Page 62 - Chemistry ICSE Class IX

50 ICSE Chemistry – 9

Oxygen Green plants use carbon dioxide present in the air, and water present
(released in the soil to make food (mostly as sugars and carbohydrates) in the
Sun into air)
presence of chlorophyll and sunlight.
sunlight
Sunlight Carbon dioxide + Water o Carbohydrates + Oxygen
chlorophyll
Carbon dioxide (from air) (from soil) (food stored in (released into
(from air) the plant) the atmosphere)
or
sunlight
6CO (g) + 6H O(l) o C H O 6 + 6O (g)
2
2
2
6
12
chlorophyll
glucose oxygen
Plants thus convert solar energy into chemical energy of the food and
oxygen gas. This synthesized food is then distributed to different parts of
the plant/tree.
Consumption and production of carbon dioxide in nature
Water
ƒ The various processes by which carbon dioxide is produced are
Š Burning of fuels, such as wood, coal, petrol, diesel, etc.
Š Volcanic eruptions.
Fig. 2.5 Photosynthesis in plants
Š Manufacturing of lime from limestone, alcohol from molasses by
fermentation, etc.
WůĂŶƚƐ ĂůƐŽ ƵƐĞ ŽdžLJŐĞŶ ĚƵƌŝŶŐ Š Decay of organic matter.
ƌĞƐƉŝƌĂƟŽŶ͕ ďƵƚ ƚŚĞLJ ƉƌŽĚƵĐĞ ŵŽƌĞ Š Due to respiration. The exhaled air contains a large amount of
ŽdžLJŐĞŶ ĚƵƌŝŶŐ ƉŚŽƚŽƐLJŶƚŚĞƐŝƐ͘ carbon dioxide.
dŚĞƌĞĨŽƌĞ ƉůĂŶƚƐ ĂƌĞ ƚŚĞ oxygen-
producer ŝŶ ƚŚĞ ŶĂƚƵƌĞ͘ ƒ The various processes by which carbon dioxide is consumed are
Š During photosynthesis, plants absorb carbon dioxide.
Š Sea water and sediments use dissolved carbon dioxide for chemical
xation of carbon dioxide.
It is in this way that a balance in the concentration of carbon dioxide
and oxygen is maintained in the nature.

Fig. 2.6 Balance of oxygen (O ) and carbon dioxide (CO ) in the atmosphere
2
2
Oxygen Cycle
The percentage of oxygen in the air remains almost constant at 20.9% by
volume. This is because
WŚŽƚŽƐLJŶƚŚĞƐŝƐ ŝƐ ƚŚĞ ŵĂũŽƌ ƐŽƵƌĐĞ ŽĨ
ƉƵƌĞ ŽdžLJŐĞŶ ŝŶ ƚŚĞ ďŝŽƐƉŚĞƌĞ͘ ƒ Removal of oxygen from the atmosphere: Oxygen (present in the
air) is consumed by
Š Plants, animals and decomposers for respiration.
Š Fossil fuels during burning.
In both the processes, carbon dioxide and water are produced.
These products escape into the atmosphere.
Š The sediments in seas and oceans.