Questions
1. Why does blotting paper absorb ink?
2. Why are lightning conductors fixed to tall buildings?
3. Why can petrol fire not be extinguished by pouring water over it?
4. Why is a small space left at the joint between the two rails?
5. Why are liquid drops spherical?
6. A glass tumbler is filled to the brim with water and a piece of ice is floating on it, As the ice melts, will the water overflow or not? Give reason for your answer.
7. When a moving train slows down quickly, will a passenger tend to fall backward or forward? Explain why?
8. Explain why it takes more time to cook meat and vegetables at hill stations?
9. When we drink soft drinks through a straw, why does the liquid go up into our mouth?
10. Explain why the moisture (water droplets) gathers on the outerside of a glass tumbler containing ice-cold water
11. Why does a metal seen colder in winter and hotter in summer
compared with a piece of wood?
12. A thick glass tumbler often cracks when a very hot liquid is poured in
it. Why?
13. How does a rainbow form? Explain the phenomenon.
14. What causes the rumbling sound of thunder?
15. Why does silver tarnish?
16. Why are mornings and evenings less warm than noon?
17. The sky appears blue. Give reason.
18. What is the difference between a planet and a star?
19. Why do stars twinkle?
20. What are chromosomes?
21. What is the difference between Supersonic and Ultrasonic sounds?
22. Why is it dangerous to have a charcoal fire burning in a badly ventilated room?
23. Copper wire cannot be used as a heating element in electric heaters. Why?
24. What do you know of the following: (a) Rocket (b) Computer
25. Why are flowers brightly coloured?
Write a short note on any three of the following
26. Dactylography and Dactyliology 27. Quantum theory
28. Radioactivity 29. Evolution theory
30. Green Revolution
Answers.
1. Blotting paper has fine pores which act like capillaries. When a portion of blotting paper is brought into contact with ink, due to surface tension (capillary action of liquids) ink enters the pores and is absorbed.
2. Lightning conductors are fixed to tall buildings to neutralise the electric charge of the cloud passing over the building before it can cause damage by suddenly finding a path to earth through the conductors.
3. Water, which is heavier than petrol, slips down permitting the petrol to rise to the surface and continue to burn. Besides, the existing temperature is so high that the water poured on the fire evaporates even before it can extinguish the fire.
4. To permit expansion of rails due to heat generated by friction of the moving train and in summer.
5. Liquid drops assume spherical shape on account of surface tension. Surface tension is the force acting on surface of a liquid, in which there is a force of tension due to intermolecular forces acting on the surface of the liquid. This tension makes the area of free surface as small as possible.
6. Level of water remains unchanged because the volume of water produced by the melting ice is exactly the same as that of the piece of ice. On melting the water will be equal to the volume of water displaced by the ice.
7. The passenger will tend to fall forward because the lower portion of his body which is in contact with the seat will come to rest quickly whereas the upper portion of the body continues to be in a state of motion. Hence, the person is thrown forward.
8. At higher altitudes, the atmospheric pressure is low as compared to that in the plains and, therefore water boils below 1000C. Hence sufficient heat is not supplied for cooking the meat and vegetables at hill stations.
9. When a person sucks air from the straw, the pressure of the air inside the straw is reduced as compared to the atmospheric pressure acting on the surface of the liquid. Therefore, the soft drink rushes up into the straw and to the mouth.
10. The water vapour present in air get cooled and appear as droplets of water on coming in contact with the cold surface of the glass tumbler.
11. Metals absorb and transmit heat more rapidly than wood.
12. The inner surface of the thick glass tumbler coming in contact with the hot liquid expands more in comparison to the outer surface which is relatively at a lower temperature. The uneven expansion of inner and outer surface may produce cracks.
13. After the rain some clouds continue to linger in the sky and they contain water droplets. Water droplets act like prisms. Sun’s rays falling on water droplets suffer dispersion and produce a spectrum. The different colours are viewed in the form of a rainbow.
14. Air is heated instantly when an electrical charge of lightning passes through it. The heat causes the molecules of air to expand in all directions. As the molecules seek more room, they collide violently with layers of cool air, and set up a great air wave that has the sound of thunder.
15. Silver is tarnished due to the presence of hydrogen sulphide in air, which forms black silver sulphide.
16. In the mornings and evenings the rays of the sun falling on earth are slanting and their distance is more. The earth gets heated up only slightly. At noon the rays of the sun falling on earth are nearly vertical and the distance is also less, with the result that the earth gets heated up considerably. Hence mornings and evenings are less warm than noon.
17. Violet and blue light have short waves and are scattered more than red light waves. While red light goes almost straight through the atmosphere, blue and violet are scattered by particles in the atmosphere. Thus we see a blue sky.
18. Stars are self-luminous celestial bodies and they have a system of their own. Planets, on the other hand, are bodies which revolve around a star and shine by the reflected light of the stars.
19. The light from the stars travels through different layers of space of varying densities. Therefore, the light rays deviate from its original path. Further, these layers are not stationary but keep on moving. This leads to the twinkling effect of the stars.
20. Chromosomes refers to rod-like or thread-shaped bodies, hundreds of which are found in the nucleus of every animal or plant cell carrying genetic materials.
21. Sounds of frequency higher than 20,000 Hz are known as ultrasonic and are inaudible. The speed greater than the speed of sound is referred to as supersonic.
22. The burning charcoal produces carbon monoxide which is a poisonous gas. If there is no escape for the gas it will fill up the room and suffocate us.
23. Copper has less melting point and if we use it in electric heater it cannot withstand high temperature.
24. (a) Rockets are projectiles driven by the reaction of gases produced by a fast burning fuel.
24. (b) Computer is a technical device designed to find instantaneous solutions of huge and complex calculations based on the information already fed.
25. The flowers contain pollen and intermixing of pollen is essential for reproduction of vegetation. Pollen is carried from one plant to another by bees and other insects which move from one flower to another. The beautiful colours tend to attract the bees and other insects towards the flowers.
26. Dactylography is the study of fingerprints for the purpose of identification.
Dactyliology is the technique of communication by signs made with the fingers. It is generally used by the deaf.
27. Quantum Theory states that changes of energy in atoms and molecules occur only in discrete quantities, each as an integral multiple of a fundamental quantity. The fundamental quantity is generally referred to as quantum.
28. Radioactivity is the phenomenon of spontaneous disintegration of unstable atomic nuclei to give more stable product nuclei. It is usually associated with the emission of alpha, beta and gamma rays.
29. Evolution is a naturally occurring process. Evolution is a slow, continuous and irreversible process of change. Complex forms of life have emerged from simpler forms over millions of years.
30. Green revolution refers to adoption of new methods of agriculture with high yielding hybrid seeds, fertilizers, pesticides and intensive cultivation, to increase agricultural production.
