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Why Is Water Slippery?

The questions that kids ask about science aren’t always easy to answer. Sometimes, their little brains can lead to big places that adults forget to explore. That is what inspired our series Science Question From A Toddler, which uses kids’ curiosity as a jumping-off point to investigate the scientific wonders that adults don’t even think to ask about. The answers are for adults, but they wouldn’t be possible without the wonder that only a child can bring. I want the toddlers in your life to be a part of it! Send me their science questions, and they may serve as the inspiration for a column. And now, our toddler …

Why is water slippery?” — Oliver B., age 4

Car tires skid on a rainy highway. Your foot slides out from under you on a wet kitchen floor. Sweaty hands lose their grip on the monkey bars at the park. Water is to blame — but we’re really only surprised by a slip because of how much friction there is in the world. Left to their own devices, objects naturally resist moving against one another. At a microscopic level, everything — from hands to highways — is a mountain range. But as strange as that is, water, one of the weirdest substances on Earth, is stranger still — it’s able to make smooth operators out of a rough world.

“When I think about friction, I think about taking the Rocky Mountains and smushing them on top of the Himalayas and dragging them across each other,” said Jim Kakalios, professor of physics at the University of Minnesota. That’s what rubbing your hands together would look like under a microscope. Even surfaces that appear smooth to the naked eye have hidden crags that snag and slow movement. Add water to Kakalios’s analogy, though, and you create a barrier — a buffer between the peaks, a filler stuffed into the valleys — that cancels out the harsh topography. Water creates a smoothness that wasn’t there before, allowing crashes and pratfalls to happen and thwarting recess gymnastics routines.

Even the slipperiness of ice is related to a thin film of liquid water on the surface, smoothing out the interaction between your feet and a frozen puddle, said Philip Ball, science writer and author of “H2O: A Biography of Water.” The idea that ice is slippery because of water on its surface had been around for a long time but wasn’t convincingly proved until 1987. And while that may seem like a very long time ago to a 4-year-old, I assure you that this date is recent enough that many grown-ups are still coming to terms with the idea that the current year is not 1988. Turns out, water is a deeply strange substance, and scientists say there are still things left to learn about the molecule that covers 70 percent of planet Earth.

How weird is water? Unlike most liquids, it is densest not at its freezing point, but at just a few degrees warmer — a unique trick that allows lakes to form a lid of ice, rather than freezing solid, Kakalios said. Water is safe for us to drink, but also so chemically reactive that it can’t be used to lubricate things like engines because of the damage it will cause inside the machine. “We’re used to [water], but if you lived in an alien world where it was a novelty, you’d handle it with care,” said Chris Shepherd, teacher support manager for the Institute of Physics, a U.K.-based society of physicists and publisher of academic research. Ball said that it’s even weird that water is liquid at all, considering that when the other elements most similar to oxygen link up with hydrogen what they form is a gas. Carbon plus hydrogen makes methane, for instance. Nitrogen and hydrogen make ammonia.

A lot of this weirdness has to do with the chemical bonds that hold atoms of hydrogen and oxygen together as H2O. Those connections are 10 times weaker than a typical chemical bond, Ball said. But each molecule of water can form four of these weak bonds at once, giving it the ability to form structures that no other liquid can. “That means that when the water molecules are linked, they form a three-dimensional network,” Ball said. “Because water does that, it makes it hard to talk about the structure at a molecular level. It’s almost like one gigantic molecule that is loose and floppy, forever making and breaking bonds. You have to talk about it as a dynamic thing that is constantly changing.”

The constant change and weirdness means there are scientific mysteries about water that don’t have an answer yet, Ball said. Among them: There’s theoretical evidence that liquid water might actually come in two different forms — one of which is denser than the other. Ball described the difference at the level of molecular bonds. In the denser form of water, the molecules of H2O would be snuggled up, holding onto each other in a hug. In the less-dense form of water, in contrast, the water molecules would be holding hands, but not getting cuddly. A glass of the denser water would weigh more than an equally full glass of the less-dense water.

Right now, these two forms of water exist only in computer models. There’s evidence to suggest that they could be real, but not necessarily evidence that they do. Proving it would require setting up an experiment in which water is cooled past the freezing point but without being turned into ice. This unlikely sounding scenario is possible, Ball said, because ice has to have something to form crystals around, what Ball called a nucleation site. That can be an impurity in the water or a spot where the still water is suddenly jostled, disrupting the molecules and giving them an opportunity to lock up into the structure of ice. You can find videos on YouTube of people who have “supercooled” water, turning it to ice instantly by pouring or bumping it. But, Ball said, we don’t yet have the technological ability to do this to the degree necessary to test the two-types-of-water theory.

If it is real, though, it could help explain some of the weirdness of water — including why that slippery surface of ice can form on the top of lakes and puddles, rather than their being frozen through. That, and other anomalies, could just be echoes of water’s secret dual life as two liquids in one. It’s slippery like that.

Maggie Koerth-Baker is a senior science writer for FiveThirtyEight.