2016SHSAT_English

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