Acid rain is defined as precipitation in any form that has acidic properties, such as nitric and sulfuric acid which is formed in the atmosphere and falls to the ground. The pH scale is used the measure acidity and alkalinity in solutions. The lower the substance’s pH value is (less than 7), the more acidic the substance is. If the substance has a higher pH value (greater than 7) then the more alkaline it is. Normal rain has a pH value of 5.6, acid rain refers to solutions with a pH below 5.6 because carbon dioxide dissolves into the rain forming a weak carbonic acid. Throughout this experiment, the acids will have a pH of 2. This is due to the reason that building material in the real world is exposed to acid rain constantly, through many years. However, with this experiment, 3 weeks is the time frame, therefore, to simulate years of exposure, the pH of the acid was lowered.
Acid deposition is another term for acid rain, but acid deposition is a broader term than acid rain as it includes all the ways acidic components leave the atmosphere and return to the ground. Furthermore, there are two types of acid deposition wet deposition and dry deposition. Wet acid deposition occurs when sulfuric and nitric acids formed in the atmosphere fall to the ground mixed with rain, snow, sleet, hail, fog, mist, dew and other precipitates. Acid particles can deposit from the atmosphere without moisture in the form of dry deposition. This form of deposition includes dust and smoke, which later dissolves in water forming acids.
Furthermore, Acid rain can be formed from man-made or natural resources, in which sulfur dioxide or nitrogen oxides are emitted into the atmosphere where they react with water, oxygen and other chemicals to form sulfuric and nitric acid. The acids then mix with water before falling to the ground. The main acids that are apart of acid rain are nitric and sulfuric acid. The small portion of these acids is produced from natural sources, as nitrogen oxides can be formed when there is an extreme heating of air during a storm in which lightning is produced. Additionally, sulfur dioxide can be emitted into the atmosphere by erupting volcanoes and rotting vegetation. Although a portion of acid rain originates from natural resources, much of the acid rain and public attention originates from man-made sources. Sulfur dioxide is produced from the burning of fossil fuels, mainly coal and oil used in power plants Furthermore, the act of burning of fossil fuels to generate electricity generates 50% percent of total sulfur dioxide emission. The reaction of sulfur dioxide to form sulfurous acid is listed below: Furthermore, sulfur dioxide can be oxidized to form sulfur trioxide, which then dissolves in water in the atmosphere to form sulfuric acid as presented below. Combustion engines are the main sources of nitrogen monoxide The burning of fuel releases heat energy, which helps combine nitrogen and oxygen. This reaction also helps combine to form Nitrogen dioxide and the oxidation of nitrogen monoxide also forms nitrogen dioxide. Nitrogen dioxide dissolves in water in the atmosphere to form a mixture of nitrous acid and nitric acid.
Moreover, acid rain is not limited to the area where the sources emitted the chemicals, as winds can blow sulfur dioxide and nitrogen oxide over long distances before being deposited, therefore, acid rain can affect many areas. Methanoic acid and Acetic acid will also be used in this experiment, as these acids are the dominant carboxylic acid in the troposphere. Both of these acids are major sources of atmospheric acidity and both contribute to more than 60% of the acidity in precipitation in remote areas and 30% in more polluted regions. These acids originate from the oxidation of volatile organic compounds and are emitted through a variety of ways. Furthermore, hydrochloric acid will be used as volcanic activity can also produce this acid. Hydrogen chloride can be emitted when coal is burned and when waste is burned, this occurs because coal and waste foods contain common salt (sodium chloride), which then reacts with hydrogen to give hydrogen chloride. Moreover, this Hydrogen chloride gas reacts with water in the air to form clouds of hydrochloric acid.
Acid rain has been linked to many detrimental effects on the environment and in human health. At higher elevation trees tend to be surrounded by acidic clouds and mists. Therefore, acidic deposition can damage the leaves of trees and it could remove minerals and nutrient from the soil, so the trees become susceptible to disease. For plants, acid rain can cause widespread damage as it has been known that acid rain can be the direct cause of slower growth, injury or death of plants. Dry deposition can negatively affect plants by blocking the pores for gas exchange. The negative impacts of acid rain can also be seen in aquatic environments, such as lakes, streams, and rivers where it can damage the wildlife there. When acid rain flows through the soil, the rain can leach aluminum particles into the river. The aluminum ions interfere with the fish’s gills and reduce their ability to take in oxygen, therefore fish communities dwindle due to high mortality and failed reproduction. A health lake has a pH of 6.5 or higher, and only a few fish can live at a pH below 5, when the pH reaches 4, the body of water is considered dead. Acid deposition also has a significant impact on building materials. Since marble and limestone are both forms of calcium carbonate, sulfuric acid can react with them, forming the calcium salt CaSO4. The reaction is shown below: Calcium sulfate has greater molar volume than calcium carbonate, therefore, when it is formed it causes stress for the material. These reactions cause the erosion of structures, as many buildings and statues are subject to the negative impact of acid rain. Furthermore, Acid rain also inflicts damage on metals, as it causes corrosion. Acid deposition reacts with metals such as iron to form salts. Therefore, ionic conductivity occurs which leads to an increase in the rate of electrochemical corrosion reactions such as rusting.
The reaction for iron is shown below:
Because of these reactions, Acid rain causes significant damage to metallic structures such as buildings, bridges, and vehicles. The focus of this investigation will be on the impact of acid rain on metals, specifically tin. Tin is a highly workable metal, it is used as sheets in the construction of buildings and roofs, for storage containers, and for soldering or joining metal parts. Therefore, it is highly beneficial to test different acids and their impact on tin. Furthermore, tin is relatively unaffected by both water and oxygen at room temperature, it does not corrode easily. Tin is attacked only slowly by dilute acids such as hydrochloric acid and sulfuric acids.