Living Science 2020 2021 Solutions for Class 8 Science Chapter 20 - Half Yearly Test Paper

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Drip irrigation is the best method of irrigation when there is scarcity of water.

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The body produces antibodies to fight the germs in the vaccine.

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Bakelite is a thermosetting plastic which can be softened by heating only once.

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The property of metals by which they can be drawn into thin wires is known as ductility.

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An animal cell does not have a cell wall. But the animal cell is protected by the cell membrane.

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Anthracite is the variety of coal which can be considered best for use as a fuel. It has 96% carbon and is considered the best variety of coal.

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The solid particles which act a pollutant of air are called suspended particulate matter. Example includes fine particle of cements given out from a cement factory.

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It ts true that a biosphere reserve can have national parks and wildlife sancturies within it. Biosphere reserves are large areas which are used to protect the flora and fauna along with the human population which live in that area.

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External fertilisation is seen in frogs. When the fusion of male and female gamete occurs outside the body, it is termed as external fertilisation.

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The stage of life when a human becomes capable of reproduction is called  puberty .
It is the stage during which an individual undergoes sexual maturation.

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Transplanting allows a farmer to select only the healthy seedlings and also space them properly while planting.

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The rule of 'metal displacement reaction' governs the displacement of a metal from its salt solution by another metal. By this rule, the more reactive metal displaces the less reactive one from its solution. 

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Petroleum is obtained from oil wells in the form of crude oil which is a mixture of many useful hydrocarbons such as, kerosene, petrol, wax etc. Refining by fractional distillation is done in order to obtain these various hydrocarbons which have different boiling points.

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Calorific value of a fuel is defined as the amount of heat released when a unit of a fuel is burnt completely in air or oxygen. It gives useful information about the heating efficiency of a fuel.

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Algae contain cell wall and chlorophyll within their cells like plants. They too manufacture their own food with the process of photosynthesis. Therefore, algae are considered to be closer to plants than to animals.

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Polymers are large molecules, consisting of long chains made from thousands of similar small molecules called monomers. Some examples of polymers are polyester, rayon and nylon.

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Endemic species are those species of plants and animals that are found exclusively in a particular area, and are not naturally found anywhere else. For example, the Asiatic lion is endemic to Gir National Park in Gujarat.

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A nucleus is the control centre of a cell. It directs the growth of the cell and controls all the activities that take place within it.

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Zygote is formed by the fusion of a sperm and an ovum in the oviduct of a female. This process is known as fertilization. A single sperm fuses with an ovum to produce a zygote.

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Hormones regulate many processes in the body such as growth, development, behaviour and reproduction. A body possesses more than 30 hormones.

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Pasteurization of milk is the heating of milk to a high temperature (70 degree Celsius) for about 30 seconds, and then cooling it down quickly. By this process, most of the bacteria present in milk get killed without affecting its flavour. 

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Silver combines with hydrogen sulphide released from petroleum refineries to form silver sulphide that leads to tarnishing of silver. Unpolluted air does not contain hydrogen sulphide and so, it does not tarnish silver.

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In adolescents, at the onset of puberty there is an increase in the activity of sweat and sebaceous glands. This leads to increased sweating which results in body odour. The skin becomes oilier which often leads to the occurrence of acne and pimples on the face.

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The three plant diseases caused by microorganisms are:
(a) Citrus canker: It is a bacterial disease that affects trees of citrus fruits and is spread by air.
(b) Yellow vein mosaic: It is a viral disease that affects vegetables such as bhindi. It is spread by insects.
(c) Rust of wheat: It is a fungal disease that spreads by air and seeds.

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Following properties make nylon a useful fibre: 
(a) Nylon is very strong and elastic. Its thread is stronger than a steel wire.
(b) It is light and wrinkle resistant.
(c) Nylon is also wear resistant. This is the reason why nylon garments last for a long time.
(d) It is easy to wash and absorbs very little water. Therefore clothes made from nylon dry quickly.  

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Coal, petroleum and natural gas are called fossil fuels, as they are naturally formed from the remains of organisms. Fossil fuels are concentrated stores of energy and give off heat and light on burning. The heat can be used to cook food or run engines (such as automobile engines). They can also be used to generate electricity in powerhouses (the most common fuel in many powerhouses is coal). Thus, fossil fuels are an important source of energy for us today.

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Binary fission is the simplest type of asexual reproduction and is seen in single-celled organisms such as Amoeba. First, its nucleus divides into two equal halves. Then the cytoplasm also divides into two parts, each part receiving a nucleus. The two daughter cells thus formed grow and attain full size before they split again. 

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The three natural methods of manuring are -

• Field fallow
• Crop rotation
• Mixed cropping

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(d) all of these

Compost is a manure obtained by the decomposition of dead plants and animal waste. It is rich in organic nutrients but lacks some important nutrients such as nitrogen, phosphorus and potassium.

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(b) mammal

All the other options are examples of microorganisms. Mammal is the odd one out.

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(a) acrylic

Acrylic is a synthetic fibre that resembles wool in terms of its properties. It is lightweight with a wool-like feel. Clothes made from acrylic are cheaper than wool.

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(d) zinc

Galvanization is the process of deposition of a layer zinc on iron. This prevents corrosion of iron.

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(b) CNG

The various products obtained by refining petroleum are LPG, petrol, kerosene, diesel, lubricating oil and asphalt.

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(d) luminous zone

The luminous zone of a candle flame is the zone of incomplete combustion. The wax vapours do not burn completely, since the supply of oxygen is inadequate in this zone. When an object is placed over this zone, it releases black soot and other residues. The carbon particles in it, glow, emitting a yellow light.

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(d) fauna
Animals found in a particular region are called as fauna of that region.
 

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(d) small living structures present inside a cell

Organelles are small living structures present inside a cell that function independently. For example, chloroplast, mitochondria and ribosomes.

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(d) the wall of the uterus

After fertilization, the zygote divides several times to form an embryo. The embryo then gets implanted in the wall of the uterus, where further development occurs.

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(c) menopause
At around 45–50 years of age, a woman is no longer reproductive, as the menstrual cycle stops. This condition is called menopause.