C9 is the most descriptive topic in chemistry. There are almost no calculations, but a great deal to describe and explain precisely. It runs in three acts: how the atmosphere evolved over 4.6 billion years from volcanic carbon dioxide to today’s nitrogen and oxygen; how greenhouse gases warm the planet and how human activity is enhancing that effect; and how burning fuels pollutes the air. The marks live in the detail: the right gas, the right process, and a clear cause-and-effect chain.
How this topic works
- Everyone: the whole of C9 (4.9) is on both Combined Science (Trilogy) and Triple Chemistry papers. There is no Triple-only or Higher-only content, and no required practical.
- The skills examined are mostly extended writing: describe and explain the changes in the atmosphere, describe the greenhouse effect using short- and long-wavelength radiation, and evaluate evidence and actions on climate change, including the 6-mark levels-of-response questions this topic is famous for.
One distinction underpins the whole topic and earns marks throughout: a greenhouse gas (water vapour, CO2, methane) warms the planet; an atmospheric pollutant (CO, SO2, NOx, particulates) is something not found in clean air that causes problems near the ground. Don’t mix them up.