Cambridge O Level Chemistry · Syllabus 5070 · Chemistry of the Environment
Global Warming
What is Global Warming?
The rise in the average temperature of the Earth's surface caused by higher levels of greenhouse gases in the atmosphere, which reduce the amount of thermal energy lost to space. Global warming in turn leads to climate change, meaning long-term changes in weather patterns. Carbon dioxide and methane are the greenhouse gases responsible in this topic.
This definition is part of the Chemistry of the Environment chapter in Cambridge O Level Chemistry.
Global Warming in context
Chemistry of the environment covers water, air and carbon as three linked systems. Water is tested for presence with cobalt(II) chloride or copper(II) sulfate, and for purity with a sharp melting point of about \(0\,{}^\circ\mathrm{C}\) and boiling point of about \(100\,{}^\circ\mathrm{C}\); a domestic supply is then treated by sedimentation and filtration, carbon filtration and chlorination. Clean dry air is about 78% nitrogen and 21% oxygen, and six pollutants — carbon dioxide, carbon monoxide, particulates, methane, oxides of nitrogen and sulfur dioxide — each have a distinct source and effect, such as acid rain or global warming. Carbon dioxide absorbed in photosynthesis links combustion back to plant growth.
Common mistakes with Global Warming
- “Clean air contains no carbon dioxide.” Defect Treats a normal component as a contaminant. Chemistry Carbon dioxide is part of the remainder of clean dry air. The adverse effect belongs to higher levels of it. Repair “The remainder is mainly noble gases and carbon dioxide.” Transfer What is the required wording for its effect? “Higher levels increase global warming and climate change.”
- “Carbon dioxide causes acid rain.” Defect The effect belongs to a different pollutant. Chemistry Acid rain is caused by sulfur dioxide and oxides of nitrogen. The named effect of carbon dioxide is increased global warming and climate change. Repair Learn the pairs in blocks: climate for \(\mathrm{CO_2}\) and \(\mathrm{CH_4}\); acid rain for \(\mathrm{SO_2}\) and NOx. Transfer A limestone statue is eroding. Which two pollutants? Sulfur dioxide and oxides of nitrogen.
- “Sulfur dioxide is the main cause of global warming.” Defect The mirror image of the previous error. Chemistry Sulfur dioxide causes acid rain. The greenhouse gases named in this topic are carbon dioxide and methane. Repair Two gases for climate, two for acid rain — and none in both. Transfer Which control targets sulfur dioxide? Low-sulfur fuels, or flue gas desulfurisation with calcium oxide.
- “A catalytic converter turns harmful gases into harmless ones.” Defect One word too strong. Chemistry Nitrogen is benign, but carbon dioxide contributes to global warming. The syllabus wording is less harmful products. Repair Write “less harmful” and, if there is room, name the residual concern. Transfer Does fitting a converter reduce carbon dioxide from a car? No — it increases it slightly, by oxidising carbon monoxide.
Questions students ask about Global Warming
What is the source and effect of each of the six air pollutants?
Carbon dioxide comes from complete combustion of carbon-containing fuels and causes global warming. Carbon monoxide and particulates come from incomplete combustion; carbon monoxide is toxic and particulates increase respiratory problems and cancer risk. Methane comes from decomposing vegetation and animal digestion and also causes global warming. Oxides of nitrogen form in car engines and cause acid rain and photochemical smog. Sulfur dioxide comes from burning fossil fuels containing sulfur and causes acid rain.
How do carbon dioxide and methane cause global warming?
Carbon dioxide and methane absorb thermal energy radiated by the warmed surface of the Earth and re-emit it in all directions, so some of that energy returns towards the Earth instead of being lost to space. This is a molecular effect spread through the atmosphere, not a solid layer or a reflective surface, and the result is a rise in average surface temperature.
Why is "planting trees" not a complete answer to reducing air pollution?
Planting trees strengthens photosynthesis, removing carbon dioxide from the atmosphere, so it addresses global warming. But a growing forest takes decades to have an effect, needs land, and does nothing about sulfur dioxide or carbon monoxide from a chimney or exhaust. A full answer names the specific strategy that matches each specific pollutant, plus its limitation.