1.     We need atmosphere for following reasons :

(i) It works like a blanket and traps the radiations reflected back from earth surface that keeps the average temperature of earth of quite steady and suitable for sustenance of life.

(ii) It provides carbon dioxide for photosynthesis and oxygen for respiration and

(iii) Prevents sudden change in temperature.

(iv) Enables us to hear sounds.

2.   Water essential for life because :

(a) All organisms are made up of cells. All cellular processes take place in a water medium. All the reactions that take place within our body and within the cells occur between substances that are dissolved in water.

(b) Substances are also transported from one part of the body to the other in a dissolved form.

(c) Essential for digestion.

(d) Helpful in excretion and egestion.

(e) Regulates our body temperature by sweating and evaporation.

3.   Plants need simple nutrients like certain elements for their proper growth and they get most of these elements from soil. With the help of these elements plants prepare their own food in the presence of sunlight. Since all other organisms are dependent upon plants for their nutrition hence we can say that organisms that live in water are not totally independent of soil as a resource. Another reason is that organic matter from soil dissolves into water and provides nutrients to the aquatic organisms.

4.   The weather predictions are made based on information obtained about general patterns of changes in temperature, humidity, winds and clouds.

5.   Isolating many human activities lead to increasing levels of pollution of the air, water-bodies and soil would help in reducing water and soil pollution but it can hardly make any difference to air pollution severity the reason is gases will spread from isolated places and reach everywhere. Instead of isolating we must stress on sustainable management of our resources and cut down or replace their use like using cleaner fuels like CNG in place of fossil fuels.

6.   Quality of air : Forests have trees and plants that absorb carbon dioxide and liberate oxygen thus maintaining their levels in the biosphere.

Quality of soil : Roots of trees hold the soil particles and prevent soil erosion from taking place. Dead trees and plants or their parts add humus and organic matter to soil thus making it fertile.

Quality of water : Forest allows easy going of the water cycle in nature with cloud formation and condensation in the form of rain.

7.   On Earth air is a mixture of a number of gases mainly nitrogen(78%) and oxygen (21%) while carbon dioxide is only 0.03% while In planets such as Venus and Mars the major component of the atmosphere is found to be carbon dioxide. In fact, carbon dioxide constitutes up to 95-97% of the atmosphere on Venus and Mars.

8.   The atmosphere covering the Earth is like a blanket because air is a bad conductor of The atmosphere keeps the average temperature of the Earth fairly steady during the day and even during the course of the whole year. The atmosphere prevents the sudden increase in temperature during the daylight hours. And during the night, it slows down the escape of heat into outer space.

9.   When air is heated by radiation from the heated land or water, it rises. But since land gets heated faster than water, the air over land would also be heated faster than the air over water bodies. Therefore, during the day, the air above the land gets heated faster and starts rising. As this air rises, a region of low pressure is created and air over the sea moves into this area of low pressure. The movement of air (wind) from one region to the other creates winds.

10.  When water bodies are heated during the day, a large amount of water evaporates and goes into the air. Some amount of water vapour also get into the atmosphere because of various biological activities. This air also gets heated. The hot air rises up carrying the water vapour with it. As the air rises, it expands and cools. This cooling causes the water vapour in the air to condense in the form of tiny droplets. This condensation of water is facilitated if some particles could act as the ‘nucleus’ for these drops to form around. Normally dust and other suspended particles in the air perform this function. As such the clouds are formed.

11.  (i) Burning of fossil fuels like coal and petroleum releases different oxides of nitrogen and sulphur in air.

(ii) Burning of wood release suspended particles and smoke in air.

(iii) Use of harmful chemicals like aerosols, CFCs etc.

12.  All organisms are made up of cells. All cellular processes take place in a water medium. All the reactions that take place within our body and within the cells occur between substances that are dissolved in water. Substances are also transported from one part of the body to the other in a dissolved form. Hence, organisms need to maintain the level of water within their bodies in order to stay alive.

13.  Major sources of water are:

(a) rain water that provides water to all other sources

(b) lakes, ponds and pools

(c) rivers, wells and tube wells

(d) dams

14.  The fertilisers and pesticides used in our farms, sewage from our towns and cities and the waste from factories, specific industries also use water for cooling in various operations and later return this hot water to water-bodies. Such activities are polluting the water bodies.

15.  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. Factors responsible are as follows :

(a) The Sun: The Sun heats up rocks during the day so that they expand. At night, these rocks cool down and contract. Result is formation of cracks.

(b) Water: Water helps in the formation of soil in two ways. One, water could get into the cracks in the rocks formed due to uneven heating by the Sun. If this water later freezes, it would cause the cracks to widen.

(c) Wind: In a process similar to the way in which water rubs against rocks and wears them down, strong winds also erode rocks down.

(d) Living organisms : The lichen grows on the surface of rocks. While growing, they release certain substances that cause the rock surface to powder down and form a thin layer of soil. Other small plants like moss, are able to grow on this surface now and they cause the rock to break up further. The roots of big trees sometimes go into cracks in the rocks and as the roots grow bigger, the crack is forced bigger.

16.  The loss of (wearing away)topmost layer of soil which contains humus and air is
called soil erosion.

17.  The methods of preventing or reducing soil erosion are as follows :

(i) prevention of deforestation and overgrazing.

(ii) afforestation and reforestation

(iii) improved methods of agriculture

18.  Water occurs in all three states of matter during the water cycle :

solid : ice , liquid : water , gas : water vapour

19.  Urea and proteins.

20.  Three human activities which would lead to an increase in the carbon dioxide  content of air are:

(a) burning of coal and petroleum

(b) combustion of wood

(c) deforestation(cutting down forests)

21.  Due to trapping of radiations by green house gases like carbon dioxide, methane etc results into increase in the global temperature excess of which leads to global warming and the phenomenon that cause it is named green house effect.

22.  Oxygen (O2) and ozone (O3).