Today, I spent a while in my front yard shoveling the foot or so (let’s use 3 decimeters…that sounds much cooler) snow out of my driveway. As I was LOLing at my neighbors who were all stuck in snow drifts because it is apparently against the law to plow my street, I started to think about how ridiculous snow is. One day you go outside and there’s a blanket of ice shards as far as the eye can see. It really is a bizarre phenomena. So, it seemed fitting to go on about how awesome ice actually is.
Thank Goodness for Hydrogen Bonding
The thing that makes ice specifically, and water in general, pretty awesome is the hydrogen bond. Regardless of how much science you’ve been exposed to in your life, everyone knows that water is H2O; two hydrogen molecules attached to a single oxygen molecule.
The whole idea of bonding in chemistry of the sharing of electrons. Hydrogen has one lone electron and an inferiority complex; it really wants to be like its buddy helium, which has two electrons. It wants this because, with two electrons, it will be essentially non-reactive (the outer shell of electrons will be full), and this is energetically favorable. Nature enjoys things that are energetically favorable. Likewise, oxygen, which has six eight electrons, really wants to be like its buddy neon because, like helium, neon’s outer shell of electrons is full, making it fairly non-reactive. So, when two hydrogen atoms and one oxygen atom get together, they strike a deal. Oxygen, the ring leader, says “Tell you what…if you two hang out close enough to me, I’ll let you ‘borrow’ a couple of my electrons, so you can be like helium, and in return, you’ll let me ‘borrow’ your electrons so I can be like neon…win-win for everyone!” Thus, we have water.
However, the shape of water is important. The fact that it is bent slightly makes the charge distribution just a little asymmetric and the molecule acts like something called a dipole. One the whole, the molecule as a whole is electrically neutral. But, since the positive and negative charges are separated a bit, the negative part can be attracted to a positive charge nearby and the positive part can be attracted to a negative charge nearby. This will cause the molecule to rotate into alignment with an electric field. Indeed, this is how your microwave works; as the microwaves pass through the object and the polarity of the wave switches back and forth from positive to negative, the water dipoles rotate back and forth, creating a kind of friction that heats the material.
If you put a bunch of water molecules together, they don’t just float around in any old way. The positive parts of each molecule attract the negative parts of other molecules. This action is known as hydrogen bonding. It is much weaker than the covalent bonding that holds the water together, but it is responsible for most of the awesome properties of water and ice. For instance, water is “wet” because of hydrogen bonding…hydrogen bonds are created between the water and the molecules of whatever it’s on, making it atomically “sticky” when you get it on your skin. If you’ve every played with mercury from a thermometer, you may have noticed that it doesn’t wet anything, it just rolls around. This is because there are no hydrogen bonds present.
Fantastic…but why does this make ice nifty? The molecules in liquid water have enough kinetic energy to constantly break the hydrogen bonds and move around. This is what makes water, well…water. It’s viscous because of hydrogen bonds, far more viscous than it would be without them. If you add heat, the molecules speed up, gaining kinetic energy, and eventually all hydrogen bonds break and the molecules fly apart…STEAM!!! In fact, one interesting fact is that if hydrogen bonding did not exist, liquid water would boil at around -90°C! Considering how important water is to life and its development, it’s a good thing this is not the case.
If you remove heat, the molecules loose energy and hydrogen bonding becomes a more dominant process. At the freezing point, the molecules no longer have enough energy to break the hydrogen bonds and “flow”. The water then crystallizes into ice.
The hydrogen bonding forces the molecules into layers of rippled hexagons; I added the little copper rods in the picture above to represent this bonding. This brings be to something waaaaaay cool.
Snowflakes
The hexagonal structure imposed in normal ice is what is primarily responsible for the hexagonal symmetry seen in snowflakes. A snowflake is an individual crystal of ice that forms when a small droplet of water in the atmosphere supercools around a nucleus, something like a dust particle or our ever-present smog. Once this nucleus forms, the surrounding vapor droplets freeze and aggregate onto the crystal. However, the hydrogen bonding imposes symmetry onto the deposition…so we get the familiar snowflake.
This image was taken of an ice crystal with a scanning electron microscope; the colors were added afterwards to increase contrast.
So, the thing that really makes water interesting is this hydrogen bonding. Without it, we wouldn’t have liquid water at the normal temperature of the Earth and we wouldn’t have bazillions of amazing snowflakes with unique shapes falling onto our tongues…and onto our walkways and roads…at 2 am…that we have to plow and shovel.
There is another interesting consequence to hydrogen bonding, though. As the water freezes and takes on this hexagonal structure, the overall effect is to push the molecules away from each other into the rigid pattern of ice. The result is that ice is less dense than liquid water and, as we all know, floats in water. This is actually quite remarkable; there are very few substances whose solid phase is less dense than it’s liquid. An example of a very not-water example of something else that has this property, check out gallium.
This fact has had profound impact on Earth. If ice was like every other solid and was more dense, then the freezing process would start from the bottom up. If this were the case, then the oceans, lakes, and streams of our planet would freeze from bottom up, destroying life. The hydrogen bonding in ice is what has made it possible for life evolve; without it, all of our water would have frozen long ago and life could never have developed in the oceans for billions of years. Erosion from freezing and thawing would also not exist, as water would not expand as it froze. Our landscape would look very different.
There’s More Than One Type of Ice
Several years ago, I got around to reading the novel “Cat’s Cradle” by Kurt Vonnegut. It revolves around the creating of this material known as ice-nine, which is a weapon of mass destruction that can turn all of the liquid water on Earth (and in your body) to ice at a much higher temperature than normal. It’s a pretty good read, albeit bizarre.
After getting through it, I was looking up ice-nine on the Interwebs to learn about it’s cultural references when I can across this line line at the top of its Wikipedia article which kind of blew my mind:
“This article is about the fictional material in Cat’s Cradle. For the metastable form of solid water, see Ice IX.”
The ice that we all know and love is actually only one possibility. It’s the form that we experience because the conditions of it’s formation, namely pressure and temperature, are common to Earth. However, it isn’t the only kind of water ice. In fact, there are more than a dozen forms of ice; “normal” ice is referred to as Ice 1h since is hexagonal. There are all sorts of crazy forms…and they have wacky properties.
Ice 1c, for instance, doesn’t have a hexagonal structure. Instead, it has the same cubic structure as diamond and forms in the upper atmosphere in small amounts.
Ice XI is probably the coolest (haha) form. It is likely the most common form of ice, because it is actually more stable than normal ice. However, it only forms below 72K, which is colder than liquid nitrogen. Despite this, we have found some in Antarctica under the right conditions, namely pressure deep below the surface. Crystalline water in space and on the surface of places like Jupiter and Saturn’s moons is most likely this form. What’s really interesting is that Ice XI is a type of material known as a ferroelectric. You’re all familiar with a ferromagnet, it is a material that can hold a permanent magnetic moment, which is responsible for permanent magnets. A ferroelectric can hold a permanent electric polarization. This means that the material as a whole would not be electrically neutral. Crystals of Ice XI floating in space would attach each other electrically, similar to the idea of static electricity, and would be “sticky”. It is thought that this contributed to holding together enough material to allow the formation of the icy bodies in the solar system, such as comets.
So, ice is pretty awesome. If it wasn’t for it’s specific properties, life and, perhaps, the entire planet, would not exist. We wouldn’t have comets and we wouldn’t have snowflakes (at least not the awesome ones we have now). I am happy, though, that the ice on Earth isn’t more exotic…it would suck even more to shovel out of my driveway.