urban areas, it can occasionally drop even lower, but $3.5 - 5.6$ represents the standard environmental range. Therefore, any rain with a pH significantly lower than $5.6$ is classified as acid rain.