Chemistry
Environmental Chemistry
7,044 words, 11 sections
Environmental chemistry is the chemistry of what humans release into air, water and soil, and of what those releases then do. The subject splits into air and water, and the air side dominates: the chapter has been asked 72 distinct times across 33 official papers between 2001 and 2022, on five boards (DUHS, Federal, KMU, NUMS and UHS), with a mean item difficulty of 1.83 on the bank scale and 319 active questions in the practice bank. It is not named in the official PMDC syllabus table, so it is examined material the table does not list, and it belongs with the physical and inorganic units it draws on. A student who has mastered it can name each major pollutant and its source, write the reactions that turn smokestack gases into acid rain, explain how a CFC destroys ozone without being used up, describe how a waterworks turns raw river water into drinking water, and say why a catalytic converter needs an oxygen monitor.
The atmosphere is layered by the way temperature changes with height, and the layers decide where each pollutant does its damage. The troposphere is the lowest layer, and the federal and Punjab books give it a depth of about 12 km. It holds the major gases, nitrogen, oxygen and carbon dioxide, it is the densest layer, and it is where weather happens: rain, lightning and hurricanes all live here. Temperature falls steadily as you climb, from roughly 17 °C at the surface to about -58 °C at the top, and pressure falls with it from about 760 torr at sea level. Sindh writes the same range slightly tighter, from 15 °C to -56 °C, and that small difference is the only variation between the books on the layer.
Above the troposphere sits the stratosphere, from about 12 km to 50 km, and here the temperature rises instead, from about -58 °C to -2 °C, because ozone absorbs high-energy ultraviolet light. That absorption is itself the chemistry of the layer. In the upper stratosphere ozone takes up the ultraviolet and breaks apart:
In the middle stratosphere, where less ultraviolet reaches, the monoatomic oxygen finds a molecule of oxygen again and the two recombine to remake ozone, an exothermic step that warms the air. The lower stratosphere receives very little ultraviolet, so no monoatomic oxygen is present and no ozone is formed there. The ozone layer itself sits 25 to 28 km high and filters most of the harmful ultraviolet before it reaches the ground. Down here in the troposphere ozone is a pollutant, but up there it is the shield.
The two outer layers are cooler and hotter respectively. The mesosphere runs from about 50 km to 80 km and temperature falls from -2 °C to -93 °C, making it the coldest part of the atmosphere. The thermosphere runs from about 80 km to 600 km and temperature climbs as far as 500 °C to 2000 °C, because it absorbs energetic ultraviolet and X-rays. The composition the questions test is simpler than the layering: air is about 78 per cent nitrogen by volume, oxygen about 21 per cent, and carbon dioxide only about 0.03 per cent. Nitrogen and oxygen sit together through an entire sky without reacting, because the nitrogen molecule is highly inactive, its two atoms held by a triple bond.
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