More Scientific Reasons
41) Why do the stars twinkle?
Ans: The light from the stars travels through different layers of space of varying densities. Therefore, the light rays deviate from their original path. Further, these layers are not stationary but keep on moving. This leads to the twinkling of stars.
42) On what days do we have equal days and nights all over the world? And why?
Ans: On 23 September and 21 March we have equal days and nights all over the world. This is so because on these days, the rays of the sun fall vertically on the equator at noon. Both the poles receive equal rays of the sun. As a result, exactly one-half of each hemisphere receives the sun’s rays. This makes day and night equal.
43) Every fourth year has 366 days. Explain.
Ans: One revolution of the earth around the sun takes 356 days and 6 hours. But we consider a year as consisting of only 365 days and ignore 6 hours. In four years the difference becomes as much as 24 hours or one day. Hence, to every fourth year we add one day. That year of 366 days is known as leap year.
44) Why are igneous rocks called primary or parent rocks?
Ans: Igneous rocks are formed when the molten material from volcanoes gets solidified. This material is liquid, hot and sticky which moves towards the surface through cracks and joints. All other rocks derived from these rocks. Hence, they are called primary or parent rocks.
45) Why are the areas lying between the Arctic Circle and the North Pole in the Northern Hemisphere and the Antarctic Circle and the South Pole in the Southern Hemisphere very cold?
Ans: These areas are very cold because the sun does not rise much above the horizon. Therefore, its rays are always very slanting which emit minimum heat. These areas being very cold are called Frigid Zones.
46) Why are the three hot regions of the world-equatorial forests, savannah lands and hot deserts-not found in Europe?
Ans: The three hot regions of the world-equatorial forests, savannah lands and hot deserts-are not found in Europe because Europe is the only inhabited continent situated entirely outside the tropics.
47) Why is the lowest layer of the atmosphere in contact with the earth’s surface, the warmest?
Ans: The lowest layer of the atmosphere in contact with the earth’s surface is the warmest because the atmosphere is heated mainly from the below.
48) Why does the temperature above the ocean and land masses vary even on the same latitude?
Ans: Temperature above the oceans and land masses varies even on the same latitude because of the differential heating of land and water, i.e., land mass is heated and cooled more rapidly and to a greater degree than water.
49) Why are marine animals able to live at great depths than marine plants?
Ans: As marine animals do not depend upon sunlight for their survival, they are able to live at great depths where there is permanent darkness.
50) Why do trees of coniferous forests possess needle-like leaves?
Ans: The needle-like leaves limit transpiration and thus enable conifers to grow in the drier areas.
51) Why do equatorial forests appear evergreen?
Ans: In equatorial forests trees often shed a few leaves or shed their leaves seasonally, but most of the trees retain their leaves for most of the time so that the forests appear evergreen.
52) Why is petroleum often called black gold?
Ans: Petroleum is often called black gold because of its great demand in the modern industry and for domestic use. It provides fuel for heat and lighting, lubricant for machinery and raw material for a number of industries.
53) Why are the kangaroos called marsupials?
Ans: Marsupial means broad-pouch. Since kangaroos have a pouch-like fold of skin near the stomach in which they carry their young ones, they are called marsupials.
54) Why is platypus considered a strange animal?
Ans: Platypus is a strange animal because it is an animal-bird that survives under water, walks on the ground and digs tunnel under the ground. It is a four-legged animal that lays eggs like a bird. It is found in Australia.
55) Why is a person in moving vehicle thrown forward when the vehicle stops suddenly?
Ans: A person in a moving vehicle is in a state of motion. When the vehicle suddenly stops his body tends to remain in a state of motion due to inertia and he is thrown forward.
56) Earth is continuously pulling moon towards its centre. Why does not the moon fall on to the earth?
Ans: it is so because the gravitational attraction of the earth provides the necessary centripetal force to the moon for its orbital motion around the earth due to which the moon is revolving around the earth.
57) Which of the two-glass or rubber is more elastic and why?
Ans: Glass is more elastic than rubber because for a given applied force per unit area, the strain produced in glass is much more than that produced in rubber.
58) Animal like camel can easily walk in the desert sand while other animals like donkeys, dogs and horses cannot. Explain.
Ans: The camel has very broad and large feet. As a result of the large surface area in contact with the ground, it exerts less pressure on the sand and sinks only slightly in it. Other animals have smaller feet which exert more pressure on the sand. As a result, they sink more in the sand and cannot walk easily in desert.
59) Why does a small quantity of liquid assume spherical form?
Ans: A small quantity of liquid assumes a spherical form due to surface tension which tends to reduce the surface area. A given mass will acquire minimum surface area if it assumes a spherical shape.
60) Why does an iron needle float on clean water but sink when some detergent is added to this water?
Ans: Due to surface tension, the free surface of liquid at rest behaves like a stretched membrane. When an iron needle floats on the surface of clean water, its weight is supported by the stretched membrane. When some detergent is added to this water, its surface tension decreases. As a result of it, the stretched membrane on the surface of water is weakened and is not able to support the weight of needle. Hence the needle sinks in such water.
61) Why is cooking quicker in a pressure cooker?
Ans: The boiling point of water depends upon the pressure on its surface. Steam produced inside the cooker builds up pressure thereby raising the boiling point of water, which results in quick cooking.
62) Why does steam cause more severe burns than boiling water?
Ans: The amount of heat (latent heat) possessed by steam is much greater than the amount of heat possessed by water at the same temperature. Therefore, steam causes more severe burns than boiling water.
63) Why does ice not melt readily when salt is sprinkled over it?
Ans: When salt is sprinkled over ice, some of it dissolves. As dissolution of the salt is accompanied by absorption oh heat. The temperature of the system falls below 0 degree Celsius. Hence, ice does not melt readily.
64) It is difficult for firemen to hold a hose, which ejects large amount of water at a high velocity. Why?
Ans: the water which comes out of the fireman’s hose carries large momentum as its velocity is very high. The equal and opposite reaction force pushes the fireman backwards with a great speed satisfying the law of conservation of momentum. As a result, it is difficult for the fireman to hold the hose.
65) In the outer space astronauts talk to each other through radios. Why?
Ans: sound waves need a material medium for its propagation. There is no air in space and hence, sound waves cannot travel. However, radio waves can travel through space. The astronauts can see each other because light, like radio waves, can travel through vacuum.
66) Sonar scanners are used by doctors. Explain.
Ans: sonar scanners send out ultrasonic waves, which are reflected by body tissue and organs. From the pattern of reflections, a computer can build up an image of the internal structure which is vital for diagnosis.
67) Why the flash of lightning is seen before the sound of thunder is heard?
Ans: the velocity of light is much greater than that of the sound. Therefore, flash of lightning is seen before the sound of thunder is heard.
68) Bats have poor eyesight but are able to home their prey with great accuracy. Also, dolphins can avoid fishing nets and can detect fish at night. Explain
Ans: Bats emit high frequency (1, 20,000 Hz) sound waves and listen with their sensitive ears for any echoes. From the time taken to hear the echo and from the nature of sound received. Bats are able to estimate the distance and the type of surroundings. This process is known as echolocation. Same process is used by the dolphins.
69) When low flying aircraft passes overhead, we sometimes notice a slight disturbance on our TV screen. Why is it so?
Ans: a low flying aircraft reflects TV signal. Due to the interference between the direct signal and the reflected signal, there is disturbance on the TV screen.
70) In automobiles, why are convex mirrors used to see the traffic from behind?
Ans: Convex mirrors are used in automobiles because they form erect and diminished images of the object. Thus, it helps the drivers to get a wider field of view of the traffic coming from behind.
71) Why is mirage formes in the deserts?
Ans: A mirage is formed owing to total internal reflection. To the observer at a distance, the reflected image of the object appears behind the reflecting surface, as if the object were in front of it, but actually it is just an illusion.
72) What will be the colour of grass in blue light?
Ans: Grass will appear blackish in colour in blue light because it has the property of absorbing all other colours except its own colour. The blue rays falling on grass will be absorbed by it, and it will appear dark coloured.
73) A bird perches on a bare high power line and nothing happens to it. A man standing on the ground touches the same line and gets a fatal shock. Why?
Ans: When bird perches on a live high power line, no current passes through the body of bird because there is no potential difference between live wire and the body of bird as the potential of bird’s body is the same as that of wire.
When a man standing on the ground touches the same wire then due to a large potential difference between his hands and feet, a large current flows through his body to the earth.
74) During lightning it is safest to be inside a car rather than under a tree or in the open. Give reason.
Ans: When a person is in the open or under the tree, the lightning passes through his body to the earth. On the other hand, for a person inside a car, the car provider shielding and the electric field inside the car is zero, thus lightning does not affect the person inside it. The lightning actually passes through the metallic body of the car to the earth without affecting the person sitting inside it.
75) Why is earthing desirable for electric appliances?
Ans: Earthing helps the current move into the earth in the event of short-circuit, without giving a shock to the user.
76) Why does a perspiring man feel relief when air flows by his side?
Ans: The air flowing by the side of a perspiring man quickens the pace of evaporation of perspiration from the body of that person, and the resultant loss of heat from his body causes the cooling sensation which provides relief to him.
77) Why is magnet always made of soft iron?
Ans: Magnet is always made of soft iron because ‘magnetisation’ and ‘demagnetisation’ both are possible in soft iron.
78) Why are telephone wires between two poles kept loose?
Ans: Telephone wires are kept loose between two poles so that they are protected from being broken due to expansion and contraction caused by the change of temperature in summers and winters.
79) X-rays penetrate through the flesh but not through bones. Why?
Ans: The penetrating power of X-rays depends upon the potential difference between the cathode and the anode of X-ray tube. The X-rays produced can penetrate through light element like flesh of human body but they are unable to penetrate through heavier elements like bones.
80) What is a black hole? Why is it called so?
Ans: A black hole is a super dense planetary material formed due to the death of a star of mass more than five solar masses.
It is called black hole because any particle or photon approaching its surface is just swallowed by it. It appears black, as radiation is neither emitted nor reflected by it.
81) Why do water pipes burst in severe cold winters?
Ans: Water freezes in pipes during winters, when the temperature goes below 0 degree Celsius (i.e., freezing point of water). It expands and in the process exerts pressure on the water pipes, thereby sometimes bursting them.
82) Most aircrafts and ships have their front shape pointed. Why?
Ans: The shape of many objects, viz. aeroplanes, rockets, ships, etc. moving through air or water, are designed in such a way that friction can be reduced between the objects and air/water. Such type of body is known as streamlined and the process is known as streamlining.
83) An athlete runs some distance before taking jump, why?
Ans: An athlete runs a certain distance before actually jumping in order to increase his speed, and thereby, his inertia of motion. This increased inertia of motion enables him to jump a longer distance.
84) It is difficult to drown in the Dead Sea. Why?
Ans: The water of Dead Sea has a salt content of 27%. Therefore, its density is much greater than that of ordinary sea water and it also offers a greater upthrust. Hence, the body weighs less than an equal volume of Dead Sea water and is thus, able to float.
85) Why aquatic animals have soft skeleton unlike those of the terrestrial animals?
Ans: The density of animals and fish living in water is almost the same as the density of water. Therefore, their weight is almost completely balanced by buoyancy. That is why they do not need massive skeleton like those of terrestrial animals.
86) Why does a swimming pool appear less deep than it really is?
Ans: When rays of light start from the bottom of a pool and travel from water to air, they are refracted away from the normal because they travel from a denser medium to a rarer medium. As a result, a virtual image of bottom is formed above the bottom. Hence, a swimming pool appears less deep than it really is.
87) Why is electrical wiring in parallel better?
Ans: electrical wiring in parallel is better because all the lines will have the same potential difference and if one line gets fused the other lines remain unaffected.
88) It is easier to swim in sea water than in river water. Explain.
Ans: The density of sea water is more than that of the river water. Therefore, the weight of sea water displaced by the swimmer is more. Thus, buoyant force of upthrust on the swimmer increases, making it easier to swim in sea water.
89) A cold compress is applied on the forehead of a person suffering from high fever. Why?
Ans: Evaporation causes cooling. As the water evaporates, it absorbs heat from the forehead and helps in reducing the temperature.
90) A man with a load his head jumps from a high building. What will be the load experienced by him.
Ans: Zero, because the acceleration of his fall is equal to the acceleration due to gravity of the earth.
91) Why is spring made of steel and not copper?
Ans: The elasticity of steel is greater than that of copper.
92) Why is it easier to spray water to which soap is added?
Ans: Addition of soap decreases the surface tension of water. The energy for spray is directly proportional to surface tension.
93) A piece of chalk when immersed in water, emits bubbles. Why?
Ans: Chalk consists of pores forming capillaries. When it is immersed in water, the water begins to rise in the capillaries and air present there is expelled in the form of bubbles.
94) Why does a liquid remain hot or cold for a long time inside a thermo flask?
Ans: Because of the presence of air, which is poor conductor of heat, in between the double glass walls of a thermos flask.
95) Why is the boiling point of sea water more than that of pure water?
Ans: Sea water contains salts and other impurities with different points which jointly rise its boiling point.
96) Why is it recommended to add salt in water while boiling grams?
Ans: By addition of salt the boiling point of water gets which helps in cooking.
97) Why is soft iron used as an electromagnet?
Ans: Because it remains magnetic only till the current passes through the coil and loses its magnetism when the current is switched off (principle of electric bells)
98) Why does ink leak out of a partially filled pen when taken to a high altitude?
Ans: As we go up the pressure and the density of air go on decreasing. Partially filled pen leaks when taken to a higher altitude because the pressure of air acting on the ink inside the tube of the pen is greater than the pressure of air outside.
99) Why does some liquids burn while others do not?
Ans: A liquid burns if its molecules can combine with oxygen of the air with the production of heat. Hence, oil burns but water does not.
100) Oil and water do not mix. Why?
Ans: i) Molecules of oil are not bigger than that of water and therefore do not mix easily.
ii) Molecules of water are polar, i.e. they have opposite charges at two ends whereas oil molecules do not; as a consequence they tend to stay away from water molecule.
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