Question.1 List any three human activities that you think would lead to air pollution. ]SAII – 2013]
Answer. Human activities that would lead to air pollution are :

1. Excessive use and burning of fossil fuels like coal and petroleum produces different oxides of nitnopfrand sulphur. These are not only dangerous to our respiratory system but also leads to acid rain.
2. Incomplete combustion of various fuels forms unburnt carbon particles which lowers the visibility, especially in cold weather when water also condenses out of air. This is known as smog and is a visible indication of air pollution.
3. Large usage of automobiles and improperly tuning of engines leads to the formation of carbon monoxide gas and other oxides of nitrogen which causes lot of respiratory problems.
4. Forest fires, excessive mining and ore refining, excessive use of chlorofluorocarbons and v excessive industrialization leads to air pollution.

Question.2 Why do organisms need water ? [SAII-2012]
Answer. Organisms need water due to the following reasons :

1. All cellular processes take place in a water medium.
2. All the reactions that take place’Within our body and within the cells occur between substances that are dissolved in water.
3. Substances are also transported from one part of the body to the other in a dissolved form.
4. Water makes-up about 70% of body weight of all living organisms.
5. It helps in the digestion of food and absorption of nutrients in the blood. Hence, organisms need to maintain the level of water within their bodies in order to stay alive.
6. It helps in maintaining body temperature. –

Question.3 Do you know any activity which may be polluting these water sources ?
Answer.There are many activities which cause pollution of water sources such as :

1. Dumping of waste from factories, sewage from our towns and cities into rivers or lakes.
2.  Discharging hot water from cooling towers into the water bodies affects the breeding capacity of aquatic organisms.

Question.4 How is soil formed ? [SAII – 2013, 2014]
Answer. Over long periods of time, thousands and millions of years, the rocks at or near the surface of the Earth are broken down by various physical, chemical and some biological processes. The end product of this breaking down is the fine particles of soil. There are many other factors which play a vital role in the formation of soil. These factors are :

1. The Sun : It causes heating of rocks which causes cracking and ultimately breaking-up into smaller pieces.
2. Water : It breaks rocks both by freezing and fast flow.
3. Wind : It causes erosion of rocks similar to as done by fast flowing water. It also carries sand from one place to the other like water does.
4. Living organisms : Lichens and moss plants grow on the rock surface and cause rock surface to powder down and form a thin layer of soil. The roots of big trees sometimes go into cracks in the rocks and as the roots grow bigger, the crack is forced bigger.

Question.5 What are the different states in which water is found during the water cycle ? [SAII – 2011]
Answer. Three different states of water can be seen during the water cycle. These states are :

1. Gaseous stateTtn the form of water vapour which evaporates from the surface water).
2. Liquid state (It is formed by the condensation of water vapour and can be seen in the form of rain).
   (iii) Solid state (It is formed by the freezing of liquid droplets in the upper layer of atmosphere which can be seen in the form of snow, hail or sleet).

Question.6 List any three human activities which would lead to an increase in the carbon dioxide content of air. [SAII – 2011]
Answer.Three human activities which would lead to an increase in the CO₂ content of air are :

1. Respiration : It is the natural process of release of CO₂  by both plants and animals. But this release is neither dangerous nor it has any adverse effect on our environment.
2. Combustion of fuels : The various types of fuels are burnt to provide energy for various – , needs like heating, cooking, transportation and industrial fuels.
3. Deforestation : Trees help in the conversion of CO₂  into organic compounds such as glucose, starch etc., by the process of photosynthesis. When these trees are cut non- judiciously, then the level CO₂  increases in our environment.

Question.7 How does nitrogen fixation take place during lightning ? How do plants make use of the nitrates and nitrites present in soil ? [SAII -2014]
Answer.

Nitrogen fixing bacteria mostly found in the root nodules of leguminous plants convert
atmospheric nitrogen molecules into nitrates.
During lightning, the high temperatures and pressure created in the air convert nitrogen into
oxides of nitrogen. These oxides dissolve in water to give nitrous and nitric acids that fall on
land along with rain.

•  Nitrogen fixing bacteria which are found in the roots of leguminous plants convert atmospheric nitrogen into  nitrate (the usable form by plants).
•  During lightning and thunder, the high temperature and pressure help to convert atmospheric nitrogen into  oxides of nitrogen and to give nitric and nitrous acids which fall along with rain.
•  The plants use nitrogen in the form of nitrates and nitrites to form amino acids and
  proteins.
•  The plants are used as food by animals.
•  After the death of plants and animals, the bacteria convert these compounds of nitrogen into nitrates and  nitrites, which are used by new plants to grow.
•  Some other types of bacteria convert nitrites and nitrates into molecular nitrogen, which escapes into  atmosphere and becomes a part of it.

Question.8 Draw labelled diagrams of :
(a) Carbon cycle
(b) Oxygen cycle and briefly explain oxygen cycle.[SAII – 2013]
Answer. (a)

(b)

Oxygen cycle:

•  Oxygen from the atmosphere is used up in combustion, respiration and
  in the formation, of oxides of nitrogen.
•  Oxygen is returned to the atmosphere through photosynthesis. This
  constitutes oxygen cycle in nature.

Question.9 What are the methods of preventing or reducing soil erosion ? [SAII -2012]
Answer. The methods of preventing soil erosion are :
(i) Afforestation : Forests reduce erosion as the roots of trees hold the soil in place.
(ii) Shelter belts : Trees planted in lines or hedges around farmland reduce erosion by
reducing the speed of the wind.
(iii) Contour ploughing : Farmers plough land so that furrows lie across the natural slope of
the land. These furrows trap water and do not allow it to flow down carrying the topsoil.
(iv) Terrace (or step) farming : A terraced hillside is less likely to tie eroded than a natural
hillside. Here a series of steps formed by horizontal strips supported by walls, catch the
descending water. It gives the water sufficient time to percolate Into the soil and nourish
the crop.
(v) Soil cover : Soil left bare after harvesting a crop is often covered with dried vegetation to prevent erosion. Steep slopes that cannot be ploughed are covered with grass or pasture crops.
(vi) Preventing overgrazing : As the grass has a tendency to bind soil molecules, so even a very little grass on a field prevents erosion of soil. But if the grass is overgrazed, it exposes the soil to erosion.

Question.10 Why is the atmosphere essential for life ? [SAII-2011]
Answer. Atmosphere is essential for life because of the following reasons :
(i) It keeps the average temperature of the earth fairly steady during the day and even during the course of the whole year.
(ii) It prevents the sudden increase in temperature during the daylight hours.
(iii) It contains all the important gases which are required for sustaining life on earth. These gases are :
(a) Oxygen for respiration of living organisms and oxidation.
(b) Carbon dioxide for photosynthesis in plants and for making food.
(c) Nitrogen foTproviding inert atmosphere and making proteins.
(iv) The stratosphere region (16-23 km from the surface of Earth) of atmosphere contains a thick layer of ozone which filters the harmful UV radiation from Sun. If these radiations reach on the surface of Earth, then they may cause cancer in animals and are also harmful for plants.

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