# What is the pH of normal rain?

Dec 15, 2016

The pH of normal rain is around 5.7.

#### Explanation:

The pH of pure water is 7.0.

However, the pH of normal rain is about 5.7.

Rainwater contains dissolved carbon dioxide from the atmosphere.

Carbon dioxide reacts with water to form carbonic acid.

"CO"_2"(g)" + "H"_2"O(l)" ⇌ underbrace("H"_2"CO"_3"(aq)")_color(red)("carbonic acid")

Carbonic acid is an acid because it reacts with water to form hydronium ions, $\text{H"_3"O"^"+}$.

$\text{H"_2"CO"_3"(aq)" + "H"_2 "O(l)" ⇌ "H"_3 "O"^"+" + "HCO"_3^"-}$

Thus, normal rain is slightly acidic.

In contrast, the pH of typical acid rain is around 4.0 (about 50 times as acidic as normal rain).