What is Acid Rain?

The term acid rain is used to describe all precipitations- rain, snow, fog, dew- which are more acidic than normal water. The normal rain is slightly acidic having a pH of about 5.6 as carbon dioxide gas reacts with it to form a weak carbonic acid. For example:

CO2 + H2O → H2CO3

Rainfall is declared as acid rain when its pH is less than 5.6 because natural and unpolluted rainwater actually has a pH between 5.6-6.5 (acidic) due to the reaction of water with the presence of CO2. The acidity of the rainwater comes from the natural presence of three substances (CO2, NOx, and SO2) found in the troposphere (the lowest layer of the atmosphere) which are emitted from the combustion of coal in power plants and gasoline in vehicles. The pH of acid rain can range between 5.6 to 3.5 and in some cases, pH can go even lower than 2.

Chemicals in Acid Rain

Chemicals in acid rain are acids like nitric acid and even sulphuric acid present in the polluted air. These chemicals in acid rain are formed when oxides of nitrogen and sulphur come in contact with rainwater.

Acid Rain Causes

The main acid rain causes are the formation of mineral acids like carbonic acid, nitric acid and sulphuric acid.

The Atmospheric Reaction of Acid Rain

1. Carbon dioxide gas molecules react with water molecules to form carbonic acid (H2CO3). The ability of carbonic acid (H2CO3). The H2CO3 (carbonic acid) molecule has the ability to deliver H⁺, this capacity of carbonic acid molecule classifies this molecule as an acid. Therefore, it is responsible for lowering the pH of a solution. 

CO2 + H2O → H2CO3

H2CO3 → H+ + HCO3-

2. Nitric Oxide (NO) molecule contributes to the natural lowering in pH (acidity) of the rainwater, it is formed during lightning storms by the reaction of nitrogen and oxygen, two common atmospheric gases. In the air, nitric oxide (NO) is oxidized to nitrogen dioxide (NO2), this, in turn, combines with water to give nitric acid (HNO3). This acid dissociates in water to yield hydrogen ions (H+) and nitrate ions (NO3-) in a reaction analogous to the dissociation of carbonic acid shown in the equation below, again lowering the pH of the solution.

NO + ½ O2 (g) → NO2 (g)

N2(g) + O2 (g) → 2NO (reaction takes place during the lightning in the atmosphere)

3NO2 (g) + H2O → 2HNO3 (aq) + NO (g)

About one- fourth of the lowering in pH of the rain is accounted for by nitric acid (HNO3).

In most of the water bodies, the lowering in pH is accounted for by the presence of sulphuric acid (H2SO4) in rainwater. Although there is a natural source of sulphuric acid production. Sulphuric acid is produced naturally in trace amounts from biological decay and volcanic activity, it is produced almost entirely by human activity, especially the combustion of sulphur-containing fossil fuels in power plants.

When these fossil fuels are burned, the sulphur contained in them reacts with water to form sulphuric acid.

SO2(g) + O2 → SO3 (g) + H2O → H2SO4

Sulphuric acid is a very strong acid. Therefore, it is considered a strong electrolyte and readily dissociates in water, to give an H⁺ ion and HSO4- ion. The HSO4- ion may further dissociate to give hydronium ion (H+) and sulphate ion (SO4-2) thus, the presence of H2SO4 causes the concentration of H+ ions to increase dramatically, and so the pH of the rainwater drops to a harmful level. 

H2SO4 → HSO4- + H+

HSO4- → SO4-2 + H+

Gases Responsible for Acid Rain

The major gases responsible for acid rain are nitric oxide, nitrogen dioxide, sulphur dioxide, and sulphur trioxide. The gases responsible for acid rain when precipitates down causes the easing of nutrients from plants, and thus it damages the leaves of plants and trees.

Environmental Effects of Acid Rain

Acid rain enhances the number of inorganic reactions and biochemical reactions with deleterious environmental effects, leading to a growing environmental problem worldwide.

  • There are many large lakes that have become so acidic (low ph lake), that fish cannot live in them anymore.

  • Metal ions are produced by the degradation of the many naturally occurring soil minerals. These metal ions are then washed away in the runoff and causes several effects:

  1. Due to the acidic condition, the mobility of the toxic ions, such as Al+3 increases into the water supply.

  2. The loss of important minerals such as Ca+2, from the soil in neutralizing the sulphuric acid which causes a deficiency of Ca+2, creates killing trees and damaging crops.

  3. Acid rain causes damage to the forest.

Did You Know?

  • Acid rain causes corrosion of metals. For example, the formation of rust on iron gates and iron frames increases due to acid rain.

  • Acid rain causes a very huge deterioration of the buildings and sculptural materials. The Taj Mahal in India faces this problem. 

FAQs (Frequently Asked Questions)

Question: Why Does Rainwater Have a pH of Less than 7?

Answer: The pH of rainwater is less than 7 due to the presence of carbon dioxide in the atmosphere. When water precipitates down in the form of rainwater, carbon dioxide reacts with the rainwater and forms carbonic acid. Therefore, it leads to a drop in the pH value.

Question: What are the Effects of Acid Rain on the Monuments?

Answer: Acid rain causes extensive deterioration to the sculptural materials or marble, limestone, slate, mortar etc, as these materials become pitted and weakened mechanically.