Hello all! It’s been a busy holiday season full of travel, friends and family, interviews, and surprisingly little chemistry. As such, this blog got pushed to the backburner so to speak. But I’m back!
I’ve heard it’s been quite cold in the Northeast this last week. While I have been rocking a t-shirt in Southern California, temperatures in the Boston area have inched into the sub-zero range, with parts of the Midwest reaching record-breaking lows. Heavy snowfall has impeded holiday travel in much of the northern parts of the country, and iced-over roads have made driving difficult.
It’s that ice part we’re going to talk about this time. We skate on it, put it in our drinks, compress it into balls and throw it at each other, and curse Old Man Winter while we chip it off the windshields of our cars. It is a ubiquitous formation on our planet, so let’s take a few minutes to learn about its structure and properties.
Ice, Ice, and More Ice
Did you know there 17 different documented forms of ice? For you Vonnegut fans out there, Ice-Nine actually does exist; however, it (thankfully) won’t destroy all life on earth. That being said, the vast majority of the ice you encounter is Ice Ih (pronounced “ice one-h”). In fact, unless you’ve been to one of the poles where temperatures can reach -100⁰ C (-148⁰ F), that’s most likely the only ice you’ve ever seen. The subscript “h” stands for “hexagonal” and denotes how individual molecules of water bond together in a crystal lattice. It turns out that ice formation is very sensitive to temperature and pressure.
In addition to the typical hexagonal lattice structure, ice can also form cubic, rhombohedral, and tetragonal geometric configurations; ice can even freeze in a completely disordered manner, forming amorphous ice.
Why is Ice Slippery?
At face value, this seems like a question a toddler might ask, just after inquiring about “why the sky is blue.” But if the answer was as simple as “because it is,” I wouldn’t be talking about it here. Believe it or not, this topic has been the subject of scientific debate since the mid-nineteenth century. It has long been known that solid water (ice) is less dense than its liquid counterpart. Indeed, water is one of only half a dozen or so substances known to exhibit this property. It therefore follows that compressing ice will lower its freezing point, causing it to melt.
The Kelvin brothers (for whom the temperature scale is named) theoretically outlined and experimentally verified that water’s freezing point is dependent on external pressure. This offers one explanation as to why ice-skating works. The blade of an ice-skate is very thin, and exerts force on only a small area, resulting in very high pressure. In 1886, an engineer by the name John Joly worked out that the pressure an ice-skater would exert on the ice would be close to 500 times atmospheric pressure, enough to lower the freezing point of ice from 0⁰ C to -3.5⁰ C. This would allow regional melting of ice directly under the ice skater’s blade.
You may notice a problem here: how is it possible for people to ice skate in temperatures below -3.5⁰ C? And never mind ice skating, how is ice slippery when you’re walking on it? Your shoes certainly don’t generate enough pressure to melt ice. As it turns out, very, very cold ice isn’t slippery on its own. If you were to run your hand over it, the texture would be similar to sandpaper. In fact, there is a fair amount of friction between the bottom of your shoe or the tire on your car, and ice. This friction generates heat, just like when you rub your hands together for warmth, and the heat can be sufficient to melt a thin layer of ice.
We now have two explanatory models for why ice is slippery, but we remain with one problem. Ever tried standing on a patch of black ice? Even though you’re not moving, it’s still slippery. Since there’s no movement, there isn’t any heat generated by friction. So there must by a third model to explain this phenomena. In 2002, scientists at the Lawrence Berkeley National Lab demonstrated that a thin layer of liquid water exists on top of ice.
This layer persists down to temperatures of about -20⁰ C (-4 ⁰ F), at which point water molecules do not have sufficient energy to break out of their crystal lattice structure. See, water molecules are held together by electrostatic forces, similar to how a balloon will stick to your hair if you rub it against something. In the case of water, this force is called a hydrogen bond. When solid ice interacts with air, the molecules in air disrupt the formation of a crystal lattice, resulting in a thin film of liquid-like water.
This layer was measured to be approximately one hundred-millionth of an inch thick, 6 water molecules across. If the sole of your shoe were scaled to be as large as the tallest building on earth (Burj Khalifa, 2,722 ft) then the water layer would still be ten-thousand times thinner than a penny on the sidewalk.
So there we have it: seventeen different types of ice, condensed matter physics, pressure, friction, equilibrium, and intermolecular forces. All that to explain the chemistry of why ice is slippery. I hope you’ve found this post interesting and informative, and I’ll do my best to keep the chemistry coming.