Why Is Ice Slippery?
Ania (left), age 7: "I think it's slippery because it is water that is frozen and because it's hard and smooth and because it doesn't have anything for your feet to grab on to. At stores they put signs up when the floors are wet, so I think it's the water that makes it slippery."
What We Learned
Ania's right when she speculates that water is what makes ice slippery, just as it does a recently washed floor. The coolest thing about ice: The surface retains a slippery, liquidlike layer that acts as a lubricant, says Jerry Seidler, associate professor of physics at the University of Washington. The water molecules on the top layer of ice aren't able to form solid ice crystals, so they act more like a liquid than a solid.
This quasi fluid is even found on very cold ice, though it thins as the temperature drops. At 279 degrees below zero, it's just 1 molecule thick. Polar explorers say that pulling sleds across ice in arctic temperatures feels like dragging them through sand — evidence that the liquidlike layer is key to slickness.
Upping the ante, friction — of a speed skate rushing across an icy track, say — produces heat. And heat undeniably melts ice. Melted ice is ... water. In high-speed winter sports, says Seidler, friction plays a big role. The faster something moves against the ice, the more heat it generates. More heat equals more melting and even more slip-enhancing water.
Explain It to Your Kids
Remind your child that adding water makes a lot of things more slippery, like a water slide. Have her run her fingers over a dry countertop, then spritz the counter with water and see how much faster and easier they glide.
You can also have her rub her hands together really fast to feel the heat produced by friction. The same thing happens between the ice and her skate blade. The heat makes the ice melt a little bit, which means even more slipping and sliding (or gliding, if you're good).