Last weeks lecture on acids and bases was extremely fun and interesting! It was great to see how chemistry is so inherent in our daily lives, and how many common bases (substances that accept a proton) such as lime water and acids (substances that donate a proton) such as lemon juice or even our blood, are so readily available around us. An important acid base reaction that we did not discuss in class but is especially important for the environment is acid rain.
Acid rain occurs due to the burning of large amounts of coal. When coal is burned, the sulphur released reacts with the oxygen in the atmosphere to form sulphur dioxide, which reacts with more oxygen and water to form sulphuric acid. When it rains, the sulphuric acid mixed with rain, not only pollutes water ecosystems but also corrodes limestone. The reaction is as follows:
CaCO3 + H2SO4 –> H2CO3 + Ca++ + SO4- –
In the above reaction, limestone (CaCO3) is the base that accepts a proton from H2SO4 and is converted into carbonic acid (H2CO3).
Usually there isn’t enough sulphuric acid present in the rain to cause its pH to be below 6.5. However, at the peak of the acid rain problem in the 70s, the pH of the rain was as low as 5.5 which could seriously damage plants, and water ecosystems.
Although the problem of acid rain is much less of an issue now in the US and Canada due to the sanctions imposed by these governments which limited the burning of coal, some of the effects of acid rain are still being felt. In Canada for example, acid rain has resulted in a whole new species gaining prominence in its lakes. Due to acid rain, calcium was washed away from the soil and drainage areas which hurt the survival rates of a prominent species of plankton that required calcium to survive. A competing plankton, that has a jelly like surface, and requires little calcium, has now gained prominence and swimmers in Ontarios lakes now find themselves coated with this jelly-like substance after swimming in the lake.
Thus, it is important to realize that chemical changes to the environment (such as acid rain) have extremely long lasting impacts that persist even though the problem may be ‘solved’.