ReadingQUESTIONS 21-50DIRECTIONS: Read each passage below and answer the questions following it. Base your answerson information contained only in the passage. You may reread a passage if you need to. Mark thebest answer for each question.5101520253035If you have ever watched someone fall onthe ice, you’ve seen slipperiness at work.But have you wondered what makes iceslippery, or why skates or skis glide acrossice so easily? The answer might seemobvious: ice is smooth. Yet smoothness initself does not explain slipperiness. Imagine,for example, skating on a smooth surface ofglass or sheet metal.Surprisingly, scientists do not fully understandwhy ice is slippery. Past explanationsof slipperiness have focused on friction andpressure. According to the friction theory,a skate blade rubs across the ice, causingfriction. The friction produces heat,melting the ice and creating a slippery,microscopically thin layer of water for theskate to glide on. The friction theory,however, cannot explain why ice is slipperyeven when someone stands completelymotionless, creating no friction.The pressure theory claims that pressurefrom a skate blade melts the ice surface,creating a slippery layer of water. Thewater refreezes when the pressure is lifted.Science textbooks typically cite thisexplanation, but many scientists disagree,claiming that the pressure effect is notgreat enough to melt the ice. Nor can thepressure theory explain why someone wearingflat-bottomed shoes—which have agreater surface area than skate blades andthus exert less pressure per square inch—can glide across the ice or even gosprawling.During the 1990s, another theory foundacceptance: the thin top layer of ice isliquid, or “liquid-like,” regardless of friction51015202530354045505560657075or pressure. This notion was first proposedmore than 150 years ago by physicistMichael Faraday. Faraday’s simple experimentillustrates this property: two ice cubesheld against each other will fuse together.This happens, Faraday explained, becauseliquid on the cubes’ surfaces froze solidwhen the surfaces made contact.Faraday’s hypothesis was overlooked, inpart because scientists did not have themeans to detect molecular structures.However, technological advances duringrecent decades allow scientists to measurethe thin layer on the surface of the ice. Forexample, in 1996, a chemist at LawrenceBerkeley Laboratory shot electrons at anice surface and recorded how theyrebounded. The data suggested that the icesurface remained “liquid-like,” even at temperaturesfar below freezing. Scientistsspeculate that water molecules on the icesurface are always in motion because thereis nothing above them to hold them inplace. The vibration creates a slipperylayer of molecules. According to this interpretationof the Lawrence BerkeleyLaboratory experiments, the moleculesmove only up and down; if they also movedside to side, they would constitute a trueliquid. Thus it could be said that people areskating on wildly vibrating molecules!The phenomenon of a slippery liquid-likesurface is not limited to ice, although ice isthe most common example. Lead crystalsand even diamond crystals, made of carbon,also show this property under certain temperatureand pressure conditions.4045505560657075FORM A40CONTINUE ON TO THE NEXT PAGE
21. Which of the following best tells what thispassage is about? A. theories about how people learn to skateB. how ice changes from a solid to a liquidC. answers to the question of what makes iceslipperyD. the discoveries of Michael FaradayE. the processes of freezing and melting22. What is the most likely reason that the authormentioned lead and diamond crystals in thelast paragraph? F. to point out that solids other than ice haveslippery surfacesG. to suggest that ice, lead, and diamonds arecomposed of the same materialsH. to cast doubt on Faraday’s theory ofslipperinessJ. to suggest that scientists shoot electronsat lead and diamond surfacesK. to suggest new uses for slipperysubstances23. According to Faraday, why do two ice cubesfuse when held together? A. Friction causes the ice to melt andrefreeze.B. The warmer ice cube melts the colder icecube.C. The liquid layers on their surfaces freeze.D. The vibrations of the molecules on theirsurfaces increase.E. Their surface areas are perfectly smooth.25. According to researchers at the LawrenceBerkeley Laboratory, why is the surface ofice “liquid-like” rather than “liquid”?A. because electrons rebound from the icesurfaceB. because the molecules vibrate only upand downC. because the ice surface is wetD. because the ice surface is slipperier thana liquid surfaceE. because the ice surface is frozen solid26. According to the passage, which of thefollowing undermines the friction theoryof slipperiness? F. a person wearing flat-bottomed shoesgliding across the iceG. two ice cubes fused togetherH. electrons bouncing off an ice surfaceJ. a person trying to skate on a sheet of glassor sheet metalK. a person slipping while standing immobileon iceCONTINUE ON TO THE NEXT PAGE24. What is the most likely reason that the authormentioned the 1996 experiment at LawrenceBerkeley Laboratory? F. to provide evidence about the surfaceof iceG. to illustrate the weaknesses of scientifictechnologyH. to show how Faraday tested his theoryJ. to suggest that the ice surface was solid,not liquidK. to explain why ice cubes freeze togetherFORM A41
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