Environmental chemistry: the atmosphere: Class 10 Chemistry

Key points

The atmosphere is the layer of gases around the Earth held by gravity. Dry air by volume is about 78% nitrogen, 21% oxygen, 0.9% argon and about 0.04% carbon dioxide, with traces of neon, helium, methane and hydrogen. Air also holds a variable amount of water vapour, dust and other particles.

Layers of the atmosphere from the ground upward:

• Troposphere: up to about 12 km. It holds most of the air and all weather. Temperature falls with height.
• Stratosphere: up to about 50 km. It contains the ozone layer, and temperature rises with height as ozone absorbs ultraviolet rays.
• Mesosphere: up to about 85 km. Temperature falls again, and meteors burn here.
• Thermosphere: above this, with very thin air and high temperature. The ionosphere lies in it and reflects radio waves.

Air pollution is the presence of harmful substances in air at levels that damage health, plants, buildings or climate. Pollutants may be primary, released directly (CO, SO₂), or secondary, formed in the air by reactions of primary pollutants (ozone, smog, acid rain). Sources are natural (volcanoes, dust storms, forest fires) and human (vehicles, factories, burning of crop residue and waste).

Main pollutants and their effects:

• Carbon monoxide (CO): from incomplete combustion. It combines with haemoglobin more strongly than oxygen does and causes headache, then death.
• Sulphur dioxide (SO₂): from burning coal and oil. It irritates the lungs and causes acid rain.
• Nitrogen oxides (NO, NO₂): from vehicle engines and power plants. They cause acid rain and smog.
• Hydrocarbons and lead compounds: from fuels. Lead harms the brain, which is why leaded petrol has been banned in most countries.
• Particulate matter (PM₂.₅, PM₁₀): soot and dust that enter the lungs.
• Chlorofluorocarbons (CFCs): from old refrigerators and aerosols.

Acid rain is rain with a pH below 5.6. Normal rain is slightly acidic, about pH 5.6, because of dissolved carbon dioxide. SO₂ and nitrogen oxides oxidize and dissolve in water to give sulphuric and nitric acids: SO₂ + ½O₂ → SO₃ and SO₃ + H₂O → H₂SO₄. Acid rain damages forests, kills fish in lakes, and corrodes marble and metals. Marble: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂.

Smog is a mixture of smoke and fog. Classical smog comes from coal smoke and SO₂ with fog. Photochemical smog forms in strong sunlight when nitrogen oxides and hydrocarbons react to give ozone and other irritants such as PAN. It causes eye irritation, cough, and breathing trouble. Heavy smog in winter has been a serious problem in Lahore and other cities of Punjab.

The greenhouse effect is the natural warming of Earth. Sunlight passes through the air and warms the ground. The ground then gives off infrared heat, and greenhouse gases (CO₂, CH₄, N₂O, water vapour, CFCs) absorb a part of it and send it back. Without it, Earth would be about 33 °C colder on average. The danger is the extra warming from more greenhouse gases.

Global warming is the rise in the average temperature of Earth due to the increase of greenhouse gases from burning fossil fuels, cutting forests, farming and industry. Effects include melting of glaciers and ice caps, rise of sea level, more floods and heat waves, and changes to farming. Pakistan has glaciers in the north and has felt serious floods and heat waves.

The ozone layer is a region in the stratosphere, about 15 to 35 km above the ground, where ozone (O₃) absorbs most harmful ultraviolet-B and UV-C radiation. Ozone is formed when UV splits O₂ into atoms which combine with O₂: O₂ → 2O and O + O₂ → O₃.

CFCs destroy ozone. In the stratosphere, UV breaks CFCs to release chlorine atoms. Cl + O₃ → ClO + O₂ and then ClO + O → Cl + O₂. The chlorine atom is regenerated, so one atom can destroy thousands of ozone molecules. The loss of ozone, mainly over the polar regions, lets in more UV, which can cause skin cancer, cataracts and damage to crops. The Montreal Protocol (1987) phased out CFCs.

Control of air pollution:

• Use of catalytic converters in vehicles to change CO, NO and hydrocarbons to CO₂, N₂ and water.
• Switching to cleaner fuels such as CNG and to solar, wind and hydro power.
• Scrubbers and filters in factory chimneys to remove SO₂ and particles.
• Planting trees, which take in CO₂ and give out oxygen.
• Regular vehicle checks, banning burning of rubbish and crop residue, and public transport.

Study tip: Learn the composition of air and the order of layers with one fact each. For pollution questions, use a fixed pattern: pollutant, source, effect, control. Practise writing the chlorine ozone reactions and the acid rain equations.

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Common mistakes

• Saying that the ozone layer is in the troposphere. It is in the stratosphere. Ozone near the ground is a pollutant.
• Mixing up the greenhouse effect and ozone depletion. They are different problems with different causes.
• Writing that normal rain has a pH of 7. Clean rain is about 5.6.
• Saying CO₂ is the main component of air. Nitrogen makes up about 78%.
• Listing only effects and not control measures when a question asks for both.

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