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In the past century, due to the burning offossil fuels in energy plants and cars, acid rainhas become a cause of harm to the environment.However, rain would still be slightly acidic even if05these activities were to stop. Acid rain would con-tinue to fall, but it would not cause the problemswe see now. The environment can handle slightlyacidic rain; it just cannot keep up with the level ofacid rain caused by burning fossil fuels.10A pH of 7 is considered neutral, while pH below7 is acidic and pH above 7 is alkaline, or basic.Pure rain water can have a pH as low as 5.5. Rainwater is acidic because carbon dioxide gas in the airreacts with the water to make carbonic acid. Since15it is a weak acid, even a large amount of it will notlower the pH of water much.Soil, lakes, and streams can tolerate slightlyacidic rain. The water and soil contain alkalinematerials that will neutralize acids. These include20some types of rocks, plant and animal waste, andashes from forest fires. Altogether, these materialscan easily handle the slightly acidic rain that occursnaturally. The alkaline waste and ashes will slowlybe used up, but more will be made to replace it.25Anthropomorphic causes of acid rain, such asthe burning of fossil fuels, release nitrogen oxideand sulfur oxide gases. These gases react with waterto make nitric acid and sulfuric acid. Since theseare both strong acids, small amounts can lower30the pH of rain water to 3 or less. Such a low pHrequires much more alkaline material to neutral-ize it. Acid rain with a lower pH uses up alkalinematerials faster, and more cannot be made quicklyenough to replace what is used up. Soil and water35become more acidic and remain that way, as theyare unable to neutralize the strong acid.
In humans, keeping a constant balance betweenacidity and alkalinity in the blood is essential. Ifblood pH drops below 7.35 or rises above 7.45,40all of the functions in the body are impaired andlife-threatening conditions can soon develop. Manyprocesses in the body produce acid wastes, whichwould lower the pH of blood below the safe levelunless neutralized. Several systems are in place45to keep pH constant within the necessary range.Certain conditions, however, can cause acids to bemade faster than these systems can react.Most of the pH control involves three relatedsubstances: carbon dioxide, carbonic acid, and50bicarbonate ions. Carbonic acid is formed whencarbon dioxide reacts with water. Bicarbonateions are formed when the carbonic acid releases ahydrogen ion. Excess carbonic acid lowers the pH,while excess bicarbonate ions raise it.55The kidneys store bicarbonate ions and willrelease or absorb them to help adjust the pH ofthe blood. Breathing faster removes more carbondioxide from the blood, which reduces the amountof carbonic acid; in contrast, breathing more slowly60has the opposite effect. In a healthy body, thesesystems automatically neutralize normal amountsof acid wastes and maintain blood pH within thevery small range necessary for the body to functionnormally.65In some cases, these systems can be overwhelmed.This can happen to people with diabetes if theirblood sugar drops too low for too long. People withdiabetes do not make enough insulin, which thebody uses to release stored sugar into the blood to70supply the body with energy. If a person's insulinlevel gets too low for too long, the body breaks downfats to use for energy. The waste produced frombreaking down fats is acidic, so the blood pH drops.If the kidneys exhaust their supply of bicarbonate75ions, and the lungs cannot remove carbon dioxidefast enough to raise pH, all of the other functions inthe body begin to fail as well. The person will needmedical treatment to support these functions untilthe pH balancing system can catch up. The system80will then keep the blood pH constant, as long as theproduction of acid wastes does not exceed the body'scapacity to neutralize them.
1. Passage 1 most strongly suggests that
2. Which choice provides the best evidence for the answer to the previous question?
3. According to the information in Passage 1, which pH level for rain water would cause the most damage to the environment?
4. As used in line 17, "tolerate" most nearly means
5. Passage 2 most strongly suggests that
6. Which choice provides the best evidence for the answer to the previous question?
7. As used in line 74, "exhaust" most nearly means
8. Which of the following plays a role in the environment most similar to the role played by excess bicarbonate ions in the blood?
9. Based on the information in Passage 2, which of the following can cause the body to break down fats to use for energy?
10. Which of the following best describes a shared purpose of the authors of both passages?
11. Both passages support which of the following generalizations?
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