`color{green}(=>)` The fossil fuels like coal and petroleum contain small amounts of nitrogen and sulphur.

`color{green}(=>)` When these fuels are burnt, nitrogen and sulphur too are burnt and this produces different oxides of nitrogen and sulphur.

`color{green}(=>)` The inhalation of these gases is very dangerous.

`color{green}(=>)` They also dissolve in rain to give rise to acid rain.

`color{green}(=>)` The combustion of fossil fuels also increases the amount of suspended particles in air.

`color{green}(=>)` These suspended particles could be unburnt carbon particles or substances called hydrocarbons.

`color{green}(=>)` Presence of high levels of all these pollutants cause visibility to be lowered, especially in cold weather when water also condenses out of air. This is known as smog and is a visible indication of air pollution.

`color{green}(=>)` Regularly breathing air that contains any of these substances increases the incidence of allergies, cancer and heart diseases.

`color{green}(=>)` An increase in the content of these harmful substances in air is called air pollution.

Water exists in various forms on earth's surface.

`color{green}(=>)` Some amount of water exists in the form of water vapour in the atmosphere.

`color{green}(=>)` Most of the water on Earth’s surface is found in seas and oceans and is saline.

`color{green}(=>)` Fresh water is found frozen in the ice-caps at the two poles and on snowcovered mountains.

`color{green}(=>)` The underground water and the water in rivers, lakes and ponds is also fresh.

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.

Terrestrial life-forms require fresh water for this because their bodies cannot tolerate or get rid of the high amounts of dissolved salts in saline water. Thus, water sources need to be easily accessible for animals and plants to survive on land.

The availability of water, temperature and nature of soil are the factors that decides the sustainability of life. But water is one of the major resources which determine life on land.

`color{green}(=>)` Fertilisers and pesticides that are used on farms are washed into the water bodies.

`color{green}(=>)` Even the sewage from towns and cities and the waste from factories are also dumped into rivers or lakes.

`color{green}(=>)` Specific industries also use water for cooling in various operations and later return this hot water to water-bodies.

`color{green}(=>)` Another manner in which the temperature of the water in rivers can be affected is when water is released from dams. The water inside the deep reservoir would be colder than the water at the surface which gets heated by the Sun.

All this can affect the life-forms that are found in these water bodies in various ways. It can encourage the growth of some life-forms and harm some other life-forms which affects the balance between various organisms that had been established in that system.

`color{green}("𝟏. 𝐓𝐡𝐞 𝐚𝐝𝐝𝐢𝐭𝐢𝐨𝐧 𝐨𝐟 𝐮𝐧𝐝𝐞𝐬𝐢𝐫𝐚𝐛𝐥𝐞 𝐬𝐮𝐛𝐬𝐭𝐚𝐧𝐜𝐞𝐬 𝐭𝐨 𝐰𝐚𝐭𝐞𝐫-𝐛𝐨𝐝𝐢𝐞𝐬 :")` These substances could be the fertilisers and pesticides used in farming or they could be poisonous substances, like mercury salts which are used by paper-industries. These could also be disease-causing organisms, like the bacteria which cause cholera.


`color{green}("𝟐. 𝐓𝐡𝐞 𝐫𝐞𝐦𝐨𝐯𝐚𝐥 𝐨𝐟 𝐝𝐞𝐬𝐢𝐫𝐚𝐛𝐥𝐞 𝐬𝐮𝐛𝐬𝐭𝐚𝐧𝐜𝐞𝐬 𝐟𝐫𝐨𝐦 𝐰𝐚𝐭𝐞𝐫-𝐛𝐨𝐝𝐢𝐞𝐬 : ")` Dissolved oxygen is used by the animals and plants that live in water. Any change that reduces the amount of this dissolved oxygen would adversely affect these aquatic organisms. Other nutrients could also be depleted from the water bodies.

`color{green}("𝟑. 𝐀 𝐜𝐡𝐚𝐧𝐠𝐞 𝐢𝐧 𝐭𝐞𝐦𝐩𝐞𝐫𝐚𝐭𝐮𝐫𝐞 : ")` Aquatic organisms are used to a certain range of temperature in the water-body where they live, and a sudden marked change in this temperature would be dangerous for them or affect their breeding. The eggs and larvae of various animals are particularly susceptible to temperature changes.

`color{green}(=>)` The outermost layer of our Earth is called the crust and the minerals found in this layer supply a variety of nutrients to life-forms.

`color{green}(=>)` 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.

`color{green}(=>)` The end product of this breaking down is the fine particles of soil.

`color{green}("• 𝐓𝐡𝐞 𝐒𝐮𝐧:")` The Sun heats up rocks during the day so that they expand. At night, these rocks cool down and contract. Since all parts of the rock do not expand and contract at the same rate, this results in the formation of cracks and ultimately the huge rocks break up into smaller pieces.

`color{green}("• 𝐖𝐚𝐭𝐞𝐫:")` Water helps in the formation of soil in two ways.

`color{green}(=>)` 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.

`color{green}(=>)` Two, flowing water wears away even hard rock over long periods of time. Fast flowing water often carries big and small particles of rock downstream. These rocks rub against other rocks and the resultant abrasion causes the rocks to wear down into smaller and smaller particles. The water then takes these particles along with it and deposits it further down its path. Soil is thus found in places far away from its parent-rock.

`color{green}("• 𝐖𝐢𝐧𝐝:")` In a process similar to the way in which water rubs against rocks and wears them down, strong winds also erode rocks down. The wind also carries sand from one place to the other like water does.

`color{green}(•)` 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.

`color{green}(=>)` Soil is a mixture that contains small particles of rock (of different sizes), bits of decayed living organisms which is called humus, various forms of microscopic life.

`color{green}(=>)` The type of soil is decided by the average size of particles found in it and the quality of the soil is decided by the amount of humus and the microscopic organisms found in it.

`color{green}(=>)` Humus is a major factor in deciding the soil structure because it causes the soil to become more porous and allows water and air to penetrate deep underground.

`color{green}(=>)` The mineral nutrients that are found in a particular soil depends on the rocks it was formed from.

`color{green}(=>)` The nutrient content of a soil, the amount of humus present in it and the depth of the soil are some of the factors that decide which plants will thrive on that soil.

`color{green}(=>)` Thus, the topmost layer of the soil that contains humus and living organisms in addition to the soil particles is called the topsoil.

`color{green}(=>)` The quality of the topsoil is an important factor that decides biodiversity in that area.


`color{green}("𝐇𝐚𝐫𝐦𝐟𝐮𝐥 𝐞𝐟𝐟𝐞𝐜𝐭𝐬 𝐨𝐟 𝐟𝐞𝐫𝐭𝐢𝐥𝐢𝐬𝐞𝐫𝐬:")`

`color{green}(=>)` Use of these fertilisers over long periods of time can destroy the soil structure by killing the soil micro-organisms that recycle nutrients in the soil.

`color{green}(=>)` It also kills the Earthworms which are instrumental in making the rich humus.

`color{green}(=>)` Removal of useful components from the soil and addition of other substances, which adversely affect the fertility of the soil and kill the diversity of organisms that live in it, is called soil pollution.

The fine particles of soil may be carried away by flowing water or wind. If all the soil gets washed away and the rocks underneath are exposed, we have lost a valuable resource because very little will grow on the rock.


`color{green}(=>)` The roots of plants have an important role in preventing soil erosion.

`color{green}(=>)` The large-scale deforestation that is happening all over the world not only destroys biodiversity, it also leads to soil erosion.

`color{green}(=>)` Topsoil that is bare of vegetation, is likely to be removed very quickly. And this is accelerated in hilly or mountainous regions.

`color{green}(=>)` This process of soil erosion is very difficult to reverse.

`color{green}(=>)` Vegetative cover on the ground has a role to play in the percolation of water into the deeper layers too.