It's in our homes, our schools, and in houses of national and international power. It's running through our country and underneath our cities. It goes into all of our food, and many of us go to sleep at night with some quantity of it by our bedside. It's water, and since we see it every day, we don't consider it to be unusual. That's a mistake. Water has some intensely strange properties.
Unlike other liquids, it gets lighter (by volume) as it freezes - which has made our oceans one of the weirdest bodies of liquid in the solar system. It's one of the most powerful corrosives we've ever discovered. And if water weren't the way it is, humans would have evolved very different bodies.
Why icebergs are exceedingly strange
One of the most surprising characteristics of water is its tendency to get less dense when it cools beyond a certain point. The more heat that is pumped into a substance, the more its component atoms and molecules vibrate. That vibration causes the substance to expand and get less dense. As it cools, it contracts and gets denser, sometimes solidifying. Water follows the pattern, to a point. However, it gets densest at around 4 degrees celsius. When cooled beyond that point, water molecules push apart and freeze into a less dense solid - ice. Since the ice is less dense than the colder water around it, it floats upwards, to the surface of the water.
What if it didn't? To start with, photographers who made their money off of taking pictures of adorable harp seals and polar bear cubs would be out of work. And the movie Titanic would never have happened, since no icebergs would lurk near the surface of the ocean. (The actual Titanic also would not have sunk, but that seems like a lesser matter considering the global scope of James Cameron's later work.) Ice that formed in oceans or lakes would drop down into the water, out of the reach of sunlight. This would make it difficult to melt. Under the right conditions, then, more ice would accumulate at the bottom of oceans and lakes. Although thermal vents at the bottom of the sea provide enormous heat, that heat drops with distance. An inch away from the vent, the water temperature is 2 degrees celsius. If water molecules didn't spread out as they froze, oceans and lakes might have been chunks of ice with livable conditions only at the very surface and the furthest depths.
Why it always takes so long to boil water
It turns out though, that water is obliging at both ends of the temperature spectrum. Other liquids in its weight class have a much lower boiling point. If water weren't so cool, calm, and collected, more of it would be whizzing around our atmosphere. Much more. To set an example, hydrogen sulfide, which is actually heavier than water, is known by the unlovely name 'sewer gas' because it's often produced by the breakdown of the, more delicately named, 'waste material.' Although it could be liquid, it's mostly encountered as a gas. If water had a similar boiling point, the oceans would be relative puddles. Much of the earth's water would be transferred into the air, and we'd be walking through very thick water vapor.
The reason for water's equanimity is its ability to form hydrogen bonds. Water is made up of two hydrogen atoms and an oxygen atom in a tug of war for the hydrogen atoms' electrons. The oxygen atom has the advantage in this war, and is able to hold the electrons closer than the hydrogen atoms can. This leaves the hydrogen end of the water molecule with a subtle positive charge, while the oxygen end is slightly negative. When water molecules get close to each other, these slight differences in charge cause attraction. The negative oxygen in one molecule will tug over the positive hydrogen in another. This allows water a special strength to resist temperature changes and stick together, as well as helping form the light crystalline structure of ice. Without it, we'd need gills or fur to withstand the world.
Why water can destroy anything
And without it, we'd need to do a whole lot more sifting to find what we need. This polarity - the existence of positive and negative sides to the molecule - allows water to be The Universal Solvent. It will dissolve almost anything. This means that, as water trickles through the earth, it will pick up substances and move them from place to place. Gold, salt, and even uranium deposits occur as the result of water picking up tiny particles from the soil and shedding them in large amounts. Water helps hollow out canyons and caverns, it helps build up deposits of metals and minerals. It even once helped make a nuclear reactor. All because it picks up pretty much everything it comes across and gives it a free ride.
So what would we do if water weren't the way it was? Die, and quickly too. There's no substance that can adequately replace all the characteristics that water has. There is, however, one really cool one that can at least partially replace the loss. Water's ability to become less dense as it freezes is not entirely unique. Diamond can do the same thing, although at vastly different temperatures. In fact, some scientists believe that outer planets like Neptune and Uranus could have seas of liquid diamond, studded with diamond ice bergs. It's unlikely that ruby fish swim through the seas' waters, or if Emma Frost waits on their shores. They probably couldn't support any type of life. It's nice to know that we have a cool back-up, though, if water ever decides to become unreliable.
[Hydrogen Bond Image Source: Michal Maňas, has translated image made by User Qwerter into English.]