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How Come?: Why is ice so slippery?

How come ice is so slippery? asks Rebecca Anderson, a student in Brookville, NY.

If you've ever careened into the sink on your way across the kitchen in the morning, you know a patch of spilled water is slippery. On a dry floor, the friction between your shoes and the floor's surface keeps you from sliding. But add water, and you glide on a liquid film, reducing your soles' contact with the floor's snaggy imperfections.

However, ice is a solid, like the floor. So what would make it slippery? One popular explanation is that the slipperiness is created by the pressure of an object (like a shoe or skate blade) on the ice.

How would that work? Water is, well, weird. Most substances shrink as they change from liquid to solid, their molecules packing more densely together. Chill water, and it also does a slow shrink as its molecules move less and less. But at 39 degrees, a parcel of water reverses course, its volume actually increasing as it cools further.

When water freezes solid, at 32 degrees, it expands dramatically. That's because water's hydrogen and oxygen molecules are locked in a crystal lattice, a kind of honeycomb-like structure with lots of empty space. This extra space is the reason a parcel of water swells by about 9 percent when it's frozen solid.

Since water molecules in solid ice are actually further apart than in liquid water, they are especially susceptible to pressure. Put the surface of ice under extra pressure, squeezing its molecules together, and it will melt at a lower temperature - less than 32 degrees.

So some reasoned that the simple pressure of shoe soles or ice skate blades causes the surface of ice to melt a little, even when the temperature is below freezing. The resulting film of liquid reduces friction, creating a slippery expanse.

However, even a thin skate blade, which exerts more pressure than a broad shoe bottom, only lowers the melting temperature by a fraction of a degree below 32 degrees. And as we all know, we can slip and fall on the ice when it's a frigid 20 degrees. So what's really going on?

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No one knows the whole answer, but in the late 1990s, scientists in the United States and Germany, doing experiments with small particles, discovered that ice apparently has a built-in layer of surface liquid, even at temperatures below minus 200 degrees. The possible explanation? Water molecules on the surface aren't bonded as strongly as those locked together in the icy depths. So surface molecules jiggle and move more than those below, creating a very thin liquid layer on the ice, even at extremely cold temperatures. While this liquid-y layer is extremely thin, friction can increase it. Move across ice on shoes or skates, and friction with the solid ice creates heat, accelerating the surface melting. Until scientists know more, the combination of a semiliquid surface and friction may explain why we go slip-sliding away, even on the coldest winter days.

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