6 Water, water, Everywhere
6.2 Acid/Base Theory
6.5 pH and pOH
6.10 Weak Acids and Bases
6.11 The Water Around Us
6.42 Learning Outcomes
Let's consider some of the chemistry of the water that is around us.
Rain is water that falls from the sky. How'd it get there? It evaporated from any of the lakes, rivers, streams, and oceans arounds us. It is a constant cycle. Clouds are just large bundles of tiny water particles - so small that they stay airborne, but big enough to reflect and scatter light so we see them as a cloud.
The droplets in the air are also large enough (macroscopically) to act as a solvent for dissolving solutes. Gases in the air tend to dissolve in those water droplets. Even though most gases are barely soluble in water due to their non-polar nature and water's polar nature, they still dissolve a bit. A good example of this is CO2. When CO2 dissolves in water is almost immediately reacts with water to make carbonic acid.
CO2(g) → CO2(aq)
CO2(aq) + H2O → H2CO3(aq)
The newly formed carbonic acid behaves like any other weak acid and dissociates into aqueous protons and the conjugate base, hydrogen carbonate.
H2CO3(aq) ⇌ H+(aq) + HCO3–(aq)
This will of course increase the acidity and decrease the pH. It is for this reason that most "pure" rain is actually slightly acidic with pH's in the range between 5 and 6.
"Acid rain" is not the same as the slightly acidic pure rain with dissolved CO2 mentioned above. Instead, acid rain is when other gases dissolve and react with water or a derivative from water to form strong acids. Those two gases are two of our air pollutants NO2 and SO3. Here is the reaction scheme for both.
NO2(aq) + ·OH → HNO3(aq) → H+(aq) + NO3–(aq)
SO3(aq) + H2O → H2SO4(aq) → H+(aq) + HSO4–(aq)
The ·OH is a hydroxyl radical which is present in the atmosphere - it comes from water molecules getting hit with ionizing radiation or reaction with other free radicals. The bottom line is that both nitric acid and sulfuric acid can be formed when NOx's and SOx's are floating around in the air. The pH of these droplets is much lower than with CO2 - generally below a pH of 5 and even down to just below 3. This can be devastating to many plants and trees and any type of carbonate based structures - limestone and marble. Acid rain will literally dissolve them away via the conversion of the carbonate back to CO2 gas and water.
CaCO3(s) + 2H+(aq) → H2CO3(aq) → H2O + Ca2+(aq) + CO3–(aq)
So that is bad news there if you are a limestone statue or some other limestone object. But this can be a very beneficial reaction as well...
Have you ever had an upset stomach and chewed up a TUMS tablet? It neutralizes acid. It can really help and give relief. Well you are crunching good ol' calcium carbonate when you do that - yes, the active ingredient in TUMS is pretty much limestone. Limestone is all over the place here in Austin, TX and in general central Texas. Most all of our rivers and lakes are lined with limestone. What does this mean? Well first, it means that our lakes tend to be slightly basic thank to carbonate, a weak base. It is NOT very soluble so very little will dissolve - but enough does such that our lakes tend to have pHs around 8.2. This also means that if anything acidic gets into the lake which could cause a big swing to lower pHs, the carbonate will neutralize that acid and cushion the blow. This type of action is called buffering. So you could say that our lakes in central texas are buffered with limestone (aka calcium carbonate) and will neutralize acid in a very natural way.
Once you get far enough north though things change, by north I mean all the way up to Michigan, Illinois, all the way over to the east coast at Massachusetts and up to Maine. Up there much of the bedrock is igneous rock - mostly granite/quartz. There is no neutralizing of acid from granite. The pH in lakes up there tends to be lower than those down here with limestone. By lower I mean around 7 and some dip into the 6's. This lack of acid buffering did not aid when acid rain was a thing up and around the rust belt - where the manufacture of iron was blowing out so much SO2 and NO2 into the air that acid rain was a real issue up there and much of their trees died and many of their lakes dipped down to the low 5's in pH. That will pretty much kill all fish once you hit a pH of 5. Many fish die even up at 6. The bottom line was that something needs to be done to save these aquatic ecosystems. There were huge fish kills and some lakes were totally dead - no fish. Action had to be taken to stop the toxic emissions that were causing acid rain. Luckily, they got a handle on it* and things gradually (over a period of years) got better and they are still better to this day. Federal safety regulations have saved lives and ecosystems - cutting back on them is generally NOT a wise move.
* Bit of an understatement there... There were literally years of fighting for the cause on this issue. It was political as you can imagine. Although the Clean Air Act had been around for years it didn't directly address the acid rain problem. President Reagan basically blew it off (imposing regulations) and it just got worse, the republicans in charge finally realized that something had to be done. In 1990 President George H. W. Bush signed an amendment to the Clean Air Act to directly address the acid rain threat, and the hole in the ozone layer. So that is what "got a handle on it" meant. Keep in mind, the first discovery of the potential destruction of acid rain was in 1963, when the original Clean Air Act was signed. With a few amendments inbetween, it still took 27 years to finally get the government to actually do something about it. Feel free to deep dive on this in Wikipedia and all the places that leads you to.
** One more thing... How do we remove pollutants from exhaust fumes and help prevent acid rain? Back up to chapter 2 section 10 (Curbing Air Pollution) of this book and read up (again) on scrubbers.