Cambridge IGCSE Chemistry · Syllabus 0620 · Chemistry of the Environment
Greenhouse Gas
What is Greenhouse Gas?
A gas in the atmosphere that absorbs, reflects and emits the thermal energy radiated by the warmed surface of the Earth, so that some returns towards the Earth and less is lost to space. Carbon dioxide and methane are the two greenhouse gases named in this topic. They are molecules spread thinly through the atmosphere, not a solid layer or a mirror.
This definition is part of the Chemistry of the Environment chapter in Cambridge IGCSE Chemistry.
Common mistakes with Greenhouse Gas
- “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.
- Supplement 10.3.7 “Greenhouse gases trap the ultraviolet rays coming from the Sun.” Defect The wrong energy, travelling the wrong way. Chemistry Energy from the Sun passes through and warms the surface. It is the thermal energy radiated outwards by the warmed Earth that greenhouse gases absorb. Repair Start the answer at the surface, not at the Sun. Transfer Why is it colder on the Moon at night? No atmosphere, so all the radiated thermal energy is lost to space.
- Supplement 10.3.7 “Greenhouse gases form a layer that reflects heat back like a mirror.” Defect A picture that predicts the wrong physics. Chemistry The syllabus does name reflection, but not a mirror. They are individual molecules spread thinly through the atmosphere; they absorb and reflect thermal energy and emit the absorbed energy again in all directions, so less is lost to space. Repair Use the three syllabus terms — absorb, reflect, emit — and the consequence: reduce the loss to space. Transfer If it were a mirror, why would incoming sunlight still reach the ground? It would not — which is how you know the picture is wrong.