Friday 29 September 2017

CBSE Class 8 - Science - Ch 18 Pollution Of Air and Water - Chapter Notes (#cbseNotes)


(Chapter Notes)
CBSE Class 8 - Science - Ch 18 Pollution Of Air and Water - Chapter Notes (#cbseNotes)

Pollution is release of harmful substances in the environment thus causing threats to our ecosystem. Even non-harmful substances when precipitated in environment can be harmful. Land, water and air are the most important natural resources. Man made factors (agriculture, industrialization, transport etc.) are the primary reasons for the pollution.

According to World Health Organizations, five millions deaths occur in India due to pollution.
Pollution can be classified in many categories like:
  • Land Pollution
  • Air Pollution
  • Water Pollution
  • Noise Pollution
  • Light Pollution
Here we discuss primarily about Air pollution and Water pollution.
Composition of Air
Air is a mixture of gases. 
By volume, Nitrogen:78%, Oxygen:21% and 1% (remaining gases like Carbon Di-oxide, Ozone, Argon, Methane etc.)

Air Pollution

Presence of one or more chemicals in sufficient quantities in the atmosphere to cause harm to humans and other forms of life is called Air pollution.

Expanding urbanization and increased industrialization in cities have resulted in degraded environments and human health problems.

Air Pollutants

The substances which contaminate the air are called air pollutants. Smoke, dust particles from fire, gases like Sulphur DiOxide, Carbon DiOxide, Carbon Monoxide, Nitorgen oxides are examples of air pollutants.

Air PollutantSourceEffects
Carbon Monoxide (CO),
Carbon DiOxide(CO2)
Burning of charcoal, coke, woodGlobal Warming, Blood Poisoning
Sulphur Di-Oxide(SO2) Burning of coal and sulphur Acid Rain, Asthma, Lung Cancer
Nitrogen DiOxide(NO2) Smoke from automobiles and factories Acid Rain
Chloroflorocarbons (CFC)gases released from air-conditioning systems and refrigerationOzone layer depletion increasing cancer risk
Suspended particulate matter(SPM) Solids suspended in smoke coming from factories and vehicles Respirator problems, Lung cancer

Broadly, there are two sources of Air pollution:Source of Air Pollution

  • Natural or Biogenic Sources: Volcanic eruptions emit gases like sulfur dioxide, carbon dioxide and hydrogen fluoride. In nearby places, sulfur dioxide gas can cause acid rain in nearby places and air pollution in downwind areas from the volcanic site. Forest Fires emit carbon monoxide, sulphuur dioxide, nitrogen dioxide, ozone, and particulate matter.
  • Man-made or Anthropogenic Sources: Man-made are the major contributors. Examples are Industrial emissions, vehicles,
    aeroplanes, power stations and burning of fuels, etc.,

Acid Rain and Taj Mahal

Hydrocarbons like coal and petroleum contain traces of Nitrogen and Sulphur. When these hydrocarbons burn (in factories, houses, automobiles), they release oxides of Nitrogen and Sulphur in the air. When these oxides mix with water vapour present in the air, form acids (nitric acid or sulphuric acid). These acids when drop down on earth along with rain, is called acid rain. Acid rain causes skin irritation, inhibits germination of plants, causes corrosion to bridges and building.

The industries located around Agra like rubber processing, automobile, chemicals and especially the Mathura oil refinery, were producing pollutants like sulphur dioxide and nitrogen dioxide, thus forming acid rain. It corroded the marble of Taj Mahal monument. The suspended particles and ash(soot) from Mathura refinery caused yellowish colour to Taj marbles. This is popularly called "Marble cancer phenomenon". Supreme Court has taken several steps to save Taj from pollution.

Green House Effect and Global Warming

What is Green House?

You might have seen a glass house in botanical garden. It is also called green house, in which plants are grown. The sunlight gets trapped by the glass and plastic walls and roofs of the house or building. The heat is absorbed by soil, plants and air inside the house. Green houses are often used for growing flowers, vegetables and plants (specially in cold region). Plants release Carbon Di-Oxide during photosynthesis. CO2 is known as green house gas which absorbs heat.

Green House Effect

Green House effect is a process by which solar radiation is absorbed by green house gases and is transmitted to all directions. This increases the temperature of the surroundings and is responsible for global warming.

In general, the sunlight enters into the earth is partly absorbed by the earth surface and remaining radiation is reflected back. Due to deforestation, urbanisation and other human activities, the concentration of CO2 is increaseing. Due to increase in Green house gases, more reflected raditaion is entrapped and thus increasing the temperature of earth's atmosphere. Other gases like Methane, ntirous oxide, water vapours also trap heat radiation like CO2 does and are called greenhouse gases.
Global warming is one of the biggest concern worldwide. Scientists believe climate shifting, melting of glaciers (e.g. Gangotri glacier), sudden floods in coastal areas are due to global warming.

The Kyoto Protocol is one such agreement made among the countries to reduce emission of green house gases.

Ozone Layer Depletion

The sunlight also contains Ultra Violet rays (UV) which are harmful for our skin. UV radiation can cause diseases like skin cancer, affect on crops and micro-organisms. The outermost layer of the earth's atmosphere is covered by a thin layer of Ozone gas. This layer of Ozone absorbs UV rays and thus protect us from its radiation. It is found air pollutant chlorofluorocarbons (CFCs) used in air conditioners, aerosol sprays and refrigeration are responsible for thinning and depletion of ozone layer. Hole created in ozone layer allows harm UV rays reach earth surface.

An international agreement called Montreal Protocol was made among countries to reduce emissions of CFCs.

Ways to check Air Pollution

Air pollution can be checked and reduced by adopting the following ways:
  1. Minimize use of automobiles and use petroleum and other fuels judciously. Wherever possible use cleaner-fuel like bio-disel, CNG etc.
  2. Use filters before releasing smoke from factories.
  3. Plant more trees to increase oxygen level and reduce CO2 concentration.

Water Pollution

Water pollution is resulted when toxic substances detriorates the quality of water. These harmful substances enter lakes, streams, rivers, oceans, and other water bodies, they get dissolved or lie suspended in water or get deposited on the bed. Water pollution degrades aquatic life in streams, rivers, lakes, and oceans.
Industrial and household wastes in the water have turned rivers into foul-smelling, polluted rivers that deliver drinking and irrigation water to several surrounding regions.
The substances that contaminate water are called water pollutants.

Source of Water Pollution

World Wide Fund for Nature (WWF) reproted that Ganga is one of the ten most endangered rivers in the world. Following are the major causes contributing to water pollution:
  1. Untreated Sewage and Gargbage from towns and cities.
  2. Increased Population and industrialization. Discharge of toxic chemicals from industries and other wastes reduces the oxygen level so much in water bodies that aquatic life cannot survive.
  3. Fertilizers, pesticides and other animal wastes from farms pollute rivers and ponds. They also seep into the ground and pollute ground water.
  4. Hot water released from power plants and factories also act as water pollutant. It is called thermal pollution. Animals and plants that live only at lower temperature cannot survive.
Question: Why do ponds looking green have almost no aquatic life?

Answer: It is due to water pollution.Excessive quantities of chemicals (fertilizers, weedicides, pesticides etc.) washed from the fields enter into ponds. These act as nutrients for algae to flourish. Once these algae die, they serve as food for decomposers like bacteria. A lot of oxygen in these ponds gets used up. This results in a decrease in the oxygen level which kills aquatic organisms.

Potable Water

Water fit for drinking is called potable water. In layman terms it is drinking water. Water which is not safe to drink can carry diseases and heavy metals. People who consume this water will become ill, and there is a risk of death. Unfortunately, 25% of the world's population is without safe drinking water.

Water Treatment and Purification

Various physical and chemical processes are done in sewage treatment plant to clean water before discharging it into water bodies.
Similalry water is also treated and purified by municipal bodies before it is sent to households. At home, water is boiled or various filters (e.g. candle type filter or reverse osmosis) are used to purify water from purities. Chlorine tablets are also added to purify water. One should not use excessive chlorine tablets which may be harmful.
Sewage is waste carried water released by houses, industries, hospitals, offices etc. It also includes rain water that has run down the streets and roads.

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