5 Characteristics of Acid Rain

Acid rain is precipitation with the presence of sulfuric acid, nitric and nitrous acid, resulting from chemical reactions that occur in the atmosphere.

All rains are acidic, even in unpolluted environments. However, rainfall becomes an environmental problem when its pH is less than 4.5.
This is due to the large amount of products from the burning of fossil fuels released into the atmosphere, as a consequence of human activities.


Some of the characteristics of acid rain are:

  • Contains high levels of acids: Acid rain contains much higher levels of acid than normal rain, which can damage vegetation, buildings and bodies of water.
  • It forms mainly in industrial areas: Acid rain is more common in industrial areas and in areas with high emissions of sulfur dioxide and nitrogen oxides.
  • Can have harmful effects on health: Acid rain can affect human health by inhaling contaminated particles in the air or by consuming contaminated food or water.
  • Can affect biodiversity: Acid rain can damage vegetation and affect aquatic ecosystems, which can have an impact on the biodiversity of an area.
  • It can have long-term effects: The effects of acid rain can last for a long time, as the acids can remain in the soil and bodies of water for years.

How acid rain is formed?

The carbon dioxide (CO2) in the atmosphere already makes rain slightly acidic, even under natural conditions. The natural pH of water is 7 and when it is in balance with atmospheric CO2 it is 5.6, that is, not very acidic.

Sulfur oxides (SO2 and SO3) and nitrogen oxides (N2O, NO and NO2) are the main components of acid rain. These compounds are released into the atmosphere through the burning of fossil fuels. When they react with water droplets in the atmosphere, they form sulfuric acid (H2SO4) and nitric acid (HNO3). Together, these two acids cause the acidity of rainwater to increase.

Chemical reactions of formation of these acids:

  1. Formation of sulfuric acid:

S + O 2 → SO 2

SO 2 + OH → HOSO 2

HOSO 2 + O 2 → H 2 O + SO 3

SO 3 (g) + H 2 O(l) → H 2 SO 4 (l)

  1. Formation of nitric acid:

O 2 + N 2 → 2 NOT

O 2 + 2NO → 2 NO 2

3 NO 2 + H 2 O → 2 HNO 3 + NO

In the presence of these acids, the pH of rainwater can reach 4 to 2, extremely acidic values.


Human activities are the main causes responsible for this phenomenon of acid rain. As we have seen, the release of gases as a result of the use of fossil fuels is the main cause of the formation of acid rain.

Therefore, it is due to the use of fossil fuels in transportation, thermoelectric plants, industries and other forms of combustion. These can also form due to natural causes, such as the release of gases during the eruption of a volcano.


Industrialized countries are the most affected by acid rain. However, pollutants can be carried by air currents to distant locations.

This occurred in the lakes of Scandinavia, which became acidic due to rain as a result of the industrial activities of Germany, France and the United Kingdom.

For nature, the consequences of acid rain are the destruction of plant cover, the acidification of soils and the waters of rivers and lakes.

An example of the consequence of acid rain was observed in Brazil. The coastal municipality of Cubatão, in São Paulo, has a large concentration of industries and acid rain destroyed the vegetation on the slope of the Sierra del Mar, exposing the soil to erosion.

When acidification reaches the soil and waters of rivers and lakes, the living beings that inhabit those places are affected. Water and soil become unsuitable for housing some organisms, condemning them to death.
Acid rain can also cause corrosion of marble and limestone and oxidation of metals in historical monuments, such as buildings and statues.