Animated Solution for Chemistry - Environmental Chemistry: When rain is accompanied by a thunderstorm, the collected rain water will have a pH value

Have you ever noticed how the air smells different after a thunderstorm? That fresh, sharp scent is a clue to the intense chemical reactions happening miles above our heads. Today, we are going to dive into the fascinating chemistry of thunderstorms and discover why they make rain more acidic.

Normal Rain

First, let's establish a baseline. Even in the most pristine, unpolluted environments, normal rainwater is not perfectly neutral. It has a pH of about $5.6$. Why? Because our atmosphere contains carbon dioxide ($C{O}_2$). As raindrops fall through the air, they dissolve some of this $C{O}_2$, forming a weak acid called carbonic acid ($H_{2}C{O}_3$). This slight acidity is completely natural and harmless.

Lightning Strikes

Now, imagine a massive thunderstorm rolling in. The sky darkens, and suddenly—CRACK! A bolt of lightning tears through the sky. Lightning is not just a beautiful display of electricity; it is incredibly hot. In fact, a lightning bolt can heat the surrounding air to temperatures hotter than the surface of the sun (around $2000\text{ K}$)!

Our atmosphere is mostly made of nitrogen ($N_2$) and oxygen ($O_2$). Normally, nitrogen is a very stable and unreactive gas because of the strong triple bond holding its atoms together. But the extreme heat from a lightning strike provides enough energy to break these bonds.

Once the nitrogen bonds are broken, the nitrogen atoms react with oxygen to form nitric oxide ($NO$). The reaction looks like this:

But the story doesn't end there. Nitric oxide is quite reactive. As it cools down and mixes with more oxygen in the atmosphere, it quickly oxidizes to form nitrogen dioxide ($N{O}_2$):

Now we have nitrogen dioxide floating in the air, and it's raining. As the raindrops fall, they sweep up this $N{O}_2$. Nitrogen dioxide reacts with the water ($H_{2}O$) and oxygen in the air to form nitric acid ($HN{O}_3$):

Nitric acid is a strong acid. Unlike the weak carbonic acid in normal rain, nitric acid completely dissociates in water, releasing a large number of hydrogen ions ($H^{+}$).

Because of this influx of hydrogen ions, the pH of the rainwater drops significantly. It becomes more acidic than normal rain. Therefore, when rain is accompanied by a thunderstorm, the collected rainwater will have a pH value slightly lower than that of rainwater without a thunderstorm. This is a classic example of how natural phenomena can drive complex chemical processes in our environment!