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Chill skills: find out how ice's properties give skaters the edge during the Winter Olympic Games.

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This February, figure skater Rachael Flatt hopes she'll be showing off her cool moves at the 2010 Winter Olympic Games in Vancouver, Canada. "I'm going to work really hard to achieve that goal," says Rachael, one of the highest-ranked skaters in the world. When she takes to the ice, her skates' thin blades glide easily over the slippery surface. And whether the 17-year-old athlete is landing a triple flip or preparing for a spin, ice makes her winning tricks possible.

But how? Read on to find out.

Chill Factor

Just like the ice cubes that cool your lemonade, the slab of ice beneath Rachael's skates is made up of water molecules. The two hydrogen atoms and single oxygen atom that make up a water molecule share electrons. "But the sharing isn't equal," says Paul Doherty, a physicist at the Exploratorium in California. One side of the water molecule ends up with a negative charge and the other side with a positive one. This causes water molecules to be attracted to one another.

When temperatures are above 0[degrees]C (32[degrees]F), water molecules jiggle and pull these links apart. The result? Liquid water.

But as temperatures dip, the jiggling motion slows down. Then, the water molecules can give in to their attraction and pack together tightly. Liquid water turns into solid ice.

That's why the ice Rachael glides over is kept at a teeth-chattering -3[degrees]C (27[degrees]F). That way, Rachael can show off her jumps and spins without landing in a giant pool of water.

Slip-N-Slide

Good thing for Rachael that ice is not only solid, but slippery too. Skaters would be met with an ear-splitting screech if they tried their stunts on other solids like glass or concrete.

Why is ice slippery? Scientists are still puzzling over this question. But physicists know one thing: The answer given in many school textbooks is wrong.

Scientists used to think that a skater's weight put pressure on the ice, causing it to melt at a lower temperature. But experiments showed that this pressure change is too small to make ice melt.

Now, many scientists think a different force holds the slippery clue: friction. When Rachael pumps her legs, her skates' blades rub against the ice. That rubbing creates friction, which releases heat. With each leg stroke, the heat warms up the ice beneath the skater's feet. When the ice's temperature climbs above freezing, a thin layer on the surface melts into liquid water (see Why Ice Is Slippery, page 14). The water acts like grease under Rachael's skates to allow for speedy spins, graceful glides, and smooth landings.

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Liquid Layer

Rachael can never be too careful on the ice rink. Whether she is landing a triple axel or making her final pose, there's a chance of slipping. Even a person who is simply standing on ice can slip. This led scientists to think that there has to be more to ice's slipperiness than friction alone.

Scientists have found that the surface of ice is naturally slick. Normally, the water molecules are lined up like cars in a parking garage, with neatly ordered vehicles on each floor. But like the cars on the top floor of the parking garage, the molecules in the top layer of ice don't have any molecules above them.

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"[Molecules in the top layer] are a bit fleer to move about," Doherty says. These molecules behave more like a liquid than a solid. "The surface is like water and allows the blades to slide," he says.

That's a good thing for Rachael, who hopes to glide across the slippery ice to win a medal in the Olympic Games.

Words to Know

Molecule--A group of two or more atoms that are joined together by sharing electrons in a chemical bond.

Electron--The particle in an atom that has a negative charge.

Pressure--A force that is applied against a gas, liquid, or solid.

Friction--A force that acts against motion when two surfaces rub against each other.

WHY ICE IS SLIPPERY

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MELTING BY FRICTION: A fast-moving blade creates friction on the ice. This force causes heat, which melts the ice crystals into a thin layer of water under the skate. But this reasoning isn't the whole story. It doesn't explain why even a person standing still on ice can slip.

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BUILT-IN SLIPPERY LAYER:

A liquid-like film exists on the surface of ice. Chains of water molecules along the top layer are exposed to air and are unable to form solid ice crystals. It's easy to slip on these molecular chains, which vibrate like liquid water molecules.

quick quiz

1. When the temperature rises above 0[degrees]C (32[degrees]F), ice changes from a to a.

(A) gas, solid

(B) liquid, solid

(C) liquid, gas

(D) solid, liquid

2. A water molecule is made of.

(A) two hydrogen atoms and an oxygen atom

(B) two oxygen atoms and a hydrogen atom

(C) electrons

(D) ice particles

3. Which explanation for ice's slipperiness is widely cited but untrue?

(A) Friction from the skate's blades melts ice.

(B) The skater puts pressure on the ice, causing it to melt.

(C) Ice is naturally slippery because of the top layer's structure.

(D) None of the above

ANSWERS

1. d 2. a 3. b

BEFORE READING

Set a Purpose

Learn why ice is slippery and how the characteristics of this common solid make sports like ice skating possible.

Background

* If you think all ice is the same, think again. Water ice can occur as up to 15 known crystalline phases of water. Everyday snow and ice fall known as hexagonal ice, the most common variety. Other types can occur under extreme pressure and temperature combinations, either here on Earth or on other planets.

* Hexagonal ice gets its name due to the fact that the water molecules line up in a hexagonal, or six-sided, pattern. That is why all snowflakes have six sides.

Discussion Question

* Have you ever slipped on ice? Can you reduce your chances of slipping by wearing certain kinds of shoes? How do you think this works? (Answers will vary, but should include that boots with ridges for traction would reduce one's chance of slipping on ice because the ridges increase friction, keeping the shoe from slipping out from under you.)

AFTER READING

Discussion Question

* For a tong time, scientists believed in the wrong explanation to the question, "Why is ice slippery?" What are some other examples of scientists finding new theories to replace incorrect beliefs? (Possible answers: Scientists used to believe the sun revolved around the Earth until the discovery that it is the other way around; scientists used to believe that heavier objects fell faster than light ones until it was proved that gravity accelerates all objects at the same rate.)

RESOURCE

* Ice and People, by Nikki Bundey (Carolrhoda Books, 2000). This book reviews what ice is and how humans and animals cope with ice and cold.

Directions: Match the word(s) in the left column with the correct phrase in the right column.
1. Pressure   a. The particle in an atom that has a negative charge.

2. Molecule   b. A force that acts against motion when two surface
                 rub against each other.

3. Electron   c. A group of two or more atoms that are joined
                 together by sharing electrons in a chemical bond.

4. Friction   d. A force that is applied against a gas, liquid, or
                 solid.


ANSWERS

1. d 2. c 3. a 4. b
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Article Details
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Title Annotation:physical science
Author:Bryner, Jeanna
Publication:SuperScience
Article Type:Cover story
Geographic Code:1USA
Date:Nov 1, 2009
Words:1260
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