Greenhouse gases — AQA GCSE Chemistry
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Greenhouse gases explained
Greenhouse gases covers 2 specification points, including Greenhouse gases in the atmosphere maintain… and Students should be able to describe the….
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Open a point below for its explanation and revision guidance.
Greenhouse gases in the atmosphere maintain temperatures on Earth high enough to support life. Water vapour, carbon dioxide and methane are greenhouse gases.
Energy from the Sun reaches Earth mainly as short wavelength radiation, including visible light and ultraviolet. Earth's surface absorbs this energy and warms, then re-emits energy as longer wavelength infrared radiation. Greenhouse gases in the atmosphere absorb some of this outgoing infrared radiation and re-emit it in all directions, including back towards Earth. This traps energy and keeps the average surface temperature higher than it would otherwise be, warm enough for liquid water and life. Water vapour, carbon dioxide and methane are the greenhouse gases named in this specification. Without this natural greenhouse effect, Earth's surface would be far colder.
Students should be able to describe the greenhouse effect in terms of the interaction of short and long wavelength radiation with matter.
The greenhouse effect depends on how radiation of different wavelengths interacts with matter. Short wavelength radiation from the Sun, mainly visible and ultraviolet, passes through the atmosphere and is absorbed by Earth's surface, warming it. The warmed surface emits longer wavelength infrared radiation. Greenhouse gas molecules in the atmosphere absorb this infrared radiation because their bonds can vibrate at frequencies matching that energy. The absorbed energy is re-emitted in all directions, including back to Earth, so less escapes to space and the surface stays warmer. This is a continuous energy balance: incoming short wavelength radiation, outgoing long wavelength radiation, and absorption and re-emission by greenhouse gases.
Your focus
- Name water vapour, carbon dioxide and methane as greenhouse gases.
- Describe how greenhouse gases absorb and re-emit infrared radiation.
- Explain that the natural greenhouse effect maintains temperatures high enough to support life.
Show all 6 objectives
- Describe the greenhouse effect using short and long wavelength radiation.
- Explain how greenhouse gases absorb and re-emit infrared radiation.
- Relate the greenhouse effect to the balance of incoming and outgoing energy.
Greenhouse gases exam tips
Marking Points
- Greenhouse gases absorb and re-emit infrared radiation emitted by Earth's surface, reducing energy lost to space.
- This trapped energy keeps average surface temperatures higher than they would be without the greenhouse effect.
- The natural greenhouse effect is essential: it maintains temperatures high enough for liquid water and life.
- Water vapour, carbon dioxide and methane are identified as greenhouse gases.
- Short wavelength radiation from the Sun passes through the atmosphere and warms Earth's surface.
- Earth re-emits longer wavelength infrared radiation, which greenhouse gases absorb.
- Short wavelength radiation from the Sun, including visible and ultraviolet, passes through the atmosphere and is absorbed by Earth's surface.
- Earth's warmed surface emits longer wavelength infrared radiation.
- Greenhouse gas molecules absorb infrared radiation and re-emit it in all directions, including back towards Earth.
- Because some infrared energy is returned, less energy escapes to space and surface temperature is maintained.
- The interaction depends on wavelength: short wavelength radiation is largely transmitted, while long wavelength infrared is absorbed by greenhouse gases.
- The process is a balance between incoming solar energy and outgoing infrared energy.
Examiner Tips
- 💡Use the words 'absorb' and 're-emit' when describing what greenhouse gases do to infrared radiation.
- 💡Distinguish incoming short wavelength radiation from outgoing long wavelength infrared radiation.
- 💡State clearly that the natural greenhouse effect keeps Earth warm enough for life before discussing any enhancement.
- 💡Use the terms 'short wavelength' for incoming solar radiation and 'long wavelength' or 'infrared' for radiation emitted by Earth.
- 💡Explain the sequence: Sun emits short wavelength, Earth absorbs and warms, Earth emits infrared, greenhouse gases absorb and re-emit.
- 💡Avoid saying heat is 'trapped' without explaining absorption and re-emission, as this shows the mechanism more clearly.
Common Mistakes
- Confusing the greenhouse effect with the ozone layer; the ozone layer absorbs ultraviolet radiation, while greenhouse gases absorb infrared radiation.
- Thinking greenhouse gases stop all heat leaving Earth; they slow energy loss, and some infrared still escapes to space.
- Believing the greenhouse effect is entirely harmful; the natural effect is necessary for life, and the issue is its enhancement by extra greenhouse gases.
- Saying greenhouse gases reflect infrared radiation; they absorb and re-emit it, which is different from reflection.
- Claiming short wavelength radiation is trapped by greenhouse gases; it is mainly the longer wavelength infrared emitted by Earth that is absorbed.
- Describing the greenhouse effect as a one-way trapping process; energy is continuously absorbed, re-emitted and partly lost to space.