class 8th Science by stariya


									                                        BURN HALL SCHOOL
Class: 8                                        Subject: Science


1. Discuss in details different types of renewable resources.
    The sources of energy which can be regenerated over a reasonable period of time are called renewable
    resources. Different types of renewable resources are
  i.    The Sun:
                 The Sun produces enormous amount of heat and light energy due to the fusion of hydrogen atom.
        For us, on the surface of the Earth, the sun is the primary sources of energy.
                 The green plants absorb the carbon dioxide gas from air and water from the soil. The CO2 and H2O
        react chemically in the presence of chlorophyll and sunlight to form starch and oxygen. Thus the solar energy
        is trapped in the plants in the form of chemical energy. This chemical energy in the food changes into heat
        energy and then muscular energy in our bodies. Even in the fossil fuels, such as coal and petroleum (which
        are formed from plants and animals) the chemical energy is basically trapped solar energy.
                 The water cycle is also caused due to solar energy. A large amount of water evaporates due to solar
        energy. When the water vapor changes into clouds and it rains, it is the solar energy which changes to
        mechanical energy of water.
                 In this way any kind of energy which we use on Earth, has come from the sun, in the form of solar
        energy. Solar energy is directly converted into electrical energy in the solar cells. Solar energy is used in the
        solar cookers and solar hot water geysers.

  ii.   Water:
                 Flowing water is a very important source of energy. These days’ big dams are built on the rivers. The
        water stored in these dams has potential energy. When the stored water as made to flow through special
        channels, the potential energy of water changes into kinetic energy. This kinetic energy of flowing water is
        made to run the turbine, which is attached to an electric generator. In the generator, the kinetic energy of
        rotating is converted into electrical energy.

 iii.   Wind:
            Strong wind has a lot of energy, which generally causes soil erosion. The windmills are erected by using
        strong aluminum vanes. When the vanes rotate with the kinetic energy of the wind, their circular motion can
        be used for drawing up underground water. These days’ generators are attached to the ark of the rotating
        vanes. In the generator the kinetic energy changes into the electrical energy.

 iv.    Wood and Biomass:
            Wood has been the most important source of energy from prehistoric times. Wood, on burning produces
        heat energy. The heat energy so generated is put to a number of uses.
            However, with the increase in population, the availability of wood is decreasing rapidly.
            Biomass consists of dried leaves, cow dung or any other non-usable product of animal or vegetable life.
        The biomass is converted into combustible gases, such as methane, in the gobar gas plants.

2. How is electricity produced in hydro-electricity plant?
                In a hydroelectric dam, water is stored in a reservoir. As the height of water in the reservoir rises, the
      potential energy of stored water increases. In order to convert this potential energy of water into electrical
      energy, the iron gat near the bottom of reservoir is raised with the help of control value. As the water rushes
      out, its potential energy changes to kinetic energy. This energy, in turn, rotates the blades of the turbine. The
      turbine is coupled to a generator. When the coils of the armature of generator turn rapidly in the magnetic
      field, electrical energy is produced in the coils. The electrical energy, so produced, is fed to set up
      transformers and then supplied to distant places through high-tension wires.

                 A solar cooker consists of a rectangular wooden box, which is lined from inside with some insulating
        material, such as thermocol or glass wool. The insulating material prevents heat losses due to conduction,
        convention and radiation. Within the wooden box is placed a tight fitting metallic box, whose inner surface is
        painted black. Black surfaces are good absorbers of heat energy. On the top of box is fitted a lid which
        consists of a wooden pane in which is fixed plane sheet glass. The glass sheet in the lid allows the solar
        energy to enter the box, but does not allow it to flow out in the form of radiant heat. To the upper side of the
        box is attached a plane mirror with the help of a clamp. The plane mirror can be adjusted at any angle so that
        solar energy could be reflected into the box.
        In order to cook food in the solar cooker, metallic-utensils are used and their external surfaces, including lids
        are blackened. The lid of the solar cooker is closed and it is turned towards the sun so that solar radiation
directly enters in it. The angle of the solar reflector is also adjusted so that more of the solar radiations get
reflected into it. Within 1 ½ hour or so, the temperature within the solar cooker rises about 100 C to 140 C,
which is sufficient to cook the food in 3 to 4 hours. [diagram form book]

         Biogas is a mixture of gases formed when the slurry of animal dung and water is allowed to ferment in
the absence of oxygen (or air). The fermentation of animal dung which takes place in the absence of air due
to the presence of anaerobic bacteria present in the animal dung is called anaerobic fermentation.
Fermentation is a mixture of methane, carbon dioxide, hydrogen and traces of hydrogen-sulphide along with
water vapor. The chief constituent of biogas is methane gas and is about 65% by volume.
The animal dung or the crop wastes contains large number of complex organic compounds, such as
carbohydrates, fats and proteins. It also contains anaerobic bacteria. When the slurry of animal dung and
water is allowed to ferment at a temperature of around 35 C or above in the absence of air, the anaerobic
bacteria slowly breaks down the complex compounds of carbon to biogas. The process of formation of biogas
from animal dung slurry is biochemical in nature.
Advantages of biogas.
     a. Biogas has a fairly large calorific value.
     b. It is a convenient and clean fuel which does not produce smoke or other harmful gases.
     c. It is cheaper than any other fuel.
     d. It has no storage problems as it can be directly supplied to a number of houses through pipelines.


Glass is non-crystalline or an amorphous substance. Glass does not have any definite composition. The
properties, quality and appearance of glass largely depend on the purity of the material used.
The raw materials required for the manufacture of ordinary glass are-
    a. Sodium Carbonate(Soda ash) Na2CO3
    b. Calcium Carbonate(Limestone) CaCO3
    c. Silica(Sand) SiO2
        The glass is prepared by heating a mixture of soda ash, limestone and sand to a temperature of
        about 1400 C in a furnace known as Rotary Furnace. A clear transparent substance looking like jelly
        is formed. On slow cooling this liquid forms glass. Different shape of glass can be made by using
        different moulds into which hot liquid glass is poured. Hot glass can be blown either directly or by
        using a blow pipe to get articles of different shapes. This process is known as glass blowing.
        Generally to make glass articles molten glass is poured into moulds and then cooled. If the hot glass
        article is cooled at once, it becomes brittle and so cracks easily. To avoid this moulded glass article is
        cooled very slowly. Thos process of slow cooling of articles is called annealing of glass.
        [Types of glass- from text book]

         The name ceramics is derived from the old Greek word Keramos meaning potters clay. Clay is
formed by the weathering of minerals Feldspar which is found in igneous rocks. Ceramics are poor
conductors of heat and can withstand high temperature such materials are known as refractory materials.
Ceramics are objects made from clay, feldspar and flint. These are mainly compounds of silicon, carbon,
oxygen and nitrogen.
         Clay used in the ceramic industries is mainly a hydrated aluminum silicate. These minerals, called
silicates are crushed, grinded and sieved to obtain fine clay. Fine clay is then mixed with water to form dough.
Dough is kneaded to make it uniform. This homogenized dough is then moulded, rolled or shaped into various
objects and allowed to dry for several days.
         After drying, the product is heated in a special type of furnaces called KILNS to a temperature of over
800 C. Heating the articles are given a glossy coating called Glaze. Glazing makes the surface smooth and
attractive. Colored ceramics are also made by adding some colored compounds.
     i.  They are poor conductors of heat and electricity.
    ii.  They can withstand very high temperatures.
   iii.  They are resistant to the actions of chemicals, acids and oils.

   i.   They are used for lining furnaces.
  ii.   They are used for making bricks, roof tiles.
 iii.   They are used as insulators in spark plugs, powodines and in television.
 iv.    They are used to store liquids, pickles, acids and other industrial chemicals.
  v.    Glazed ceramics are used to make bathroom tiles, sinks, and drainage pipes etc.

The substance used for cleaning are called detergents. There are two types of detergents.
    a. Soapy and
    b. Non- soapy.
We call soapy detergents as soaps and non-soapy detergents as synthetic detergents or just detergents.
Soaps: are the alkali salts of higher fatty acids. The fatty acid,
Usually stearic acid (C17 H35 COOH), Olcic acid (C17 H33 COOH) or palmitic acid (C15 H31 COOH). These acids
are obtained from plants or animal fat and hence called fatty acid.

Soap is prepared by heating vegetable oil or animal oil with potassium or sodium hydroxide. The oil or fats react with
alkali solution to give soap and glycerol.
Oil + Alkali ------------- Soap + glycerol

C15 H31 COO CH2                              CH2-OH
                  I                             I
C15 H31 COO CH + 3NaOH-------- 3C15H31COONa + CH-O
                  I                Soap         I
C15 H31 COO CH2                              CH2-OH
The process of splitting the oil or fat on its hydrolysis with alkali is called saponification. The soap produced is
separated from the solution by the addition of common salt. Salt decreases the solubility of soap and being lighter
starts floating on its surface. The solution left behind contains common salt and glycerol. The latter (glycerol) is
recovered from this solution and put to various uses e.g. in making cosmetics, drugs, explosive and paints.
Soap is made hard by adding some filler like sodium carbonate, sodium silicate etc. Further it is mixed with desired
coloring agent and perfume and then cast into various shapes.


             A molecule of soap is made up of two parts: a long hydrocarbon part and a short ionic part containing-----
COO Na group. The soap molecule is said to have a tadpole structure which can be represented as
/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/Na short ionic part
Long hydrocarbon
             The long hydrocarbon part of soap molecule is water repelling (hydrophobic) and is therefore, insoluble in
water while it’s ionic part water-attracting (hydrophilic) and hence soluble in water. With regard their solubility in oil or
grease, hydrocarbon end is soluble while ionic end is insoluble. When these clothes are dipped in water and soap is
added, the greasy or oily particle attach themselves to the hydrocarbon group of the soap and ionic part remains
attached to water. On agitation of the dirty cloth in soap solution, the dirt particles got attached to the soap molecules,
due to pull of ionic part attached to water and consequently the cloth gets cleaned.

    A plastic is a synthetic material which can be moulded or set into any desired shape when soft and then hardened
to produce a durable articles e.g. Nylon, polyester, polythene PVC etc. Plastics are actually synthetic polymers.
Plastics are of two types.

   i.    Thermoplastic:
    A plastic substance which can be melted repeatedly by heating and can be moulded again and again into different
shapes is called thermoplastic substance or thermoplastic polymer. In other words thermoplastic become soft on
heating and this property allows them be set in a desired shape again and again, which can be retained on cooling
e.g. polythene, poly vinyl chloride (PVC), Polystyrene and Nylon.

   ii.   Thermosetting plastic:
  A plastic substance which once set, does not become soft on heating and cannot be moulded a second time is
called thermosetting polymer. In other words, the irreversible plastics which cannot be melted or softened once set in
a given shape and solidified are called thermosetting plastics or thermosetting polymers e.g. Bakelite, Melamine etc.
Plastics are polymers having large molecules. They are made up of 100 or even 1000 small molecules. The small
molecule of a single unit is called monomer.
The polymer is obtained by linking together these monomers. The process of linking together of monomers to form
polymers is called polymerization.

n(monomer)----------------(monomer) n
n(C2 H4)------------ [H2C---- CH2]n
Ethylene        polythene
Properties of plastic:
   a. They are strong and resistant to corrosion.
   b. They are bad conductors of heat and electricity.
   c. They may be transparent, translucent or opaque.
   d. They can be moulded into any shape when hot.

The most commonly used plastics are
      a. Polythene:
      It is tough, but flexible and strong. It is translucent, solid and thermoplastic. It is also poor conductor of electricity.
   i.       It is used as a water-proofing material.
  ii.       It is used for making bags for wrapping food. It is used for making containers and pipes for storing and
            transporting water, oil etc.

      b. Polyvinyl Chloride (PVC)
      It is a thermoplastic and is tougher than polythene.
   i.       It is used for making hand bags, bottles.
  ii.       It is used as insulating covering for electrical wires.
 iii.       It is used for making soles of shoes, sandals.
 iv.        It is used for making gramophone recorder.

      c. Polystyrene:
      It is thermoplastic which is far lighter than polythene and more easily moulded. This property enables it to be
blown up into very large foam containing air bubbles. In this form, it is called Styrofoam or thermocole.
 i.It is used as packing material for securing delicate and fragile objects.
ii.It is used as thermal insulator in the hollow walls of refrigerators and coolers.

        d. Acrylic or perplex:
It is a clear, transparent plastic which is used to replace glass in some situations. However, being soft, it is easily
scratched. It also dissolves in organic.
     i.    It is used for street light fittings and telephones.
    ii.    It is used for making windscreens of cars and windows in aircraft.

        e. Bakelite:
It is a thermosetting plastic which is a good electrical insulator.
     i.    It is used for making plugs, switches, telephone cases.
    ii.    It is used for making radio and television carting.
   iii.    It is used for making artificial leather.
   iv.     It is used for making handles of tea kettles and electric presses.


         Manure is an organic substance obtained from the decomposition of vegetables and animal wastes through
the action of microbes. They contain a mixture of various nutrients recycled from biomass wastes. They also restore
soil texture for better retention of water and aeration. However, being voluminous it is not convenient.
Fertilizer is a water soluble salt rich in one, or more essential plant nutrient. They are in concentrated form, are easy
to transport, store and apply to crops. They are available both in solid form as well as in solution. Being soluble in
water, they are readily absorbed by crops. Granular

Manures are of three kinds:
   i. Farmyard manure:
      Consists of cattle dung, wine, remnants of straw and farmyard wastes. All these are mixed and allowed to dry
      in the sun. Subsequently it is used as manure.
  ii. Compost:
      Is manure obtained by the decomposition of dead plant and animal wastes.
 iii. Green manure:
      Consists of green plants which are ploughed along with soil. Generally quick growing leguminous plants are
      used as green manure.
                                      BURN HALL SCHOOL
 Class: 8                                     Subject: Science


 Long Answer Type Questions:

 1. Describe the five categories of planets which are useful to humans.
       The five categories are as follows:

                          1. Food producing

                          2. Fire producing

                          3. Timber yielding

                          4. Medicinal

                          5. Ornamental and other useful planets.

 1. Food Producing Planets:

        We are usual grains as staple food in our diet. We also use some root and tuber crops as food such
 as sweet potato and tapioca (Cassava). All these food products are rich sources of carbohydrate which
 give us energy. They also contain some vitamins. Leguminous plants have nitrogen fixing bacteria living in
 their root nodules. So all food products obtained from leguminous plants are rich in protein. We also use a
 large number of plants products such as seeds or parts of fruits, leaves, etc to extract oil, sugar, use as tia,
 copper and as vegetables. Mustard, sunflower, coconut and rice hunk are all used to extract oil for cooking.
 Vegetables are good sources of vitamins, minerals, and roughage materials. Banana gives us able of
 energy and some minerals. Guava and all citrus fruits are rich in vitamins B and C. Tamarind and mango
 fruits are also used as flower giving agents in our cooking.

 2.   Fiber Producing Plants:
      Cotton fiber and coir are obtained from the parts of the fruits of cotton and coconut trees. Cute fiber is
obtained from the stalk of the fruit plant. Cotton is mostly cultivated in the black soil found in Bengal and
Coconut is largely a coastal and peninsular plant. Silk used as clothing.

 3. Timber producing Plants:
         All been can yield wood which can be used as fuel when they are derived. In complete burning of
 wood produces charcoal which is again used as fuel. Wood from certain kinds of trees such as deodar,
 pine, furniture such as cots, cupboards, chairs, etc. Bamboo and other tree wood are used for construction
 of buildings. Also many tools and implements such as wheels, plough, etc are made from timber. Wood is
 also the raw material in paper industry.

 4. Medicinal Plants:
        Many plants have great medicinal value. There systems of medicine namely Ayurveda, Unani and
    Siddha, use medicinal plants excessively. Even allopathy or the modern system makes many deigns
    from plants. The common plants such as neem (Margosa) leaves, amla (Gooseberries) fruits, oil
    extracted from eucalyptus, gum of papaya and of course tusk leaves and ginger all considered to have
    medicinal value.
5. Ornamental and Other Useful Plants:
       Many people make gardens keep indoor plants like cotton for aesthetic purpose. Some of these
   indoor and outdoor ornamental planets are economically important such as orchids. Orchids are grown
   in cool climate and hills, forest areas and are sold for high value. Cacti are natural desert plants but
   they are used extensively as indoor ornamental plants. Bonsai are made to fulfill aesthetic needs.
   Rubber, dyes, gum and resins, rose water and essence and cork are all plant products which we use in
   our daily life. Sandal wood tree is a very expensive plant resource available in southern states of India.
   The tamarind tree (Imli) is an essential component that gives taste to our food. There are many other
   ways in which we use or need plants. They give us shade, help in preventing soil erosion, help in
   counting rainfall and also maintain O2 – CO2 balance in nature.

     2. What is Animal Husbandry? Discus its factors.

         The keeping breeding of animals for specific purpose is called domestication. All domesticated
   and useful animals constitute livestock. Each domesticated animal has its own need of food, shelter
   and health care. The study of all there aspects is called animal husbandry. There are four main factors
   of animal husbandry. There are: (1) Breeding, (2) Feeding, (3) Weeding, (4) Heeding.

   1. Breeding: Breeding is done to obtain animals with deserved characters. Mating is done selectively over
      generations so as to improve the quality of the animal or stock. Just as we develop and use high yielding
      grain crops, we can also develop high milk yielding and high meat yielding cattle. A breed of cows called
      sindhi red is an excellent draught and dairy animal. The sahiwal cows are excellent milk producers and the siri
      bread from Bengal and sikhim is a mountain animal excellent as drought animal but unfit for the plains. There
      are six main breeds of Buffaloes. Buffaloes are mainly bred for milk and not as draught animals.
   2. Feeding: Each set of animals has a different feeding habit. The type of food, the mode and time of eeding
      the animals are important. For cattle, the feed is stran, hay and fodder, fresh pasture grass and oil seed
      cakes. Chickens need grains with special feeds. Cattle are classified as dairy breeds (which yield milk)
      draught breeds (which are used in work) and dual breeds where the female is the milk producer and the male
      used for work. The time of feeding and quantity of food are important. In a farm, cattle are feel in the mornings
      and then put to use or milked. The cows are left to gaze until the wining feed. Reduction in the amount of
      feed leads to weakness in the animal and reduced productivity. Special breading and feeding and care are
      therefore necessary.
   3. Weeding: The method of eliminating uneconomical breeds is called weeding. Though we have the largest
      number of animal, we do not produce enough milk. Part of the reason is improper feeding.
   4. Heeding: Heeding means proper care and management of animals. Some very sample practices are
      followed by farmers on heeding their animals. These are.
           a. Feeding the animals regularly and properly.
           b. Cleaning out their drinking trough and putting in clean water.
           c. Removing dirt, cleaning the cattle shed regularly.
           d. Watching the animals carefully to see if they are normal and healthy.
           e. Taking precautions against diseases.
           f. Working calmly and treading them gently.
   5. Write notes on.
           a. Apiculture: Honey bees collect nectar from flowers and make honey. They keep it in their nest
               (beehive). Honey consists of water, sugar, minerals and enzymes. It is an antiseptic and is used in
               making medicines. Beehives can bee seen hanging from fall trees and huge buildings. To rear bees
               at home and extract honey in large amount at regular intervals, large seale beekeeping can be done,
               this is known as apiculture.
           b. Pisciculture: A major source of animal protein are fish. This grow both in fresh water sources such as
               ponds, lakes and rivers and sea water. Food habits depends on the availability of there fish. E.g
               coastal area people eat marine fish and other sea food where as inland people are used to fresh
               water fish. Catla catla, labeorohita, cirrhina mrigala example of marine sources arithina and cod. Fish
               are not only cooked and eaten right after the catch, they are eaten in dried form, pickled form and
               caned form. Rearing and management of fish for large scale production is known as pisciculture. In
               piacicuture hatching of fish eggs to small fries, putting the right number of fries in properly kept water
               source with proper fish food and then allowing sufficient light and oljgen and keep harvesting grown
               fish at proper times are all important processes.
           c. Sericulture: Silk threads are extracted from the cocoons of the pupae. Silk moths are reared in large
                                                         0      0
               scale by hatching the eggs at about 22 c – 27 c. these eggs can be stored in cold for long term
               storage clean and dry mulberry leaves are given to these hatched larvae. When these larvae form the
               cocoons, they are used to extract silk thread. The silk moth larvae are not able to spin if they feed on
               other leaves encept a few plants. The rearing and manage mint of silk moths and making silk out of
               them is known as sericulture.
6. Short answer type questions.
      a. Why is a sheep dipped in a insecticidal solution after it is sheared?
      A sheep is dipped in a insecticidal solution so as to prevent infection by parasites like lice or ticks.
      b. What are the six elementary steps in animal care?
                i. Good housing with plenty of space. An airy shed with good light is much safer than a damp
                   and over crowded one.
               ii. Good food that helps the animal resist infection
              iii. Clean water, since dirty water carries disease causing germs.
              iv. Control on rats and mice. They not only eat up the food but also carry and spread diseases
               v. Control insects such a flies and ticks
              vi. Washing and cleaning of the animals regularly.
      c. What useful products do we get from forest plants?
                   Do your self

        d. In what ways are animals harmful to us?
           Most animals are also harmful to us. Different animals have different methods of depending
           themselves against their attackers. An animal like a snake or a bee bite generally when they are hurt
           or disturbed. Many small animals also harm by destroying crops or causing diseases. A large number
           of insects and their larval feed on plants . when they feed on crop plants, they can damage crops
           severely and reduce yields. These insects are called pests. Aphids and mites are pests that such the
           sap off the leaves and damage the plant severely. Stalk boring caterpillar are the weevil. These are
           all tissue eating insects. The locust eat plant leaves and grains.

        e. What is fodder and what plants are used for fodder?
           Do your self.

7. Objective type questions.
      a. Name three cereals rice, wheat, maize.
      b. Four factors of animal husbandry are breeding, feeding, weeding, heeding.
      c. Perls are obtained from an oyster shell.
      d. Plants help in preventing soil erosion.
      e. Dwarf plants are known as bonsai.
      f. The four types of silk found in india are Tasar, Endi, Muga, Mulbery
      g. Domestication is keeping and breeding of animals.
      h. Ivory products are made from teeth of elepheant.
      i. Sheep, goat, cow, buffalo are called as domestic animal.
      j. Ornamental plants add to the aesthetic value.

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