What is acid rain?
Acid rain is defined as rainwater with a pH of less than 5, or any rainfall that has an acidity level beyonf what is expected in a non-polluted rainfall. To give a better understanding on what this number "5" entail, I would give you the pH of some other substances. Pure water has a pH of 7 while rainwater, a pH of 5.6 due to the slightly acidic carbon dioxide in the air.
Acid rain is classified as regional air pollution problem, and it causes major damages to our environment. Not only does it kill plants, since acid rain makes soil more acidic and hurts leaves of plants, which in turn lead to destruction of sensitive forests, it also affects the respiratory systems in human and other animals. In addition, it acidifies the lake water with toxic effects especially to fish, and corrodes the exposed structures, electrical relays, equipment and ornamental materials. The hydrogen ions from the acid rain dissolve the limestone (CaCO3) and thus cause damage to marble structures and galvanized iron sheets. Furthermore, air pollutants from one country can produce acid rain in another. For example, China burns large amounts of coal, and gases from this burning are carried to Japan and other countries where acid rained are formed, such as North Korea and South Korea and Japan. Let us now look at another case study.
Case Study:
Acid rain not only kills fish in rivers and lakes (since the increased acidity reduces ability of aquatic life to take in oxygen, salts and nutrients for survival), it also dirties and corrodes buildings, dissolves minerals like aluminum which is toxic to plants in the soil and also make plants lose their leaves, limiting photosynthesis and die. For example, toxic gases are blown by wind from industrialised Northwest Europe to Norway and Sweden, resulting in death of salmon and trout in Norwegian and Swedish lakes and rivers; whereas in the United States of America, the Sphinx of Egypt and the Statue of Liberty have been severely damaged by acid rain or corroded.
How is acid rain formed?
There are mainly two main pollutants, which are namely sulfur dioxide and nitrogen dioxide.
Sulfur dioxide in the air reacts with oxygen to form sulfuric acid, which is represented in the following equation:
2SO2 (g) + O2 (g) +2H2O (l) --> 2H2SO4 (aq)
The sulfuric acid dissolves in rainwater, making the rainwater slightly acidic and giving it the ability to corrode substances.
Nitrogen dioxide, on the other hand, forms nitric reaction, although undergoing a similar reaction in the air:
4NO2 (g) + O2 (g) +2H2O (l) --> 4HNO3 (aq)
Acid rain with a pH of about 4 is about 40 times more acidic than unpolluted rainwater, although only with a mere difference of 1.6 in the pH value. Some acid even have a pH of less than 1.5! This is very VERY acidic.
How to reduce the effects of acid rain?
People could burn fuels which contain little or no sulfur. This is why natural gas is preferred instead of coal or oil for power stations. For example, in Singapore, more and more public buses are changing from fuel to natural gas. Furthermore, people use catalytic converters in cars and and also reduces acidic gases in power stations. Lastly, the acids in lakes and soil could be neutralise by using slaked lime, which is also known as calcium hydroxide. Slaked lime is added to soil while some countries add calcium carbonate powder to lakes. Indeed, Norway and Sweden have successfully done this to help restore lakes and streams. However, what is worth mentioning is that, it is not suitable to add alkali to the affected areas, since we do not know when would the affected area be "neutralized". Thus, if we add in excess, the area would turn alkali instead of becoming neutralized. It is hence decided to use insoluble base, slaked lime, since even if added in excess, it would not change the pH of the place to above 7 since no reaction would take place once all the acids have reacted.
Sulfur dioxide in the air reacts with oxygen to form sulfuric acid, which is represented in the following equation:
2SO2 (g) + O2 (g) +2H2O (l) --> 2H2SO4 (aq)
The sulfuric acid dissolves in rainwater, making the rainwater slightly acidic and giving it the ability to corrode substances.
Nitrogen dioxide, on the other hand, forms nitric reaction, although undergoing a similar reaction in the air:
4NO2 (g) + O2 (g) +2H2O (l) --> 4HNO3 (aq)
Acid rain with a pH of about 4 is about 40 times more acidic than unpolluted rainwater, although only with a mere difference of 1.6 in the pH value. Some acid even have a pH of less than 1.5! This is very VERY acidic.
How to reduce the effects of acid rain?
People could burn fuels which contain little or no sulfur. This is why natural gas is preferred instead of coal or oil for power stations. For example, in Singapore, more and more public buses are changing from fuel to natural gas. Furthermore, people use catalytic converters in cars and and also reduces acidic gases in power stations. Lastly, the acids in lakes and soil could be neutralise by using slaked lime, which is also known as calcium hydroxide. Slaked lime is added to soil while some countries add calcium carbonate powder to lakes. Indeed, Norway and Sweden have successfully done this to help restore lakes and streams. However, what is worth mentioning is that, it is not suitable to add alkali to the affected areas, since we do not know when would the affected area be "neutralized". Thus, if we add in excess, the area would turn alkali instead of becoming neutralized. It is hence decided to use insoluble base, slaked lime, since even if added in excess, it would not change the pH of the place to above 7 since no reaction would take place once all the acids have reacted.
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