Acid rain is formed when sulfur dioxide SO2 and nitrogen oxides NOx are released into the atmosphere by burning fossil fuels, volcanic eruptions, or other industrial processes. These compounds react with water, oxygen, and other substances in the atmosphere to form sulfuric acid H2SO4 and nitric acid HNO3 , which then fall to the ground as acid rain.When acid rain enters freshwater streams, it lowers the pH levels of the water, making it more acidic. The pH scale ranges from 0 to 14, with 7 being neutral. Freshwater streams typically have a pH level between 6 and 8. Acid rain can cause the pH levels to drop below 6, which can have detrimental effects on aquatic ecosystems and organisms.The chemical reactions that occur when acid rain interacts with aquatic organisms depend on the specific species and their tolerance to acidity. Some general reactions include:1. Dissolution of calcium carbonate CaCO3 in shells and exoskeletons: Acid rain can cause the dissolution of calcium carbonate, which is a primary component of shells and exoskeletons of many aquatic organisms such as mollusks, crustaceans, and some plankton species. The reaction is as follows:CaCO3 s + H2SO4 aq CaSO4 aq + H2O l + CO2 g This reaction weakens the shells and exoskeletons, making the organisms more vulnerable to predation, disease, and other environmental stressors.2. Disruption of physiological processes: Acidic conditions can disrupt the normal physiological processes of aquatic organisms, such as respiration, reproduction, and metabolism. For example, fish gills can be damaged by the acidic water, impairing their ability to extract oxygen from the water and leading to suffocation.3. Mobilization of toxic metals: Acid rain can cause the release of toxic metals, such as aluminum Al , from the soil and rocks into the water. The reaction is as follows:Al OH 3 s + 3H+ aq Al3+ aq + 3H2O l These toxic metals can accumulate in the tissues of aquatic organisms, leading to toxic effects and potentially causing death.4. Changes in nutrient availability: Acid rain can alter the availability of essential nutrients in the water, such as phosphorus and nitrogen, which can impact the growth and reproduction of aquatic organisms.Overall, the chemical reactions caused by acid rain can have severe consequences for aquatic ecosystems, leading to a decline in biodiversity and the collapse of food webs. Efforts to reduce SO2 and NOx emissions from industrial processes and transportation are crucial to mitigating the effects of acid rain on freshwater streams and their inhabitants.