H2O: A Water Odyssey
Science (Is) All Around You.
Describe the weirdest substance that first comes to mind. Is it a hazy gas, viscous gel, or impenetrable solid? Is the molecule a sprawling chain of atoms? What purposes does it fulfill? Is edible, medicinal, or used in construction? There are plenty of molecules that would meet any of the above qualifications. Take, for example, Silly Putty. If you slowly pull the ends, you can stretch it out for a few feet before it breaks. If you give it a yank, however, you end up with two small pieces. Or consider spandex: Its basic units make for a long chain, yet fabric made of the fibers is clingy and flexible. These molecules certainly are odd, but the title of “Weirdest Substance” goes to a three-atom (triatomic) molecule: water.
The Many States of Water
Yes, water is weird. Most of us don't give it much thought due to the ubiquity of the stuff. I will explain.
First, consider the three basic phases of matter: solid, liquid, and gas. Which substances you encounter are what phase? Air is a gas. If it is cooled enough, it can become a liquid. It can be frozen if cooled even more, but most of us have never experienced it, and the conditions required to make it are extreme. On the other end of the spectrum is a rock. It is hard, heavy, and dense. It can be liquefied, but liquid rock is found in volcanoes, where it goes by either magma or lava. I do not know where to look for gaseous rock, if it exists.
Water, on the other hand, exists in all three states on Earth with ease. Shoot, all three states often exist together in a single house, from the ice cubes in the freezer, to the water dispensed from the faucet, to the steam emitted by the kettle. This is due, in part, to the fact that the freezing and boiling points of water (known as its fiducial points) are a moderate distance apart. Water freezes at 32 degrees Fahrenheit (F) and boils at 212, so the difference is 180 degrees.
Not only the range matters, but also where the fiducial points are relative to the temperatures we experience. Ammonia, a slightly lighter molecule, melts at -108 degrees F and boils at -28 degrees, for a difference of 80 degrees. We, however, rarely experience cold enough temperatures that it would even condense. Helium is even worse; it melts at 458 degrees F and boils at -452 degrees. The fiducial points of water, on the other hand, are such that they are easily reached by the average person either by external conditions or using basic equipment, such as a fire or hot plate. Liquid water is abundant on the planet; liquid helium was not produced until 1908.
An Everlasting Bond
Water is liquid at most temperatures we experience. That is quite the feat for such a dinky molecule. Consider a slightly lighter weight molecule, ammonia, NH3. It boils at a balmy -27.4 degrees F. Even a heavier molecule, propane, C3H8, boils at a lower temperature, -43.8 degrees F. Why? The answer can be found in the special type of intermolecular bond (called a hydrogen bond).
The oxygen atom in water has two pairs of electrons on it in addition to bonds with the two hydrogen atoms. These electron pairs give the oxygen a partial negative charge, while the hydrogens have a partial positive charge. The difference in charge induces a weak bonding between the oxygen and the hydrogens of up to four other water molecules. The interaction between molecules is weak, but on the scale in which we live, the effect is noticeable. Ammonia also can undergo the same type of bonding, but the interaction is weaker because the nitrogen has only one electron pair but three hydrogens. So the packing is not as tight because not all of the atoms are contained in one plane, like they are with water. Propane cannot undergo this type of bonding at all because it does not have either oxygen or nitrogen.
Consider what happens when you put solid water in liquid water; the solid water floats. Yes, this is odd. In most substances, the solid is denser than the liquid; if this were the case with water, ice cubes would sink in your beverage. This doesn't happen because of what occurs to water molecules when it freezes. In the most common form, ice is formed from hexagonal packing of water molecules. As water cools, the bonds between adjacent water molecules are locked into an orderly configuration. This configuration pushes water molecules from each other, so that a given volume of space can contain fewer water molecules.
The end result is that ice is about 8.3 percent less dense than water (0.917 g/cm3 versus 1.000 g/cm3). The density of ice can increase with further cooling; at -180 degrees F, the density is 0.934 g/cm3. If that isn’t crazy enough, think about this: Ice has fifteen different variations depending on temperature and pressure.
Some foods are 90 percent water. Our world is covered by about 70 percent water. We humans are about 50 to 60 percent water. When we think about the water, we usually consider what is in it and whether it is beneficial or harmful. Water in itself, however, has plenty of properties that make it interesting. It defies our expectations for usual chemical behavior; its solid form floats on its liquid form. Take some time to appreciate the neat, yet beneficial and offbeat, behavior of water.
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