made by Kanupriya Garg chemistry class 9

1. WATER CYCLE AND
NITROGEN CYCLE
MADE BY: KANUPRIYA GARG

2. WATER CYCLE
◦ There is a constant exchange of water
between air, land, ocean and the living
organisms. The whole process by which
water evaporates and falls on the land as rain
and later flows back into the sea river is called
the water cycle or the hydrological cycle.

3. PROCESSES
INVOLVED IN
THE WATER
CYCLE
◦ Stage I: Evaporation : The sun’s energy heats up the lakes, rivers, oceans,
swamps and other water bodies which subsequently increase the temperature of
the water present in them. Consequently, some water evaporates as vapour into
the air. The rising air currents take the vapour up in the sky.
◦ Transpiration: Simultaneously, plants and trees also lose water to the
atmosphere in the form of vapour which rises up in the sky.
◦ Sublimation: It occurs when ice or snow changes directly into water vapours
without changing into water
◦ Stage II: Condensation As the vapours rise high, the cooler temperatures make
them cool down and turn back into liquid – condensation. Wind and air currents
move the moisture around, leading to the formation of clouds.
◦ Stage III: Precipitation Wind movements cause the clouds particles to collide.
As they become water laden, they develop into rain bearing clouds and fall back
onto the earth’s surface by the process known as precipitation. This may occur in
the form of rain, hail, snow or sleet depending upon the temperature conditions.
◦ Stage IV: Runoff and Infiltration The precipitation either runs off into oceans,
rivers and ground surface or is absorbed into the soil (infiltration).

4. NITROGEN
CYCLE
◦ It is the cyclic process by which nitrogen
passes from its elemental form present in the
atmosphere into simple nitrogenous compound
present in the soil and water that can enter the
living beings and forms complex molecules in
them. These molecules are then broken down
again to release nitrogen back into atmosphere.

5. STEPS
INVOLVED IN
THE PROCESS
OF NITROGEN
CYCLE
◦ (i) Nitrogen fixation: It is the initial step of the nitrogen cycle. Here,
Atmospheric nitrogen (N2) which is primarily available in an inert form, is
converted into the usable form -ammonia (NH3).
◦ Types of Nitrogen Fixation
◦ Atmospheric fixation: A natural phenomenon where the energy of
lightning breaks the nitrogen into nitrogen oxides and is then used by
plants.
◦ Industrial nitrogen fixation: Is a man-made alternative that aids in
nitrogen fixation by using ammonia. Ammonia is produced by the direct
combination of nitrogen and hydrogen and later, it is converted into
various fertilisers such as urea.
◦ Biological nitrogen fixation: Bacteria like Rhizobium and blue-green
algae transform the unusable form of nitrogen into other compounds that
are more readily usable. These nitrogen compounds get fixed in the soil by
these microbes.

6. CONTINUED...
◦ (ii) Nitrogen Assimilation: It is carried out by plants. Plants absorb
nitrate and nitrites and convert them to amino acids. They are used to
make proteins. Other complex compounds containing nitrogen are
formed in other biochemical pathways. Animals can take organic nitrogen
from plants directly or indirectly.
◦ (iii) Ammonification: It is the process of production of ammonia
(compound of nitrogen). It occurs by the decomposition of dead plants
and animals.
◦ (iv) Nitrification: It is the process of conversion of ammonia into
nitrites and then into nitrates, by nitrifying (e.g. Nitrosomonas and
Nitrobacter).
◦ (v) Denitrification: It is the process of reducing nitrates or ammonia,
present in the soil to molecular nitrogen (N2) that goes back into the
atmosphere. This is done by microorganisms such as Pseudomonas.

7. IMPORTANCE OF NITROGEN CYCLE:
◦ Importance of the nitrogen cycle are as follows:
◦ Helps plants to synthesise chlorophyll from the nitrogen compounds.
◦ In the process of ammonification, the bacteria help in decomposing the animal and plant
matter, which indirectly helps to clean up the environment.
◦ Nitrates and nitrites are released into the soil, which helps in enriching the soil with
necessary nutrients required for cultivation.
◦ Nitrogen is also cycled by human activities such as combustion of fuels and the use of
nitrogen fertilisers. These processes, increase the levels of nitrogen-containing compounds
in the atmosphere. The fertilisers containing nitrogen are washed away in lakes and rivers
and results in eutrophication.