Solved examples for class 8 science : Chapter 18

    Pollution of Air and Water

    I. Multiple Choice Questions (MCQs).

    Q.1. A highly poisonous gas produced by the incomplete combustion of fuels is
    a) carbon dioxide
    b) sulphur dioxide
    c) carbon monoxide
    d) none of these

    Answer. c) carbon monoxide
    Q.2. Which may cause respiratory problems including permanent lung damage?
    a) $SO_{2}$
    b) $CO_{2}$
    c) $NO_{2}$
    d) CFC
    Answer. a)$ SO_{2}$

    Q.3. Which of the following pollutants are causes of acid rain?

    a) Oxides of carbon
    b) Oxides of sulphur
    c) Oxides of nitrogen
    d) All of these

    Answer d) All of these

    Q.4. The second most important life supporting substance is known as
    a) food
    b) air
    c) water
    d) none of these
    Answer. (c) water

    Q.5. Which of the following is not a pollutant unless present in excess?
    a) oxides of nitrogen
    b) carbon monoxide
    c) sulphur dioxide
    d) none of these

    Answer. d) none of these

    II. Fill in the blanks by selecting appropriate words from the given list:

    List: haemoglobin, marine, Carbon dioxide, Water, solid

    Q.1. ………………………….helps in germination of seeds and transport of nutrients.
    Q.2. Carbon monoxide combines with…………………… and decreases oxygen carrying capacity.
    Q.3. The increased concentration of ……………….. traps solar radiation and causes global warming.
    Q.4. Smoke, dust and suspended particles are………………………. air pollutants.
    Q.5. Oil spills that are accidental release of oil by tankers in oceans may cause………………….. water pollution.

    Answer.
    1- Water,
    2- haemoglobin,
    3- Carbon dioxide,
    4- solid,
    5- marine,

    III. Match the items of Column A with those of Column B.


    Answer. 1-e, 2-a, 3-d, 4-b, 5-c,

    IV. Write ‘T’ for the true and ‘F’ for the false statements.
    1. It is very essential that air and water available to us should be free from any harmful component.
    2. Air pollutants cannot come from natural sources.
    3. Sulphur dioxide (SO,) can cause respiratory problems including permanent lung damage.
    4. Some heavy metals such as mercury, lead, cadmium, etc., do not cause any disease.
    5. The algal bloom consumes most of the dissolved oxygen from the water, results in a lack of oxygen in water.

    Answer. 1-T, 2-F, 3-T, 4-F, 5-T,

    V. Very Short Answer Questions.
    Q.1. Write natural sources, which may produce air pollutants.
    Answer. Smoke and dust arising from volcanic eruptions and forest fire

    Q.2. Write health problems due to that occur oxides of nitrogen in air.
    Answer. Oxides of nitrogen in air combine with other air pollutants and fog to form smog. The smog causes breathing difficulties such as asthma, cough and wheezing in children.

    Q.3. Write the air pollutants which damage the ozone layer of the atmosphere.
    Answer. CFCs damage the ozone layer of the atmosphere.

    Q.4. Write three main greenhouse gases.
    Answer. Carbon dioxide, methane and nitrous oxide

    Q.5. Write two damaging effects of acid rain.
    Answer. Damaging effects of acid rain
    (i) Acid rain is harmful to aquatic life because it kills fishes and other aquatic animals and plants.
    (ii) Acid rain decreases the soil fertility.

    VI. Short Answer Questions.

    Q.1. What do you mean by pollutants? Explain solid and gaseous air pollutants in your own words. Ans. Pollutants are the substances which contaminate air, water, etc.

    (i) Gaseous air pollutants: The pollutants present in the form of gases, are called gaseous air pollutants. e.g., carbon monoxide, oxides of nitrogen and sulphur etc.

    (ii) Solid air pollutants: The pollutants in the form of solid are called solid air pollutants. For example, smoke, dust, suspended particles, etc.

    Q.2. How can you control air pollution? Explain such methods briefly.
    Answer. Air pollution can be controlled by the following methods: o Reduce the use of fossil fuels by switching to alternative energy sources. The other sources could be solar energy, hydropower and wind energy.
    o Scrubbing down industrial emission with water before releasing them into the air helps to reduce air pollution.
    o Some cars have a special device called catalytic converter which can change harmful exhaust gases like carbon monoxide and nitrogen dioxide into harmless carbon dioxide, nitrogen and water.
    o Switching to unleaded petrol or compressed natural gas [CNG]. o Replacing wood, coal and kerosene with LPG for domestic use can reduce the pollution.
    o One can plant trees and nurture the ones already present in the neighbourhood.
    o Composting of dried leaves and twigs is a better option rather than burning.

    Q.3. Define greenhouse effect.

    Answer. Excess carbon dioxide in air has the property of trapping the heat of sunlight. An effect occurring in the atmosphere because of the presence of gases like carbon dioxide and methane that trap sun’s heat, known as greenhouse effect. The greenhouse effect leads to an increase in temperature of the earth.

    Q.4. Why CNG and LPG are said to be cleaner fuel?
    Answer. CNG and LPG emit significantly less pollutants such as carbon dioxide, nitrogen oxides, sulphur oxides and particulate matter as compared to petrol and diesel. Hence it is referred to as cleaner fuel.

    Q.2. Define water pollution.
    Answer. The water pollution may be defined as the adverse change in the composition or condition of water such that it becomes less suitable for which it would be suitable in its natural state. The substances that pollute water are called water pollutants.

    Q.3. What do you understand by potable water? Discuss some physical and chemical methods for purification of water.
    Answer. Water which is suitable for drinking is called potable water.
    Purification of water involves both physical and chemical methods.
    (i) Filtration: Suspended impurities present in water can be removed by filtering water through a fine muslin cloth.
    (ii) Boiling: Filtered water can be made germ-free by boiling for 10-15 minutes.
    (iii) Treatment with chemicals: Filtered water can also be made germ-free by adding certain chemicals e.g., potassium permanganate or chlorine tablets.

    VII. Long Answer Questions.
    Q.1. Discuss main air pollutants and their sources briefly.
    Answer.


    Q.2. Describe harmful effects of air pollutants in your own words.

    Answer. The harmful effects of various air pollutants are as described below:
    i. Carbon monoxide: It combines with haemoglobin and decreases oxygen carrying capacity. As a result, blood becomes oxygen deficient and causes unconsciousness or death.

    ii. Carbon dioxide $(CO_{2})$: The increased concentration of CO2) traps solar radiation which in turn increases atmospheric temperature, called global warming.

    iii. Sulphur dioxide $(SO_{2})$: it can cause respiratory problems including permanent lung damage.

    iv. Oxides of nitrogen: Nitrogen present in fuels, on burning, reacts with oxygen to produce their oxides. These oxides combine with other air pollutants and fog to form smog. The smog causes breathing difficulties such as asthma, cough and wheezing in children.

    v. CFCs damage the ozone layer of the atmosphere, which absorbs harmful ultraviolet radiation coming from the sun and protects us.
    vi. Particulate matters present in the atmosphere reduce visibility. Inhalation of these particles causes respiratory diseases.

    vii. Oxides of nitrogen and sulphur react with water vapour present in atmosphere to form nitric acid and sulphuric acid. The acids drop down with rain making the rain acidic. This is called acid rain.
    a) Acid rain is harmful to aquatic life because it kills fishes and other aquatic animals and plants.
    b) Acid rain decreases the soil fertility.
    c) Acid rain corrodes the marble of the historical monuments such as Taj Mahal.

    viii. The greenhouse effect: It leads to an increase in temperature of the earth. This is called global warming. The increase in temperature of the earth would lead to the melting of polar ice caps and glaciers, ultimately resulting in the rise in sea level. This is expected to lead flooding of coastal areas by sea water.

    Q.3. Discuss sources of water pollution and their effects briefly.

    Answer. Sources of water pollution
    • Discharge of untreated domestic sewage into rivers Leads to water pollution.
    • Excessive use of fertilisers and pesticides in agriculture also causes water pollution.
    • Discharge of toxic waste from factories and refineries, etc., pollute water.
    • Oil spills that are accidental release of oil by tankers in oceans may cause marine water pollution.
    • Inappropriate disposal of Litter such as plastic bag, wrappers and bottles, Lead to water pollution.

    Effects of water pollution
    Water pollution affects both animals and plants and Leads to adverse impact on the aquatic ecosystem. The major effects of water pollution are the following –

    (a) Water borne diseases: Sewage gives the maximum opportunity to the pathogenic and nonpathogenic microorganisms to grow. These pathogenic (disease-causing) microorganisms are responsible for several water-borne diseases like diarrhoea, typhoid, cholera, dysentery, jaundice, hepatitis, etc.

    (b) Toxic waste material: Toxic waste material from the industries such as heavy metals, pesticides, cyanides, many organic and inorganic wastes releases directly into the river, Lakes, ocean, affecting the species present in these aquatic ecosystems, ultimately affecting the human health.

    (c) Eutrophication: Eutrophication is an increase in chemical nutrients containing nitrogen or phosphorus in an aquatic ecosystem. This promotes excessive plant growth and decay, causes severe reductions in water quality. In aquatic environment, the excessive growth of algae i.e., an algal bloom disrupts normal functioning of the ecosystem. This algal bloom consumes most of the dissolved oxygen from the water, results in a lack of oxygen in water. This makes the aquatic organisms, unable to survive.

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