Acidity is measured using a pH scale, with the number 7 being neutral. Consequently, a substance with a pH value of less than 7 is acidic, while one of a value greater than 7 is basic. It is also worthwhile to note that the pH scale is logarithmic; that is, a substance of pH of 6 is 10 times more acidic than another with a pH of 7. Generally, the pH of 5.6 has been used as the baseline in identifying acid rain, although there has been many debates over the acceptance of this value. Interestingly enough, a pH of 5.6 is the pH value of carbon dioxide in equilibrium with distilled water. Hence, acid ran is defined as any rainfall that has an acidity level beyond what is expected in non-polluted rainfall. In essence, any precipitation that has a pH value of less than 5.6 is considered to be acid precipitation. The following is a visual illustration of the pH scale:
WHAT CAUSES ACID RAIN?
One of the main causes of acid rain is sulphur dioxide. Natural sources that emit this gas are volcanoes, sea spray, rotting vegetation and plankton. However, the burning of fossil fuels, such as coal and oil, are largely to be blamed for approximately half of the emissions of this gas in the world. When sulphur dioxide reaches the atmosphere, it oxidises to first form a sulphate ion. It then becomes sulphuric acid as it joins with hydrogen atoms in the air and falls back down to earth. Oxidation occurs the most in clouds and especially in heavily polluted air where other compounds such as ammonia and ozone help to catalyse the reaction, converting more sulphur dioxide to sulphuric acid. However, not all of the sulphur dioxide is converted to sulphuric acid. In fact, a substantial amount can float up into the atmosphere, move over to another area and return to earth unconverted.
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