Dei'ah veDibur - Information & Insight

A Window into the Chareidi World

7 Nisan 5765 - April 4, 2006 | Mordecai Plaut, director Published Weekly









Produced and housed by
Shema Yisrael Torah Network
Shema Yisrael Torah Network












Observations: Science Still Does Not Know Why Ice Is Slippery
by M. Plaut

Physicists still disagree over the answer to the seemingly simple question of why ice is slippery and how ice skaters can glide across it on their skates.

According to a recent article in the New York Times, ("Why is Ice Slippery?" By Kenneth Chang, February 21, 2006), the usual answer that everyone knows is wrong.

The standard — and wrong — explanation is based on an unusual property of water: that the solid form, ice, is less dense than the liquid form. That is why ice floats on water, while a cube of frozen alcohol — which has a freezing temperature of minus 173 degrees Fahrenheit — would plummet to the bottom of a glass of liquid alcohol.

The lower density of ice also means that the melting temperature of ice can be lowered below the usual 32 degrees by squeezing on it and thereby increasing its density.

Therefore, according to the incorrect explanation, the pressure exerted by the blade lowers the melting temperature of the top layer of ice, so the ice melts and the blade glides on a thin layer of water that refreezes as soon as the blade passes.

However ice, said Robert M. Rosenberg, an emeritus professor of chemistry at Lawrence University in Appleton, Wis., and a visiting scholar at Northwestern University, "is a very mysterious solid."

Dr. Rosenberg wrote an article about the slipperiness of ice in the December issue of Physics Today, among other reasons because it annoyed him that the wrong explanation is so widespread.

"People will still say that when you ask them," Dr. Rosenberg told the New York Times. "Textbooks are full of it."

The explanation fails because the pressure-melting effect is too small. A 150-pound person standing on ice wearing a pair of ice skates exerts a pressure of about only 50 pounds per square inch on the ice. (A typical blade edge is about one- eighth of an inch wide and about 12 inches long.) That amount of pressure lowers the melting temperature only a small amount, from 32 degrees to 31.97 degrees. Yet ice skaters can easily skate when the outside temperatures are much colder.

The pressure-melting explanation also fails to explain why someone wearing flat-bottom shoes, with a much greater surface area that exerts even less pressure on the ice, can also slip on ice.

Slipperiness is just one of the puzzles about ice. Besides everyday ice, there are about a dozen other forms, some of which experts suspect exist in the hot interior of Earth or on the surface of Pluto. Scientists expect to discover more variations.

Two alternative explanations have been proposed. One says that the rubbing of a skate blade or a shoe bottom over ice heats the ice and melts it, creating a slippery layer.

The other, which emerged a decade ago, suggests that the surface of ice is always a slippery liquid. The argument is that water molecules at the ice surface vibrate more because there are no molecules above them to help hold them in place. They thus remain an unfrozen liquid even at temperatures far below freezing.

Scientists debate whether friction or the liquid layer plays the more important role. Dr. Rosenberg, asked his opinion, answered: "I say there are two major reasons."

A liquid layer on the ice surface is not a new idea. It was first proposed by the physicist Michael Faraday in 1850 after a simple experiment: he pressed two cubes of ice against each other and they fused together. Faraday argued that the liquid layers froze solid when they were no longer at the surface. Because the layer is so thin, however, it is hard for scientists to see.

In recent years more sophisticated experiments have also produced results that suggest that there is a liquid on the surface of ice. In 1996, Gabor A. Somorjai, a scientist at Lawrence Berkeley Laboratory, bombarded the surface of ice with electrons and noted that their scattering pattern indicated a surface liquid at temperatures down to minus 235 degrees. More recently a team of German scientists found results that corroborated the Lawrence Berkeley findings.

But a colleague of Dr. Somorjai's at Lawrence Berkeley, Miquel Salmeron argues that the liquid layer, while it is there, is not enough to explain the slipperiness.

In 2002, Dr. Salmeron and colleagues performed an experiment that showed, according to him, that while the top layer of ice may be liquid, it is too thin to contribute much to slipperiness except near the melting temperature. In his view, friction is the primary reason ice is slippery.

Dr. Salmeron concedes, however, that he cannot definitively prove that his view is the correct one. "It's amazing," he said. "We're in 2006, and we're still talking about this thing."

In everyday ice, the molecules of water line up in a hexagonal pattern. That is why snowflakes all have six-sided patterns. Water — H2O — is two hydrogen atoms connected to a central oxygen atom in a V-shape. In everyday ice, which scientists call Ice Ih, the water molecules line up in hexagonal structures. This is why snowflakes all have six- sided patterns. (The "h" stands for hexagonal. A variation called Ice Ic, found in ice crystals floating high up in the atmosphere, forms cubic crystals.)

At high pressures, the hexagonal structure breaks down, and the bonds rearrange themselves in denser crystal structures, labeled with Roman numerals: Ice II, Ice III, Ice IV and so on.

At a pressure of about 30,000 pounds per square inch, Ice Ih turns into a different type of crystalline ice, Ice II. Ice II does not occur naturally on Earth. Even at the bottom of the thickest portions of the Antarctic ice cap, the weight of three miles of ice pushes down at only one-quarter of the pressure necessary to make Ice II. But planetary scientists expect that Ice II, and possibly some other variations, like Ice VI, exist inside icier bodies in the outer solar system, like the Jupiter moons Ganymede and Callisto.

Some scientists calculated that the pressure of 700,000 pounds per square inch found in rocks 100 miles beneath the surface of the Earth could be great enough to transform any water there into a solid phase known as Ice VII. But no one knows whether ice can be found inside Earth.

The most recently discovered form of ice, Ice XII, was found just a decade ago, although hints of it had been seen years earlier. John L. Finney of University College London, one of the discoverers of Ice XII, said that trying to understand all the different forms of ice was important for an understanding of how the water molecule works, and that was important in understanding how water interacts with all the biological molecules in living organisms.


All material on this site is copyrighted and its use is restricted.
Click here for conditions of use.