Living Science 2020 2021 for Class 8 Science Chapter 19 - Pollution Of Water And Air

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The natural process by which water is recycled is called water cycle. This process encompasses evaporation and condensation processes, which help in the recycling of water.

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Fertilizers are introduced into the water bodies through run off caused due to rain. These fertilizers increase the growth of algae in the water, which lowers down the amount of dissolved oxygen in the water. After the death of algae, the decomposers, which further decrease the dissolved oxygen. As a result, the aquatic plants and animals die because of low oxygen. 

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The water which is completely free of all dissolved and solid impurities is not necessarily potable. This is because germs may still be present in such water, which makes it unfit for drinking. 

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Biodegradable fertilizers and pesticides are readily decomposed by the microbes. Moreover, these fertilizers and pesticides contain thatch which prevent the runoff of water. By preventing the runoff, the level of water table can be maintained which helps in conservation of water. Unlike chemical fertilizers and pesticides, biodegradable fertilizers and pesticides do not promote the growth of algae and do not affect the life of aquatic organisms. Thus, by using biodegradable fertilizers and pesticides, water pollution can be reduced.

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The most important cause of air pollution is burning of fossil fuels. This is because almost everyone depends on fossil fuels to obtain energy and the burning of these fuels releases gaseous pollutants which degrade the quality of air.

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Carbon dioxide is considered as a pollutant. Although, carbon dioxide is a natural component of air, its concentration in the air has dramatically increased. This is because of the large scale burning of fossil fuels, which increase the amount of carbon dioxide in the air. An increase in the amount of carbon dioxide causes an increase in the temperature of Earth called global warming. Due to this reason, the carbon dioxide is considered as a pollutant.

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Acid rain can be caused due to oxides of sulphur and nitrogen. For example, sulphur dioxide and nitrous oxide.

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Global warming can lead to the flooding of the coastal areas because of the following reasons:

1. Global warming can increase the average global temperature by 2ºC. This can cause the melting of ice in the polar regions, which can result in flooding.

2. Moreover, global warming will cause expansion of glaciers, which will further increase the chances of floods in coastal areas. 

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(d) all of these

Sewage contains organic matter; industrial waste, fertilizers and pesticides contain hazardous chemicals which pollute water bodies.

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(b) filtration through porous pot

Boiling, reverse osmosis and exposure to ultraviolet light remove germs from water, whereas filtration through a porous pot can only filter out solid impurities, but is unable to remove germs.

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(c) cutting vegetation so that less water is lost by transpiration

In drip irrigation, water is supplied to the roots of plants directly so there is no need to fill the entire field with water;thus it saves a lot of water. Recycling also conserves water, and planting more trees increases the ground water level. But cutting vegetation does not help in the conservation of water.

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(b) carbon dioxide

Carbon dioxide is considered a pollutant only when its level of concentration in the air is too high. Whereas sulphur dioxide, carbon monoxide and nitrogen dioxide are hazardous gases and are considered pollutants even at low levels of concentration.

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(a) carbon monoxide

Carbon monoxide is the gas which can combine with blood and prevent it from carrying oxygen, which can be fatal for human beings.

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(c) sulphur dioxide

Sulphur dioxide causes acid rain by reacting with oxygen present in the atmosphere and then combining with rain water.

${\mathrm{SO}}_2\left(\mathrm{g}\right) + {\mathrm{O}}_2\left(\mathrm{g}\right)\to \mathrm{S}{\mathrm{O}}_3(g)$
${\mathrm{SO}}_3\left(\mathrm{g}\right) + {\mathrm{H}}_2\mathrm{O(l)} \to {\mathrm{H}}_2{\mathrm{SO}}_4(l)$
                                   

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Water is recycled in nature through the process of water cycling.

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The substances that pollute water or air are called pollutants.

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Sewage that is untreated causes water pollution.

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True.

Hot water raises the temperature of water bodies which reduces the amount of dissolved oxygen, which is needed for respiration by aquatic life.

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Eutrophication is the term for 'Enrichment of water of a lake or pond by nutrients that leads to excessive plant growth'.

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Yes, one can safely drink potable water because it is free from impurities and germs.

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Ultraviolet light can kill germs.

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Using less water and preventing water from getting polluted is called conservation of water.

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Carbon monoxide binds with blood and prevents it from carrying any oxygen.

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False.

Methane, nitrous oxide and water vapours are also greenhouse gases.

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Chlorofluorocarbons damage the ozone layer in the atmosphere.

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Rain mixed with sulphuric acid or nitric acid is called acid rain.

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Lead compound when added to petrol to prevents engines from knocking and causes air pollution.

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Compressed Natural Gas (CNG) is a fuel that causes very little pollution.

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Untreated sewage contains many harmful microorganisms like bacteria, parasites, viruses and fungi. When dumped into the river untreated, it can potentially infect human beings with diseases such as diarrhoea, dysentery, cholera, typhoid and jaundice.

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Hot water discharge into rivers increases the temperature of rivers, which reduces the amount of dissolved oxygen in water. This makes it difficult for aquatic plants and animals to respire. Thus, hot water discharge is also considered as a water pollutant.

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Filtered water may becomes free of all suspended impurities but still it is not potable, because after filtration it may have harmful germs like bacteria,viruses etc., which can make us ill.

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At home water can be made potable by:

1. Boiling - Boiling the water for 15-20 minutes will kill all the germs in it.
2. Chlorination - Adding adequate quantity of chlorine tablets or bleaching powder to water can kill germs.

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Main causes of water pollution are:

1. Untreated sewage
2. Industrial wastes
3. Fertilizers and pesticides

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Compressed Natural Gas (CNG) is a smokeless fuel. In comparison to diesel, CNG emits lesser carbon particles and harmful gases (such as carbon monoxide) on ignition. Hence, CNG is being encouraged these days as fuel in vehicles over diesel.

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The function of a catalytic converter is to check vehicular pollution levels. It can convert harmful gases, such as carbon monoxide and nitrogen dioxide, into comparatively harmless gases like carbon dioxide, nitrogen gases and water vapour.

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The names of three greenhouse gases are: methane, nitrous oxide and carbon dioxide.

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Sewage and industrial waste should be treated before they are disposed into rivers to reduce water pollution. Sewage should be treated with chlorine to kill all the germs in it.

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Rivers have always been used to dump waste. In older times, the amount of waste disposed was not enormous. Natural processes were sufficient to degrade all the dumped waste to harmless material and manage the pollution level. But nowadays, increasing population and rapid growth of industries have created massive amounts of waste that have poisonous compounds and therefore, cannot be degraded naturally. Thus, such waste accumulates and pollutes the river.

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Eutrophication is caused by the fertilizers and pesticides in fields. When they either seep into the ground and get mixed with groundwater, or are washed away with rain and drain into the rivers, it results in increase in concentration of these nutrients in water bodies.This is called eutrophication.
Eutrophication causes growth of a layer of algae on the surface of water, which reduces the level of dissolved oxygen in water.When other aquatic animals and plants die due to lack of oxygen, their dead remains serve as food for bacteria which further reduces oxygen content in water bodies.

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The methods of conserving water are:

1. Avoiding wastage of water in domestic chores.
2. Recycling of water in factories for reuse.
3. Using drip irrigation method, in which water is supplied to the roots directly drop by drop instead of filling the entire field with water.
4. Water harvesting of rain water and its usage for various domestic purposes like washing.

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Solid particles of very minute size that are present in air are known as SPM (Suspended Particulate Matter).
SPM pollution is harmful in the following ways:

1. SPM can cause lung cancer because our nose hair is not able to filter such minute particles.
2. SPM is deposited on plants, which restricts the process of photosynthesis.

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Carbon monoxide is considered to be such a dangerous pollutant because it competes with oxygen to bind with blood in human beings. Thus, less oxygen circulates inside the human body, and very little is supplied to human organs.This may prove fatal. Also, it is an odourless gas and therefore, cannot be detected through smell, which makes it even more dangerous.

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Carbon dioxide is required by plants to make food. Hence, a certain level of it is essentially required in the atmosphere. But it becomes a pollutant due to its excessive concentration in the atmosphere; when its level is much more than what is required.
Carbon dioxide can trap radiation coming from sunlight which increases the average temperature of the earth. This causes global warming. This results in melting of ice in the polar region, which leads to expansion of oceans (raising the water level), which ultimately results in flooding in coastal regions. Also, this can cause climate change, affect rain patterns, agriculture, plants and animals.

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When rain water in clouds get mixed with sulphur dioxide and oxygen present in atmosphere, this mixture (i.e., sulphuric acid) is called acid rain.
It is caused mainly due to burning of coal which contains sulphur as impurity, and emits sulphur dioxide on burning. This sulphur dioxide in the atmosphere reacts with oxygen to form sulphur trioxide which in turn, reacts with the water in the clouds to form sulphuric acid.

2SO₂(g)              +      O₂(g)        $\to$   2SO₃(g)
Sulphur dioxide        Oxygen              Sulphur trioxide

SO₃(g)                +      H₂O(l)      $\to$  H₂SO₄(l)
Sulphur trioxide       Rain water           Sulphuric Acid (Acid Rain)

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Four ways to conserve air are:

1. Planting more trees, as they absorb carbon dioxide and give out oxygen.
2. Using efficiently-designed smokeless stoves instead of burning coal or kerosene.
3. Using smokeless fuels, such as CNG (Compressed Natural Gas) and LPG (Liquified Petroleum Gas) in vehicles. Such fuels emit less harmful gases.
4. Scrubbing effluent gases, in factories and refineries, with water in chimneys before they escape into the atmosphere. This removes poisonous gases like sulphur dioxide and nitrogen dioxide.

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Excessive use of fertilisers and pesticides in fields can lead to their seepage into the ground and mixing with ground water. They can also be washed away with rain and carried to water bodies, where they can contaminate the water and the aquatic animals, especially fish. Human beings consuming such contaminated fish might be fatally poisoned.
Also, rivers polluted by fertilisers allow the growth of algae that use dissolved oxygen in the water for their sustenance. This leads to oxygen deficiency in water, which can cause the death of other aquatic animals. This enrichment of nutrients and depletion of oxygen is called eutrophication.

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There are different methods available to us for making water potable by killing germs, such as:

1. Boiling - Boiling is a very common method applied in households to kill the germs in water. In this process, the water is made to boil for 15-20 minute.
2. Ultraviolet Light - This light is capable of killing all microorganisms in water and make it fit for drinking.This technique is used in several water purifiers available in the market.
3. Reverse Osmosis (RO) - In this technique water is made to pass through a semi-permeable membrane from low concentration to high concentration of water. Thus, this semi-permeable membrane allows only water to pass through its fine pores but not the impurities and germs.
4. Chlorination - Water can be treated with chlorine tablets or bleaching powder to kill the germs in it.

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Water at the waterworks is treated to make it potable by the following steps:

1. Sedimentation and Decantation - In this process, large-sized impurities are made to settle down at bottom of the tank, which is called sedimentation. The water becomes free of the large particles through the process of decantation.
2. Loading - To remove smaller-sized particles, water is treated with alum which makes these particles heavier, and consequently they settle down at the bottom and can be separated from the water.
3. Filtration - Next, the water is filtered by passing it through sand filters, which makes the water free of impurities.
4. Chlorination - Finally, to remove germs from water, it is treated with chlorine.

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Following are the main pollutants of air:

1. Suspended Particulate Matter(SPM) - These particles are too minute to be filtered through our nose hair. Thus, they can easily reach lungs and cause lung cancer in humans. Also, they get deposited on plants and interfere with the process of photosynthesis.
2. Carbon Monoxide (CO) - It is a dangerous pollutant which competes with oxygen and binds with the blood. It causes deficiency of oxygen in our body, which is fatal.
3. Excessive Carbon Dioxide (CO₂) - Although, carbon dioxide is present in atmosphere naturally, if it prevails excessively in the atmosphere it traps more heat from the sunlight which causes global warming.
4. Sulphur Dioxide (SO₂)- This gas reacts with atmospheric oxygen and rain water in clouds and forms acid rain.
5. Nitrous Oxide(NO₂)- It irritates the eyes and affects lungs.
6. Chloroflorocarbons(CFC) - They are responsible for the depletion of the ozone layer.
7. Lead Compounds - They cause mental disorders and brain damage.

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Yes, acid rain definitely affects crops in the following ways:

• It damages the roots of crops, which slows down their growth.
• It turns the stems and leaves of crops brown.
• It destroys the crops.

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Delhi Metro is expected to reduce pollution further, as it runs on electricity and so it does not emit harmful gases like nitrogen dioxide, sulphur dioxide, carbon monoxide, etc. Also, the Metro contributes towards the reduction in emission of green house gases. According to a study, the Metro prevents the emission of 6,30,000 tonnes of greenhouse gases, 90,000 tonnes of which is carbon dioxide gas.

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Groundwater found on the surface is more polluted as hazardous and poisonous chemicals seep through the earth's surface and mix with the water. On the other hand,water present deep within the earth  needs to pass through various rock beds in between and undergo high pressure, which actually filters the impurities from the water and makes it lesser polluted than surface water.

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Afforestation aids in air, water and soil conservation in following ways:

1. Air Conservation - Plants take in carbon dioxide from the atmosphere and give off oxygen. Also, they reduce dust particles from the air.
2. Water Conservation - Plants increase the level of groundwater.
3. Soil Conservation - The roots of plants hold the soil firmly which prevents soil erosion through rain water or wind.

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Potable water is not pure water without any impurities, but it satisfies the standard or permissible limit of purity of water that is safe to drink. Potable water cannot be considered as water without any foreign material. Still, this water can be used as there is low risk of it causing any harm.

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1. The waste water which is discharged from the sewage treatment plants has a higher temperature than the temperature of the river. In summers, the surrounding temperature is already higher and the release of this hot water further accelerates the pollution of water bodies. It severely affects the flora and fauna of that body.

2. During the monsoon months, the rain washes the pollutants of the atmosphere and surrounding environment and it is a continuous process. It seems, that the pollution levels are down during this time. But the problem starts after the ending of monsoon .

3. After the monsoon months, the run off water along with all kinds of wastes reaches the water bodies. These wastes lead to increased water pollution.