Water

Question 1 How does water get polluted?

Question 2 Why is water essential for life?

Question 3 Define water pollution?

Question 4 What are the sources of water pollution?

Question 5 What is the full form of BOD?

Question 6 What are the harmful effects of water pollution?

Question 7 What is BOD?

Water
The ocean,rivers,streams,lakes,ponds,pools,polar ice caps,water vapour form hydrosphere.
75% of extra surface is covered with the hydrosphere.Major part of water is found in seas and oceans and is saline.Fresh water is found frozen in the ice caps at the two poles and on snow covered mountain.
The underground water and water in rivers,lakes and ponds is also fresh.

Importance of water
1)All cellular processes take place in water medium.
2)All reactions that take place within our body and within the cell 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 formed.
4)Terrestrial life forms require fresh water for this because their bodies cannot tolerate or get rid of high amount of dissolved salts in saline water.
The availability of water decides not only the number of individuals of each species that able to survive in a particular area,but it also decides the diversity of life there.Other factors like temperature and nature of soil is also important.

Water pollution
An undesirable change in the physical,biological or chemical qualities of water(Due to addition of foreign organic,inorganic,biological or radioactive substances)that adversely affect the aquatic life and makes water unfit for use is called water pollution.

Sources of Water pollution
1)Sewage:Organic waste are contributed by domestic and commercial sewage and affluents of food processing plants,tanneries,houses,animal excreta.
2)Industrial waste:Effluents of mills and industries contain large quantities of harmful chemicals that are discharged into water bodies.
3)Synthetic soaps and detergents is discharged from houses and factories.
4)Fertilisers and pesticides are used to increase crop production.These are washed by rain water into water bodies and pollute them.
5)Leakage of petroleum oil during drilling and shipping operation results in water pollution.
6)Rain erodes soil and carries silt to water.Tiny suspended particles of clay and dust also settle in water from air.

Harmful Effects of Water Pollution
1)Human diseases:
Pathogens(viruses,bacteria,fungi,protozoa etc)are biological pollutants of water.They cause typhoid,cholera,dysentery,jaundice,hepatitis.
2)Removal of desirable substance from water bodies:
With increase in the amount of organic waste the bacteria multiply rapidly and use up the available oxygen.Lack of oxygen kills fish and other animals.
BOD:Biological oxygen demand.It indicates the quality of waste water.It refers to the amount of dissolved oxygen needed by bacteria in decomposing organic waste present in water.
3) Effects of thermal pollution or change in temperature:
Aquatic organisms are used to certain range of temperature,a sudden marked change in the temperature would be dangerous  for them ie it affects breeding of aquatic animals.
4)Destruction of useful microorganism:
When untreated industrial waste get mixed with water in lakes and rivers,the acids,alkalies,heavy metals present in them kill useful organism present in water bodies.As these micro-organism are natural cleansing agents of water therefore self purification process is hindered in these water bodies.