Living Science 2020 2021 for Class 8 Science Chapter 21 - Annual Test Paper

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Kharif crops are harvested after the monsoon season i.e. in September/October.

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Viruses do not have a cellular structure.

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Metal X replaces metal Y but metal Y is not able to replace metal X, this means that metal X is higer up in the reactivity series. A metal which is higher up in the reactivity series can replace another metal which is placed below in the reactivity series but the opposite of this is not possible.

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A substance with a very low ignition temperature can catch fire easily. Such a material is called a _flammable_ material.

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The word used for 'the variety of plants, animals and microorganisms commonly found in a region' is biodiversity.

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Rolling friction is smaller than sliding friction. A rolling friction is the force of friction between two surfaces, that takes place when one surface rolls over the other, whereas sliding friction is the force of friction between two surfaces that occurs when one surface is dragged over the other.

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To increase loudness of sound by four times, the amplitude of vibration needs to be doubled.

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Sound that is above the frequency of 20 KHz (or 20,000 Hz) is called ultrasonic.

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Cones are sensitive to colour.

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It takes approximately 8 minutes and 20 seconds for light from the sun to reach earth.

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The main advantage of drip irrigation is that it prevents the wastage of water and also increases the crop yield.

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Vaccines are preparations which contain dead or weakened microorganisms against a particular disease. Introduction of these vaccines causes the body to produce antibodies which protect the body from any attacks from the same microorganisms in future.
It helps the body to develop immunity. 

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A non-metal on reacting with oxygen produces an acidic oxide. This acidic oxide will turn blue litmus paper red. However, there will be no change in the colour of the red litmus paper, as it changes colour only under alkaline conditions.

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The pointed heel on the left leg will exert more pressure on the ground. This is because force exerted per unit area will be more with the heel as compared to the flat shoe, as the area on which the force is exerted is smaller in the former. Smaller the area over which force is applied, greater the pressure.

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Polishing a rough surface makes it smooth, and hence it reduces friction.

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Electric current can bring about chemical changes. This is known as the chemical effect of electric current. For example, electrolysis of water can break water into hydrogen and oxygen, thus bringing about a chemical change.

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With the aid of a periscope, the sailor in a submarine can see objects on the surface of the sea and also the enemy's ships from inside the submarine.

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Light is dispersed by a glass prism because it bends the light by different angles.​ Therefore, light gets dispersed into its different coloured components. 

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The Pole Star appears to be stationary in the sky because it lies exactly on the axis of rotation of the earth.

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Germs in water can be killed at home by boiling, ultra violet rays, reverse osmosis and chlorination.

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Petroleum is a mixture of various components. These components are separated from each other on the basis of difference in their boiling points. The components of petrol are termed as fractions and the technique used for their separation is called fractional distillation. 
The process of separating the various components of petrol by fractional distillation is called refining.

The difference in the boiling points of the components of petrol is due to the number of carbon atoms present in them. More the number of carbon atoms, higher will be the boiling point.

During this process, the crude oil is heated to about 400^oC in a furnace. The vapours which are formed during this process are passed into a fractionating tower. The component with the highest boiling point will condense first and collect near the base of the tower.

More and more vapours will be produced with lower boiling points and will condense at different heights. These components are separated as different fractions which are finally collected. 

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Electroplating is done either to make the metal look attractive, or to protect it from corrosion. For example, gold is electroplated on cheap metal jewellery to make it look attractive, and zinc is coated over iron when making bridges in order to protect it from rusting.

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The number of images formed by the two mirrors at angle of 10° can be calculated by formula:

$\frac{\mathrm{n}}{\mathrm{\theta }}-1 = \mathrm{number} \mathrm{of} \mathrm{images}$
In this case n = 360/10 - 1 = 35
Therefore, the number of images seen is 35.

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Excess greenhouse gases in the earth's atmosphere can lead to global warming that will further increase earth's temperature. This would result in the melting of ice in polar regions, which would subsequently lead to a rise in the sea level. This will lead to flooding of coastal areas with sea water. It is believed that global warming could also alter climate patterns.

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A force can have following effects on an object:

• It can bring a stationary object into motion and thus, bring about a change in its previous position of rest. For example, kicking a ball which is at rest moves it away from its earlier position.
• Speed of a moving object can be changed by force. For example, a goalkeeper stops the ball by applying force.
• Force can bring about a change in the direction of motion of a moving object. For example, when a batsman hits a ball with his bat, the ball's direction alters.
• Force can also change the size and shape of the object. Example, when a sponge is squeezed, it shrinks in size, and its shape changes too.

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To electroplate copper on an iron nail, copper sulphate solution is taken in a beaker. A wide copper strip and the iron nail are connected with a battery and dipped into the beaker.
Copper sulphate when dissolved into water forms positively charged copper ions and negatively charged sulphate ions.
When an electric current is passed through the solution, the copper ions (cations) move towards the cathode (iron nail) and get deposited over it. At the anode, the copper strip loses electrons to form copper ions in the solution.

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A swimming pool full of water appears to be less deep than it actually is. This is because an optically denser medium (i.e., water) when seen from an optically rarer medium (i.e., air), appears less deep than it actually is. When a ray of light passes from air to water it bends towards the normal, but when the ray passes from water to air, it bends away from the normal. Thus, emergent rays come from virtual image which is above the bottom of pool and therefore, it appears to be less deep.

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The uses of artificial satellites are:

• They aid in television and radio transmission.
• They enable telecommunication.
• They help in the study and forecast of weather by taking pictures of clouds from outer space and sending them to earth.
• They help locate mineral ores and enable the assessment of yield in agriculture by taking photographs from the sky. This is called remote sensing.
• They help in the collection of information about space and increase our knowledge of the universe.

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(a) The sexual changes that occur in male humans during adolescence are -

• increase in the size of testes and penis
• production of sperm begins

• enlargement of breasts
• initiation of menstrual cycle
• production of egg/ovum begins

• increase in height 
• change in body shape
• change in voice 
• development of hair
• activity of sweat and oil glands
• changes in behaviour and attitude
• sexual feelings

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The rabi crop is harvested in the month of march.

Hence, the correct answer is option C.

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Bakelite comes under the category of thermosetting plastics. Thermosetting plastics can be softened by heating only once.

Hence, the correct answer is option C.

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Petroleum gas has the lowest boiling point i.e. below 40^oC whereas heavy oil has the highest boiling point.

Hence, the correct answer is option A.

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Chloroplasts are a type of plastid which contain chlorophyll. They are found in plant cells and help in the process of photosynthesis.

Hence, the correct answer is option B.

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Some animals undergo the process of metamorphosis. During, this process an individual undergoes complete change during its development from a young one to an adult.
Frog, butterfly and mosquitoes are examples of animals which undergo metamorphosis.

Hence, the correct answer is option D.

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Tyres of heavy vehicles are broader than lighter ones, so that they can uniformly distribute their huge weight and exert less pressure on the road.

Hence, the correct answer is option D.

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Friction can be increased by making the surfaces rough. The rougher a surface, greater the friction. 

Hence, the correct answer is option B.

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Richter scale is not a linear scale. An earthquake that measures 5 on it is 10 times more powerful than an earthquake that measures 4.

Hence, the correct answer is option D.

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Iron plates are coated with tin to make food containers. This prevents iron from reacting chemically with the food.

Hence, the correct answer is option B. 

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A plane mirror produces a virtual erect image of the same size as of the object.

Hence, the correct answer is option B.