Acid Rain

Here’s something you might not have known: All rain is acid. I mean, assuming it’s passing through our atmosphere somewhere – polluted or not.

Here’s the deal. Pure water, not tap water or river water, but the purest water you can get, has a pH of 7. That is what we call ‘neutral.’ Any liquid with a pH greater than 7 is called a base, or basic. Any liquid with a pH less than seven is considered acid, or acidic.

The nastiest acids, like hydrochloric acid (you know, that stuff in your stomach), has a pH of something like 4, or even lower. There are lots of acids you come in contact with every day, like vinegar (acetic acid), or citric acid which is in fruit. There are also lots of bases around that you see every day, like ammonia, many detergents, and lye. You can read a bit more about acids and bases here.

What makes an acid an ‘acid’ is having a hydrogen atom as part of the molecule that will easily break away from the original molecule and react with other things, often in a way that damages those other things. This breaking away of hydrogen typically happens in water. So the hydrogen in hydrochloric (HCl) separates from the chlorine, and goes off and does potentially nasty things.

A base has a hydroxide group (oxygen and hydrogen together, OH) that is prone to break away from the parent molecule in water. So lye, also called sodium hydroxide or NaOH, becomes separated Na and OH in water. The hydroxide group is just as capable of doing nasty things to other compounds as free hydrogen, so I don’t recommend sticking your fingers in a bottle of lye.

Now, if you mix an acid and a base together, you get water and some sort of salt. From our previous example:

HCl –> H⁺ + Cl^–

NaOH –> Na⁺ + OH^–

H⁺ + Cl^– + Na⁺ + OH^– = H₂O + NaCl

NaCl is table salt. H₂O is water. But please don’t do this experiment. The reaction would be vigorous, hot, and potentially deadly. Don’t do it.

Back to our original point. All rain is acid, as long as it falls through our atmosphere. Why is that?

It turns out that carbon dioxide, CO₂, dissolves in water. When it dissolves, it makes an acid called carbonic acid, H₂CO₃. Though it’s a weak acid, it still is an acid. It will lose one of its hydrogens when dissolved in water, making that water more acidic. Since there is carbon dioxide in our atmosphere, it dissolves into any precipitation, changing the pH from a neutral pH of 7, closer to a pH of 6, or even lower.

As carbon dioxide concentrations in the atmosphere increase, the pH of rainwater will decrease, becoming more and more acidic. Given certain trends in the amount of carbon dioxide in the atmosphere this might be a concern. And this doesn’t even take into consideration pollution.

Increasing the acidity of natural surface waters affects all the organisms living in that water. Corals are suffering in part because the more acid waters literally dissolve their calcium carbonate (CaCO₃) skeletons. Take a walk in a graveyard and notice how many headstones have had the words weathered away. Those headstones are limestone, which is also calcium carbonate. Acids will dissolve calcium carbonate, and the stronger the acid, the more rapid the destruction.

Interesting, isn’t it?