Page 63 - Chemistry ICSE Class IX

Chemical Changes and Reactions 51
ƒ Addition of oxygen into the atmosphere: Oxygen is produced when
Š Green plants make food from carbon dioxide and water in the
presence of sunlight. This process is called photosynthesis. The
released oxygen escapes into the air.
Š Ozone decomposes to produce oxygen.
2O (g) o 3O (g)
3
2
Diagram of the oxygen cycle
Based on the processes both natural and man-made, which consume and
those which produce oxygen, the following diagram describes the oxygen
cycle:

Fig. 2.7 The oxygen cycle

Carbon Dioxide Cycle

Carbon dioxide is consumed and produced in many natural and man-made
processes. The average concentration of CO in the atmosphere remains
2
constant at about 0.03% by volume. This shows that there exists a balance
between the consumption and production of carbon dioxide.
The complete pathway showing the consumption and production of
CO is called the carbon dioxide cycle.
2
Before we draw the carbon dioxide cycle, let us describe the various
processes or phenomena.
ƒ Removal of the atmospheric CO
2
Š Due to photosynthesis in plants
chlorophyll
6CO (g) + 6H O(l) o C H O + 6O (g)
12
6
2
2
2
6
sunlight
glucose oxygen
Š Due to chemical xation of CO in sea water and sediments
2
H O(l) + CO (g) o H CO (aq)
2
3
2
2
sea/ocean water carbonic acid
Ca(OH) 2 + H CO (aq) o CaCO (s) + 2H O
2
3
3
2
calcium hydroxide calcium carbonate
(limewater) (sediment)
CaCO (s) + CO (g) + H O o Ca(HCO )
3 2
2
3
2
soluble in water
ƒ Addition of CO into the atmosphere
2
Š From the burning of fuels such as wood, coal, petrol, natural gas,
and diesel
C(s) + O (g) o CO (g)
2
2
from coal released into the atmosphere
CH (g) + 2O (g) o CO (g) + 2H O(v)
4
2
2
2
natural gas (from air)
or
compressed natural gas