Problem 46 - Chemistry The Molecular Science

Question

Explain why rain with a pH of 6.7 is not classified as acid rain.

Step-by-Step Solution

Answer

Rain with a pH of 6.7 is not acidic enough to be classified as acid rain.

Step 1: Understand pH Scale

The pH scale ranges from 0 to 14 and measures how acidic or basic a substance is. A pH value of 7 is considered neutral. Values less than 7 indicate acidity, while values greater than 7 indicate basicity.

Step 2: Define Acid Rain

Acid rain typically refers to precipitation with a pH lower than 5.6. This value is lower than the natural pH of rainwater due to the presence of dissolved carbon dioxide, which forms carbonic acid.

Step 3: Evaluate pH of 6.7

Rain with a pH of 6.7 is above the threshold of pH 5.6 that characterizes acid rain. Therefore, it is closer to neutral and cannot be considered acid rain.

Key Concepts

pH Scale

The pH scale is a critical tool to measure the acidity or basicity (alkalinity) of solutions. It ranges from 0 to 14. The scale is logarithmic, meaning each whole number change represents a tenfold change in concentration. This means that a substance with a pH of 6 is ten times more acidic than one with a pH of 7.
At the low end of the scale, close to 0, you find highly acidic substances. These might include things like battery acid. On the high end, near 14, are basic substances such as lye.

• Acidic solutions: pH less than 7
• Neutral solutions: pH of exactly 7
• Basic solutions: pH greater than 7

Knowing where a solution falls on this scale helps us understand its chemical properties and behavior. When considering rain, understanding its pH can also tell us about its potential environmental impact.

Neutral pH

A neutral pH is defined as a pH of exactly 7. This middle point on the pH scale represents a perfect balance between acidity and basicity. Pure water is the most common example of a neutral substance.
In context, when rainwater is close to neutral, it is less likely to cause harm. However, rain is naturally slightly acidic because of dissolved carbon dioxide forming carbonic acid.
Although the neutral point is 7, rain is often closer to a pH of 5.6 in an unpolluted environment, due to the natural formation of carbonic acid. Therefore, understanding the baseline of rain's natural pH helps us determine what qualifies as acid rain.

Carbonic Acid

Carbonic acid plays a crucial role in the natural acidity of rain. It forms when carbon dioxide gas from the atmosphere dissolves in water. This chemical reaction is expressed as \( CO_2 + H_2O \rightarrow H_2CO_3 \).
The presence of carbonic acid lowers the pH of natural rainwater, making it slightly acidic, typically around a pH of 5.6. This is normal and expected.

• Carbonic acid is a weak acid, meaning it does not dissociate fully in water.
• It gently lowers the pH from the neutral point of 7.

When discussing acid rain, it is valuable to know that carbonic acid is not the primary contributor to harmful acidity. Instead, pollutants like sulfur dioxide and nitrogen oxides create stronger acids in rain.