Read passage and answer questions If you have ever watched someone fall on the ice, you’ve seen slipperiness at
work. But have you wondered what makes ice slippery, or why skates or skis glide
across ice so easily? The answer might seem obvious: ice is smooth. Yet
smoothness in itself does not explain slipperiness. Imagine, for example, skating
on a smooth surface of glass or sheet metal.
Surprisingly, scientists do not fully understand why ice is slippery. Past
explanations of slipperiness have focused on friction and pressure. According to
the friction theory, a skate blade rubs across the ice, causing friction. The friction
produces heat, melting the ice and creating a slippery, microscopically thin layer
of water for the skate to glide on. The friction theory, however, cannot explain
why ice is slippery even when someone stands completely motionless, creating no
friction.
The pressure theory claims that pressure from a skate blade melts the ice
surface, creating a slippery layer of water. The water refreezes when the pressure
is lifted. Science textbooks typically cite this explanation, but many scientists
disagree, claiming that the pressure effect is not great enough to melt the ice. Nor
can the pressure theory explain why someone wearing flat-bottomed shoes—
which have a greater surface area than skate blades and thus exert less pressure
per square inch— can glide across the ice or even go sprawling.
During the 1990s, another theory found acceptance: the thin top layer of ice is
liquid, or “liquid-like,” regardless of friction or pressure. This notion was first
proposed more than 150 years ago by physicist Michael Faraday. Faraday’s simple
experiment illustrates this property: two ice cubes held against each other will

fuse together. This happens, Faraday explained, because liquid on the cubes’
surfaces froze solid when the surfaces made contact.
Faraday’s hypothesis was overlooked, in part because scientists did not have the
means to detect molecular structures.
However, technological advances during recent decades allow scientists to
measure the thin layer on the surface of the ice.
For example, in 1996, a chemist at Lawrence Berkeley Laboratory shot electrons
at an ice surface and recorded how they rebounded. The data suggested that the
ice surface remained “liquid-like,” even at temperatures far below freezing.
Scientists speculate that water molecules on the ice surface are always in motion
because there is nothing above them to hold them in place. The vibration creates
a slippery layer of molecules. According to this interpretation of the Lawrence
Berkeley Laboratory experiments, the molecules move only up and down; if they
also moved side to side, they would constitute a true liquid. Thus it could be said
that people are skating on wildly vibrating molecules!
The phenomenon of a slippery liquid-like surface is not limited to ice, although ice
is the most common example. Lead crystals and even diamond crystals, made of
carbon, also show this property under certain temperature and pressure
conditions.

QUESTION

1. Which of the following best tells what this passage is about?
A. theories about how people learn to skate
B. how ice changes from a solid to a liquid
C. answers to the question of what makes ice slippery
D. the discoveries of Michael Faraday
E. the processes of freezing and melting

2. What is the most likely reason that the author mentioned lead
and diamond crystals in the last paragraph?
F. to point out that solids other than ice have slippery surfaces
G. to suggest that ice, lead, and diamonds are composed of the
same materials
H. to cast doubt on Faraday’s theory of slipperiness
J. to suggest that scientists shoot electrons at lead and diamond
surfaces
K. to suggest new uses for slippery substances

3. According to researchers at the Lawrence Berkeley Laboratory,
why is the surface of ice “liquid-like” rather than “liquid”?
A. because electrons rebound from the ice surface
B. because the molecules vibrate only up and down
C. because the ice surface is wet
D. because the ice surface is slipperier than a liquid surface
E. because the ice surface is frozen solid

4. According to the passage, which of the following undermines
the friction theory of slipperiness?
F. a person wearing flat-bottomed shoes gliding across the ice
G. two ice cubes fused together
H. electrons bouncing off an ice surface
J. a person trying to skate on a sheet of glass or sheet metal
K. a person slipping while standing immobile on ice

5. According to Faraday, why do two ice cubes fuse when held
together?
A. Friction causes the ice to melt and refreeze.
B. The warmer ice cube melts the colder ice cube.
C. The liquid layers on their surfaces freeze.
D. The vibrations of the molecules on their surfaces increase.
E. Their surface areas are perfectly smooth.