Page 29 - 新思维科学学生用书9 样章

1 Photosynthesis and the carbon cycle

Continued

Self-assessment
The instructions for this experiment are quite long.
How carefully did you follow the instructions?

How fully do you feel you understand why the indicator changed to different colours
in the different tubes?

Fossil fuels and combustion

There is one more very important set of processes to add to the diagram showing how carbon moves
from the air, through organisms, and back to the air again.

When organisms die, they are not always broken down quickly by decomposers. Sometimes, their
bodies fall into places where there is no oxygen, such as deep in the ocean. In these places, the
decomposers cannot respire, because there is not enough oxygen for them. Instead, the organisms’
bodies get gradually buried, as more and more sediment builds up on top of them. High pressure and
heat change their remains into fossil fuels, including coal, oil or natural gas.

Changing dead organisms to fossil fuels takes a very long time. Most of the fossil fuels that we use on

Earth today were formed hundreds of millions of years ago.

Oil and natural gas formed when tiny marine organisms died
and fell to the sea bed. This oil rig, in the sea off West Africa,
has pipes that go deep into the sea bed where deposits
of liquid oil are present. The oil is brought up through

the pipes, and taken ashore to be used as fuel.

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