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Summaries · Grade 7 Natural Sciences › How fossil fuels form and the Sun's energy
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How fossil fuels form and the Sun's energy

How coal, oil and gas form, why they are ‘stored sunlight’, and how the Sun powers all life. Read one part at a time and tap “I know this”. Then test yourself in the quiz.

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How coal forms

Swamp plantsdieMud coversthem (littleoxygen)PeatMillions ofyears of heatand pressureCoal(anthracitethe hardest)
From swamp to anthracite.

The coal layers in the Karoo rocks contain fossils of plants such as Glossopteris: proof that it formed from such swamp plants. Coal forms in stages: peat → lignite (brown coal) → bituminous coal → anthracite (the hardest, with the most carbon). Coal holds chemical potential energy; when it burns in a stove, this becomes heat and light.

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Stored sunlight

Rule

carbon dioxide + water → (sunlight) → glucose + oxygen

Ancient plants captured the Sun’s energy by photosynthesis. That is why we call fossil fuels ‘stored sunlight’. When they burn, the carbon dioxide the plants took out of the air long ago goes back into the air.

The Sun’s energy: from ancient ferns to today’s stove.
The Sun’s energy: from ancient ferns to today’s stove.
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The Sun powers everything

  • In grass → impala → lion, the lion’s energy originally comes from the Sun; at each step most is lost as heat.
  • Wind power is also solar energy: the Sun heats the air unevenly.
  • Hydro power depends on the Sun: the Sun drives the water cycle.
  • Uranium is not a fossil fuel.
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Remember this and test yourself

Remember this: your summary
  • Coal: plants in swamps, covered by sediment, changed by heat and pressure over millions of years.
  • The plants did not rot completely: mud covered them, so little oxygen got in. The first stage is peat; anthracite has the most carbon.
  • Oil and gas: tiny sea organisms on the sea floor; trapped in porous rock under a seal of non-porous rock.
  • The Sun’s energy comes from hydrogen joining to form helium in its core; it reaches us as radiation.
  • Sasol at Secunda makes petrol from coal; natural gas was found under the sea near Mossel Bay.

Write these points in your book. Read them again the night before the test.

Memory trick

Coal = old forest, oil = old ocean. Both = stored sunlight, baked under pressure for millions of years.

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Important words

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