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Environment - class 8 science revision notes

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The Meaning and Major Components of Environment

Environment describes all the things and conditions that make up our surroundings on earth.

The major natural components of environment consist of living things and non-living things i.e. Water, Soil, Air, Plants and Animals


Water is a very important component of the environment in that all living things depend on water. Water forms part of the environment for fish as fish get oxygen from the water and feed on water plants or some other fish found in the water. All other living things that live in water get oxygen from the water.

Other water sources found in the environment is a dependent for other living things on the land e.g. animals get water from rivers, lakes, rain and dams. People also use rain water for domestic purposes. They harvest the water using tanks and drums. They also
fetch water from rivers and wells.

Without water plants can not grow well. We already know that the conditions necessary for germination are: water, air and warmth. During photosynthesis, plants use water to make their own food.


Another important component of environment is soil which makes the homes of some small animals. Plants get nutrients from the soil so as they can grow well, get water from the soil through absorption and are finally held by the soil. Also soil is important to
animals in that they feed on plants that grow on the soil e.g. zebra, cows and goats. They are called herbivorous because they feed on green plants and vegetable materials that grow on the soil. Human beings cultivate the soil to produce food crops that they eat.

Many small animals live in the soil like ants, termites, moles, groundhogs and some bacteria which decompose dead plant and animal materials.


One of the major components of the environment is air which all living things cannot survive without. We already know that air is a composition of many gases; oxygen which makes 21% of the air is needed for respiration.

Note that living things breath in oxygen and those animals that live in water get their oxygen from the dissolved oxygen in the water. Carbon dioxide in air is 0.03% which is used by plants in making their own food in presence of sunlight, a process called
photosynthesis. These plants give out carbon dioxide. It is from the atmosphere where the plants and animals on the land get their oxygen and those in water get theirs in the water.

Animals take in oxygen through breathing and give out carbon dioxide. On the other hand plants take in carbon dioxide during photosynthesis and give out oxygen. Leguminous plants e.g. beans, ground nuts and peas use nitrogen which makes 78% part
of the air to make proteins.


Another major component of the environment is plants. Trees provide shelter for some animals such as monkeys and insects. Animals get their food from plants either directly or indirectly. Plants do also hold soil together thus preventing soil erosion.
Plants give out oxygen to the atmosphere during photosynthesis and animals take in oxygen during breathing. Dead plants decay and decompose releasing nutrients to the soil making it fertile.


Some animals help in pollination such as bees and butterflies. They help in cross pollination of flowers in plants. Animals also help in seed dispersal. Waste matters from animals add nutrients to the plants and this way soil becomes fertile. Small animals like
bacteria decompose materials in the environment to release nutrients to the environment. Animal breath out carbon dioxide into the atmosphere which in turn is used by plants to make food.

The Meaning and Effects of Soil Pollution

Soil pollution is the presence of substances that affect the quality of the soil also known as land pollution. It affects the usual use of soil and is dangerous to the health of human beings, other animals and plants. In other words, soil pollution means making soil impure. We say that when certain substances are present in the soil they pollute it. Substances that make soil or other components of the environment impure are called pollutants (contaminant). Some of examples of such pollutants are plastics, polythene papers, fertilizer, pesticides and herbicides.

NB: Pesticides kill pests while herbicides kill weeds.

If oil is spilled on the soil it pollutes it. The following are some causes of soil pollution:

  • Domestic waste disposal improperly
  • Improper disposal of raw industrial waste
  • Excess use of fertilizers, herbicides and pesticidesin the activities of poor agriculture
  • Spilling oil on the soil
  • Mining activities destroysthe soil structure and leave excessminerals on the top soil.

Effects of soil pollution

When soil is polluted, its fertility is affected and this too affects the soil productivity leading to the living components of the environment i.e. plants and animals getting affected as well.

Effects of soil pollution on plants


Most materials such as plastics and polythene papers do not decay. If such materials are not properly dumped, they may cause damage to leaves and stems of plants when deposited on the soil as they decay. Through this way, they interfere with growth of the plants as follows:

  • Absorption of water and mineral salts.
  • Growth of roots of the plants since they block the roots.
  • Air circulation in the soil.

Negative Effects of Soil Pollution on Animals.

Soil being homes of many small animals, such as worms, ants and termites, they are negative affected when soil is affected (polluted). Some small animals like bacteria make soil to be rich by decomposing dead vegetable and animal materials. Thisis the way humus is added into the soil making it more productive. Other small animals like earthworms and millipedes dig in the soil and this allows air and water circulation in the soil. The improved soil aeration and drainage of the soil allows the roots of the plants to penetrate into the soil easily.

The presence of oil, chemicals in the soil such as herbicides and pesticides and other harmful pollutants make it difficult for the small animals to survive and when they die the soil losesits quality and lowers productivity.

NB: Small animals in the soil improve soil air aeration and drainage.

Methods of Soil Conservation.

Animals and plants depend on soil. Plants grow on the soil while animals feed on the plants hence they need to conserve soil. To conserve soil means to protect it from losing its natural properties and productivity. There are various methods of conserving soil. This section briefly discusses these methods.

Table 3.1: Summary of the main methods used in soil conservation

Methods used in soil Conservation

 Soil conservation methods  
 Controlled use of agricutural chemicals  Planting trees
 Ensruring proper disposal of waste  Afforestation 
 Avoid burning vegetable cover  Contour farming 
 Mulching  Building gabions
 planting ground cover  Proper stock ing or controlled grazing 
  1. Control use of agricultural chemicals
    Agricultural chemicals include the following: fertilizers, pesticides and herbicides. If used uncontrollably they can lead to soil pollution and even water pollution. Their use can also be harmful to crops and those who consume the crop which include animals and human beings.
    Farmers should strictly follow the manufacturer’s instructions while using the agricultural chemicals. The instructions which come with agricultural chemicals clearly advise on the following:
    • Type of the chemical to use.
    • Purpose of the chemical.
    • Direction of usei.e. the amount to use and mode of application.
    • Precautionary measures to observe.
  2. Ensuring proper disposal of waste
    A lot of waste is produced by domestic and industries. Both wastes can be harmful or useful. So that soil is not polluted by these wastes it is important to have good ways of disposing them. This is called waste management.
    1. Domestic Waste
      Domestic waste means the garbage people discard from their homes. This may be of organic refuse or inorganic refuse.

      Organic refuse: This refers to those that can rot and includes food remains, vegetables, and fruit peelings.

      Inorganic refuse: This does not rot and some may take long time to decay.
      Examples of such are plastics, broken glasses, metal parts and cans. Domestic
      waste should not be dumped on the soil surface.

      Methods of disposing domestic waste
      Converting waste into compost manure
      A compost pit should be dug at home for all garbage that can rot e.g. food leftovers. This can be made into compost manure

      Using local authority service
      Waste in urban areasis deposited into the garbage bins provided by the local authorities who arrange for its collection and thereafter proper disposal.
      Table 3.2:common methods of waste disposal
       Method   Description 
       Incinerator  this is burning the inorganic refuse at very high temperatures in a machine
       Recycling This is the most effective method. It involve collecting waste e.g. papers, plastics, broken glasses and food cans, treating them with chemicals and reusing them to make more of the same products
      Depositing in pit latrines Inorganic waste should be thrown into a pit latrine or buried deep in the soil. Also it can be burnt in an improvised incinerator as shown below.
      Reusing Reusing means using an item more then once or for a different function from the one it was meant for at the beginning. Many such as cooking fat and oils, honey and jam are packed in reusable jars and cans. These can be used at home to store other products e.g. salt, sugar or small foods. Honey jars can be used as drinking glasses.
    2. Industrial Waste
      A lot of waste is produced in the industries. Such wastes include oil, contaminated acids and metal waste. Water is contaminated with chemicals, waste rubbers and waste papers. Some of these chemicals are not only hazardous to the soil but to the living component sin the environment.
       Method    Description 
       On-site treatment This is where waste is treated harmless at an industry at the point where it is produced. This in turn reduces the danger of polluting the environment during the transportation process to the point where it is appropriate to be disposed.
       Reusing Some industries use some products more than once or for different function from the initially meant for e.g. commercial industries, the solvent they use such as petrol to dry clean garments is not thrown away but filtered and reused.
       Recycling Companies that make bottles do not throw broken bottles away but recycle them to make new ones. There are other companies that buy waste paper and recycle them to make tissues e.g. toilet rolls, facial tissues and serviettes.
       Waste Exchange Programme  One industry may produce waste that can or may be needed by another industry for its raw materials. This can be considered as useful waste. To explain this lets look at furniture manufacturer where the saw dust is produced and wood shaving as waste products. These can be used by a company that manufactures papers.
      Treating hazardous waste This is where the new methods are used to treat hazardous waste to make them non-hazardous. Harmful pollutants are destroyed so that they do not pose any danger to the environment.
      raw materials
      This is the process where the raw materials that produce harmful wastes are replaced with other that produces less harmful waste.
      Changing Manufacturing process  A process or stage which produces waste during the manufacturing process may be changed or eliminated so that the waste is no longer produced.
      Incineration This is the process of burning waste using machines such as incerators and furnaces.
      Reducing its generation This is the best method of reducing waste. It involves simply preventing waste generation.
      Government regulations In this, government has put in place laws to control the disposal of waste. Such control aims at preventing illegal dumping of harmful waste.
  3. Avoid burning vegetable cover
    When we want to clearland for farming, we should slash or uproot the unwanted vegetations but not burning them. This is because cleared vegetation left to rot increase humus and adds nutrients to the soil.

    NB: Vegetation cover guards soil against exposure to the agents of soil erosion.

    When we burn vegetation, we kill the living organisms in the soil and no humus goes into the soil. Vegetation cover also prevents soil from agents of soil erosion such as wind, water and animals.
  4. Mulching
    This is covering the soil with dead plant materials such as dry grass and leaves. This prevents excessloss of water from the soil through evaporation. This also reduces splash erosion. The mulch decays afterwards and adds humus to the soil.
  5. Planting ground cover
    It is advisable to plant ground coversince land should not be left bear. Ground cover may include ground cover crops that spread out overthe soil surface and cover it. Examples of such cover crops include grass and sweet potatoes. These crops or plants hold the soil firmly with their roots helping or preventing the soil from being carried away by agents of soil erosion such as wind and rain. Cover crops also trap soil as water flows through the garden thus conserving soil.
  6. Terracing
    Terraces dug along the contours on the slopes reduce the speed of run-off water. This reduces soil erosion and this way soil is conserved.
  7. Planting trees
    Planting of trees can be either afforestation or re-afforestation.
  8. Afforestation
    This is planting of trees in areas where none has been planted.
  9. Re-afforestation
    This is planting trees where forests have been cleared. Trees are important in preserving soil:
    • Reduce wind erosion by breaking the wind
    • The roots hold soil particlesfirmly together. This helps the soil from being carried away by agents of soil erosion.
    • They provide shade thus reducing the amount of water evaporation.
    • They reduce the speed of running water. This reduces the strength of water to erode the soil.
    • The leaves fall off and decompose thus increasing the amount of humus in the soil.
  10. Contour farming
    In this crops are planted along the contours on ridges. This helps reduce soil erosion thus conserving the soil.
  11. Building gabions
    As already known, gulley erosion where running waterforms V or U - shaped channels. These gullies can be blocked by building structures called gabions across them. Gabions are heavy boxes made of wire mesh that are filled with stones. As water flows through the gabion, soil is trapped thus reducing soil erosion and repairs the soil structure.
  12. Proper stocking or controlled grazing
    Farmers should keep livestock that a piece of land can hold. This is called proper stocking which leads to soil conservation. When animals overgraze they uproot the vegetable cover e.g. grass. This way the soil becomes exposed to agents of soil erosion such as wind and water.

Air Pollution

Air is an important component of the environment. Pure air is a mixture of 21% oxygen, 78% nitrogen, 0.03% carbon dioxide and 0.97% rare gases, water vapour, and dust particles. Presence of harmful substances in the air is called air pollution. The substances are called pollutants. Pollutants endanger human health and also affect other living components in the environment i.e. plants and animals. Others which arte indirectly affected by air pollution are water and soil.

Major causes of air of Pollution

  1. Tobacco smoking
    This is one of the major forms of air pollution. It is normally smoked as cigarettes. In addiction to nicotine and tar, tobacco contains carbon monoxide as well which is a highly poisonous gas as it interferes with the ability of the blood to transport oxygen to the body organs. Smoke from cigarettes does not only affect the active smoker but also the passive smokeri.e. any one who inhales the cigarette smoke (polluted air) unintentionally.
  2. Burning tyres and plastic materials
    The combustion of tyres and plastic materials produces harmful emissions and poisonous gases and especially when they do not completely burn. Incomplete combustion produces gases such as carbon monoxide which poses threat to humans health and to the survival of animals and plants. Similarly, carbon dioxide is a product of incomplete combustion. As we already know the normal carbon dioxide in the air is 0.03% excess carbon dioxide in the air as pollutant. Other pollutants from combustion are tiny particles of smoke and soot.

    Rubber and plastics when burnt produces black sooty flame and emits smoke that makes the air smoggy and also emits a foul smell. Charcoal burning is also a threatto the environment
  3. Emission of Gases from Vehicle Exhaust
    Vehicles use fuels like petrol and diesel to run. The combustion of these fuels in the engine of the vehicles produces harmful gases e.g. carbon dioxide, carbon monoxide and other gases. These gases produced by vehicles contain poisonous gases then contribute to pollution of the air. Exhaust gases from vehicles thus contribute to pollution of air and especially in cities and big towns where traffic is heavy.

    Another harmful substance that may be contained in burning fuels is lead which is dangerousto human health.
  4. Spraying Farm Chemicals
    Agricultural or otherwise farm chemicals include the following: Herbicides, Acaricides and Pesticides

    Farm chemicals are dissolved in water and sprayed to either crops or animals using a sprayer. As spraying farm chemical is done, wind blows some of the chemicals thus polluting the air. The person spraying the chemicals should take the following precaution measures to avoid inhaling the chemicals:
    • Wearing protective clothessuch as gas mask, gloves and protective coats.
    • Spraying in the same direction the wind is blowing to and not against the wind.
  5. Aerosol sprays
    These are packed in cans or other containersfrom which liquid is forced out in form of a fine mist. The perfume or pesticide is dissolved in solvent which remains in the atmosphere after spraying and pollutes it. The solvent may contain harmful chemical substances which interfere with the ozone layer.

    NB: Ozone is a gasin the upper part of the atmosphere. It forms a protective blanket known as the ozone layer which protects the earth from harmful rays from the sun. Harmful gases result in tuning of the ozone layer. When this is depleted, the survival of human being, animals and plants is at a threat.
  6. Industrial Waste Gases
    Most industries release waste gases into the air which may be harmful. Some of these industries produce excess carbon dioxide which can pollute the air. Some otherindustries produce a gas called sulphur dioxide which is also harmful. Other industries too produce dust and small particles which are harmful. These waste industrial gases must be treated so that they are made safe.

Effects of Air Pollution on Living Things and Non-Living Things

Polluted air is a threat to human, animals and plant life. Also it is dangerous to nonliving components of the environment e.g. water and soil. It corrodes materials such as corrugated iron sheets and marble.

  1. Effects of air pollution on living things

    1. Effects of air pollution on plants
      In order to grow healthy, plants need clean air. Polluted air affects plants in the following ways:
      • Leaves are covered with dust particle and thus blocking the sunlight and so affecting photosynthesis, the process by which plants make their own food. Soot and dust block the stomata this may affects plants.
      • Some waste gases such as sulphur dioxide dissolve in water droplets to form acidic solutions which damage leaves of plants. Presence of these gases in the air could also lead to formation of acid rain which make the soil to be acidic thus affecting the availability of nutrients to plants. High acidity on the soil may interfere with the survival of organisms that are very important to plants growth. When absorbed through roots, acid rain interferes with proper development of plant and it may wither and die. When deposited in water sources like pond, lakes and oceans, water plants that feed on water animals are affected.
    2. Effects of air pollution on animals
      Air pollution is a threat to humans, domestic and wild animals through the following ways:
      • Causes respiratory problems. When animals inhale dust and other harmful gases, they can develop respiratory such as coughing and sneezing which may result to respiratory diseases.
      • May cause allergies resulting in coughing, sneezing, irritation of eyes and breathing problems.
      • When acid rain gets deposited into water sources like lakes, ponds, and oceans, water animals e.g. fish are affected.
  2. Effects of air pollution on non living things

    • Sulphur and carbon dioxide are among gases that cause air pollution when dissolved in the rain water. They form acid rain which corrodes metals such as corrugated iron sheets and stones like marble.
    • Acid rain causes weathering of rocks.
    • The view of environment is also destroyed by smoke and smog which are a mixture of gas particles.
    • Smog destroys materials made of rubber too.
    • Dust particles soil our clothes and settle on surface of tables, window sills and furniture thus making then dirty.

Ways of Controlling Air Pollution

Air pollution is controlled in several ways: These include;

  • Avoiding smoking cigarettes
  • NOT burning tyres and plastic materials

Table 3.4: Air Pollution Control Measures

 Measure  Description 
Avoid smoking cigarettes  In an effort to control air pollution banning of cigarette smoking in public places e.g. hospitals, schools, and in public transport vehicles has been done. These areas have been declared smoking free zones and billboards displaying the ban in these areas have been erected.
NOT burning tyres and plastic materials Polythene papers and tyres plus other plastics should be recycled or buried deep in the soil. They should not be burnt

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