Environmental Chemistry

Acid Rain

We have all learned about Acid Rain before. We know that it is caused due to a high level of pollution in the atmosphere. However, the actual chemistry of how acid rain precipitates onto the Earth is slightly more complicated. Let us take a detailed look at the chemistry and causes of acid rain.

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Acid Rain

Firstly let us understand exactly what acid rain is. The pH level of rainwater is supposed to be 5.6. While the pH value of water is 7 which is what we consider neutral, rainwater has a more acidic nature. This is because there are some acidic compounds that are naturally mixed in with rainwater. These compounds, such as Nitrous oxide, carbon dioxide etc, are found in the lower levels of the atmosphere and mix in with the rainwater.

But if the pH of rainwater falls below 5.6, it is now acidic in nature. This is when we term as Acid rain. Acid Rain is highly toxic to all living organisms in the world, and a major cause of land and soil pollution.

Acid Rain and Causes of Acid Rain(epa.gov)

Causes of Acid Rain

Now we are already aware that acid rain is a result of heavy air pollution. Here we will study the two main culprits that are majorly responsible for acid rain – Sulpher Dioxide (SO2) and Nitrous Oxide (NOx).

Both SO2 and NO are released into the atmosphere by human activities. These pollutants are transported into the atmosphere by wind and air currents. Here they react with the water in the atmosphere and oxygen in the air to form Sulphuric Acid and Nitric Acid respectively. These acids then lower the pH value of rainwater, making it acidic and harmful.

Let us look at the chemical reactions that occur in the atmosphere.

2SO2 (g) + O2 (g) + 2H2O (l) → 2H2SO4 (aq)

4NO2 (g) + O2 (g)+ 2H2O (l) → 4HNO3 (aq)

Now let us look at the major causes of acid rain that release these pollutants ( SO2 and NO) into the air

  • The biggest cause is electricity production by burning coal. This thermal electricity plants are responsible for about 70% of Sulphur Dioxide release in the world. They also release huge quantities of NO about 20% in total.
  • Any type of fossil fuels that we burn release both these pollutants. So the burning of petrol and diesel to run automobiles, crafts and aeroplanes is one of the major causes of this pollution.
  • Also, every production and manufacturing plant that uses pollutants is also at fault. Especially if they do not have an effective pollution control system in place.

Forms of Acid Rain

  • Wet Deposition: This is the most common form of acid rain. The sulphuric acid and nitric acid are the main causes of acid rain. They get mixed with the rain and fall on the earth’s surface as acid rain. They may also precipitate in other forms such as sleet, snow, hail, or even fog. All these are just another form of wet acid deposition via acid rain
  • Dry Deposition: Now it is not necessary that the acids mix with the moisture and fall down as rain. They may also deposit on the earth’s surface in a dry form. This happens more in an area that receives an infrequent and low amount of rainfall, like deserts and arid areas.

Dry deposition occurs when the pollutants form particles in the air. These particles settle on the earth’s surface on land and lakes, ponds etc. They even settle on plants and trees. Then when rainfall eventually occurs, the particles mix in with water and form acids. These polluted waters then pollute the land and all living organisms in the biosphere.

Solved Question for You

Q: Which ions among the following are present in elevated amounts in acid rains?

  1. Calcium Ions
  2. Carbonate Ions
  3. Hydrogen Ions
  4. Hydroxide Ions

Answer: The correct answer is “C”. Acid Rain is acidic in nature. Hydrogen ion is the characteristic ion present in an acid. Hence hydrogen ions are present in elevated amounts in acid rain.

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