If someone were to stop you on the street and ask you to name, on the spot, a naturally occurring substance that epitomized ordinary—something entirely lacking in strangeness—there is high probability that you would eventually say “water.” You might note, as an aside, that it is a very important ordinary substance, essential to life and refreshing to the parched, but nothing about water strikes you as being particularly odd.
But you would be wrong. I mean, really, really wrong. The behavior of water, at least when compared to other natural materials, is a bit bizarre. In particular, there are at least five different properties of water that, if you were stumbling upon it for the first time, might strike you as strange.
Take, for instance, those ice cubes blissfully floating at the top of your iced tea. We think of this as quite normal until we test virtually every other substance known to man and find that a solid cube of copper or wax or rock each sinks straight to the bottom when dropped into a tub of its liquid form. In normal substances, the atoms of a cooling liquid tuck in closer together when solidifying into a solid. More atoms in a smaller space makes the solid denser than the liquid, such that solid forms sink. Unless you are water, of course, in which case the unique arrangement of molecules in ice actually expands the volume compared to the liquid form, and ice floats.
Then there is that transparency thing. We take it for granted that light passes through water, but what other naturally occurring substance can be collected into a fish tank and provide a clear view of everything inside? There are substances that are indeed transparent when isolated and melted, but we rarely find them in pure form in nature. But that is only the start of water’s weirdness. As it turns out, water is opaque (meaning light does not pass through) to a wide range of electromagnetic waves—except for a tiny window in the visible light range. Water absorbs short wavelengths like ultraviolet light and longer wavelengths like microwaves, but allows red, yellow, green, and blue light to pass right on through.
Oh, did I mention that water is a liquid at average Earth surface temperatures? That may seem like a pretty obtuse observation until we start to think about what else is liquid in the same temperature range. There are substances like mercury, but it is not found in large quantities, and it’s quite toxic. The planet does have a fair amount of liquid oil, but interestingly, oil would not exist if water had not been around long ago for oil-generating critters to frolic in. Virtually every substance we encounter on Earth’s surface is either solid, like rocks and metals, or gas, like oxygen and carbon dioxide.
There is also an odd property of water that gives rise to the old adage that “a watched pot never boils.” Though referring to our own impatience, the saying derives from the fact that it really does take an unusual amount of time and energy to get water up to a certain temperature. Imagine a home experiment where you are given a small pot of water and a block of copper of the same mass. Set them both on the same burner of your stove, turn up the heat, and place one finger in the water and another on the copper. Then see how long it takes before each is too hot to touch. It would be natural to think that with the same mass and the same heat input, it would take the same amount of time for each to get hot. But that is not how nature works. Your finger will lift off the copper much sooner. In fact, you will likely still be bathing your finger in tepid water when twice the time has passed since the copper became too hot to touch. The heat capacity of water—the heat energy required to raise the temperature of a mass by one degree—is two to three times higher than that of most other common substances.
Finally, there is the shape of the water molecule. With one oxygen in the middle and a hydrogen on either side, it would be reasonable to expect it to be linear, with oxygen sitting along a straight line between the two hydrogen atoms. Carbon dioxide (CO2) is quite well behaved in this sense, forming the expected straight line between the two oxygen atoms and one carbon. Not so for water! The water molecule is bent, forming an angle of 104.5 degrees. Oxygen, being the playground bully that it is, pulls electrons away from the weaker hydrogen atoms. Because the molecule is bent, the imbalance of electrons on the oxygen side produces a polarity, where one side has a negative charge (the oxygen side) and the other has a positive charge (the hydrogen side). If the molecule was not bent, oxygen would still tug on those electrons, but the resulting positive charge in the hydrogens would be equally balanced on opposite ends and water would be non-polar.
All this would be a bit esoteric if it weren’t for one additional observation. Water isn’t just weird. It is weird in very peculiar ways that make life as we know it possible. Let’s work back through those strange behaviors.
Solid water is less dense than liquid water. Because ice floats, sinking blocks of ice do not choke shallow lakes or clog up rivers. In fact, the ice cover on a northern lake insulates the underlying water when the temperature plunges, limiting the thickness of ice formation and allowing complex organisms to survive the winter.
Transparent only to visible light. It is not just odd that water is transparent only for a narrow window of electromagnetic waves. It is amazing. On one side of the transparency window, water absorbs high energy ultraviolet waves that cause damage to living tissues. On the other side, water absorbs microwaves, turning it into the heat that gives a microwave oven its purpose. In between? Imagine a world where light is not transmitted through water. Aquatic plant life requiring light to conduct photosynthesis would not exist. Scavengers and predators could still hang out in sightless form, finding their way around by feel or by chemical or electrical signals, but there would be no purpose for eyes, nor for the brilliant array of underwater colors and patterns. Still more remarkable, the transparency window just happens to match the peak energy output of the sun!
Liquid at Earth surface temperatures. For an organism to transport nutrients into its cells and transport wastes out, a liquid carrier is needed. One might be tempted to think that on Earth, water serves this purpose because it is the available substance that is liquid at temperatures pertinent to this planet. Perhaps on warmer or colder planets, some other substance will be liquid and serve the necessary function there. But at temperatures high enough to melt most Earth-solids, organic molecules disintegrate, and at temperatures cold enough to liquefy most Earth-gases, biological processes slow down to a near halt. Carbon-based life needs a liquid in the right range—the one filled by water.
High heat capacity. Think those hot summer days or frigid winter nights are hard to bear? The fact that water takes a lot of energy to heat and releases a lot of energy as it cools prevents those highs and lows from being much more extreme. Water has a moderating effect on climate that keeps the summer highs and winter lows in check. Just compare the temperature extremes of inland deserts with the more modest swings of coastal regions, and imagine what the entire world would be like if the heat capacity of water were “normal.”
Water is a polar molecule. The presence of a positive and a negative side to a water molecule results in electrostatic attraction to solid surfaces. Without such attraction, more water would drain from the pores in soils after a rain, leaving less available for roots, and dust storms would become the norm instead of limited to arid lands. Polarity also results in attraction between adjacent water molecules, especially at the air-water interface, where the negative side of one molecule aligns with the positive side of another to form a weakly bonded layer right at the surface. Without these bonds, water would escape to the gas phase more easily, resulting in rapid evaporation of lakes, and water striders and spiders that scurry across the surface of water would sink. Finally, polarity makes water much better at dissolving ions. A non-polar linear arrangement of atoms, like CO2, would be much less effective for transporting nutrients and wastes.
So water isn’t just weird, it is weird in fantastic ways that happen to align amazingly well with the needs of carbon-based life. And there’s lots of the odd stuff available on a planet that has just the right orbit to maintain a surface temperature suitable for water to be in liquid form. A planet that also happens to spin about a star with peak electromagnetic emissions that match the range of wavelengths least absorbed by water. Of course, all this could have come about by some incredible stroke of fortuitous luck, unaided by divine agency. But for those with less faith in such dazzling improbabilities, something far more grand seems evident. It is almost as if God was having fun with his creation, making water in oh-so-special ways that would supply just the right medium in which life in all its diversity could thrive, that would also effectively bear his signature upon every drop.
Gregg Davidson is professor of geology and geological engineering at the University of Mississippi.
