Why Is Rain Water Slightly Acidic?

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Hydrogen and oxygen elements combine in the ratio of 2:1; that is, two hydrogen atoms merge with an oxygen molecule to form water (H2O). Water flows freely on the ground surface to build water reservoirs like lakes, oceans, and swamps. Through evaporation, mostly from these water bodies, water vapor rises from the ground to the sky, and the vapor condenses to form clouds. Clouds are dense masses of condensed water. Thus, air containing water vapor combines with other molecules and particles to form clouds. These particulate matters and other combining substances might be contaminated. Water from the sky drops down in varying sizes when cloud weight exceeds wind pressure and can either be liquid (rainwater) or solid (ice and hail). Thus, the only difference between solid and liquid water is the temperature level, but other chemical composition remains the same. Some contamination might also happen at the ozone layer.

Pure rain water is slightly acidic at a pH level of 5.6. Contaminated rainwater can have a pH level of between 4 and 5. In most areas of the U.S., the pH level of rainwater has been ranging from 4-5. The contamination level in the environment causes these unfavorable pH levels. Chemical reactions that take place from the time water condenses until the time it reaches the ground as rainwater causes the acidity. That is at the ozone layer, in the sky, and in the atmospheric space. The pH measures the hydrogen ions in a substance or a liquid. A range of 0-14 is used to measure the pH values on a scale. A pH of 7 means neutral, and, thus, no hydrogen ions are present; a pH of more than 7 means negative ions are present; while a pH of less than 7 shows the presence of active hydrogen ions in the liquid. Consequently, any reaction that causes a rise in the hydrogen ions in water causes the rain water to be acidic. Natural or man-made factors may cause the reactions.

Specifically, Acidity in Rain Water Can Be Attributed to the Following Elements:

Major contributing factors

  1. Carbon Dioxide

Carbon dioxide gas is abundantly available in the atmosphere. It is essential for a healthy ecosystem as it is used by plants to process their food. It is a by-product emitted by living things during metabolism of foods. Water reacts with carbon dioxide to form carbonic acid. Due to high carbon dioxide emission in oceans, it explains why sea waters are acidic.

CO2+H2O →H2CO3

Carbonic acid then breaks down into hydrogen ions and hydro carbonates ions.

H2CO3→HCO3+ H+

The hydrogen ion raises the water pH and, thus, causes the rainwater to be slightly acidic.

  1. Nitrogen Oxide

During lightning storms, nitrogen and oxygen react to form nitrogen oxide.


Oxidation of nitrogen oxide produces nitrogen dioxide that in turn reacts to form nitric acid with water.



The acid dissociates in water to form hydrogen ions and nitrate ions.

HNO3→H++ NO3

The hydrogen ions are responsible for the rise in acidic level in the water.

  1. Sulfur Dioxide

It is a result of a chemical reaction between sulfur and oxygen. Sulfur is readily available in the air. Combustion of fossil fuels deposit sites produce sulfur. During the heating process, a lot of sulfur oxide is emitted. Sulfur dioxide plus rainwater forms sulfuric acid.



Sulfuric acid dissociates in water to give hydrogen ions. These hydrogen ions cause the pH level to raise, causing acidity.

H2SO4→HSO4+ H+

Minor contributing factors

Most oxides formed from the non-metal element of groups 14-17 form acidic oxides with oxygen. When the oxides react with water, they form solutions that emit hydrogen ions that are responsible for the varying pH levels.

  1. Volatile Organic Compounds (VOC)

These are the chemical elements that evaporate with ease. They are readily present in the atmosphere; and in the presence of sunlight, they combine with nitrogen and sulfur oxides to form ground-level ozone or smog.



The sulfuric acid then gives rise to hydrogen ions and causes slight acidity in rainwater.

  1. Carbon Monoxide

Burning fuels in a limited oxygen supply produces carbon monoxide. This may be from the combustion of machines and moving engines. Oxygen reacts with carbon monoxide to give out carbon dioxide. Carbon dioxide water results in carbonic acid. It in turn causes acidity in rainwater as discussed above.

  1. Hazardous Air Pollutants

Hazardous air pollutants are all elements in the air either in the form of gases or nongaseous substances. Just as the name suggests, they pose risks to the atmospheric natural balancing mechanism. These may form oxides with oxygen and finally react in water to form acidic solutions. These solutions emit hydrogen ions that cause acidity in rainwater.

Other elements and factors that may cause acidity in rainwater include lead-laden ozone layers, chlorofluorocarbons, and other atmospheric particulate matters. A contaminated ozone layer will make harmful particles and molecules stay in the air and merge with water molecules to form solutions, which are acidic.

Elimination of acidity in rainwater is almost impossible, but artificial measures should be put in place so negligible levels of acidity can be achieved. Bottled water has no carbon dioxide. Due to this, it has a pH level of 7. Acidity in rain causes adverse effects in buildings through corrosion and peeling of paints. It can also cause withering or even drying of green vegetation.

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