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An acid is any hydrogen-containing substance that is capable of donating a proton (hydrogen ion) to another substance. A base is a molecule or ion able to accept a hydrogen ion from an acid. Acidic substances are usually identified by their sour taste.
Importance of pH: Acidity is an important factor for living things. For example, many plants grow best in soil that has a pH between and 7. Fish also need a pH close to 7. Some air pollutants form acids when dissolved in water droplets in the air. This results in acid fog and acid rain, which may have a pH of 4 or even lower. Acid fog and rain can kill trees and other plants. It can also lower the pH of surface waters such as ponds and lakes. As a result, the water may become too acidic for fish and many other water organisms to survive. Even normal rain is slightly acidic. That's because carbon dioxide in the air dissolves in raindrops, producing a weak acid called carbonic acid. When acidic rainwater soaks into the ground, it can slowly dissolve rocks, particularly those containing calcium carbonate. This is how water forms underground caves.
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