Water, Part Two. Two Reasons to Get Excited About Hydrogen Bonds

29 07 2009
A gorgeous iceberg off the coast of Greenland. You're about to find out why it floats!

A gorgeous iceberg off the coast of Greenland. You're about to find out why it floats!

And we’re back! Gosh, did that last article on water make you thirsty for more? Ha-ha, get it! Thirsty! Thirsty for water!

As I was saying, you couldn’t exist without water, or more specifically, without hydrogen bonds. Right now I’m going to give you two of the reasons why. There are more, but we’ll get to those another time. I could write about this stuff all day, but I know you’re busy and you have important business to attend to.

Reason number one you better be thankful for hydrogen bonds: hydrogen bonds are the reason that ice floats.

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You may know that water expands as it freezes. This makes it less dense than liquid water, which means it floats (remember, you are buoyant if you displace at least as much water as your weight). But why? Why does water expand when it freezes, when every other thing that freezes contracts as it gets cooler?? You may have seen this answer coming. It’s hydrogen bonding.

A lattice of hexagonal arrangments of water molecules. Note the very regular spaces between molecules. Each is precisely one googly from its bonding partners. See more illustrations like this at the excellent site of Martin Chaplain. http://www.lsbu.ac.uk/water/ice1h.html

A crystal lattice of water molecules. Note the very regular spaces between molecules. Each is precisely one googly from its bonding partners. See more illustrations like this at the excellent site of Martin Chaplain. http://www.lsbu.ac.uk/water/ice1h.html

You see, hydrogen bonds form at a specific distance. I don’t know what exactly this distance is, so for the sake of this article let’s say they form at a distance of exactly one googly. That means that every time a hydrogen atom of one water molecule binds to an oxygen atom of another, it is one googly away. Okay, great. How does that relate to the density of ice? When water is has very low molecular energy (at, say, 0°C or 32°F), it lacks the energy to break these bonds, and so all the molecules stay apart at this distance of one googly. But when the molecules are sliding around in liquid form, they have higher energy and move around more speedily.* They can move closer together than one googly.

Think of it this way: If you had two bar magnets, and you slowly pushed the positive ends of each magnet together, you would find that there was a certain point past which the magnets would start to repel each other. And at that point, the only way you can get them to move closer is if you use some energy, for instance by pushing them together using your muscles. They don’t like to be this close together, but given some energy, you can force them to be. This is analogous to water. When water has some energy in it (from its heat), the molecules can push in closer than the distance of one googly. But when water gets cold enough, its molecules lacks the energy to push in that far, and so each molecule is locked at a distance of one googly. As a result, the water molecules are closer in liquid form than in solid form, and liquid water is denser than solid water. The end product is ice floating around, like an ice cube in your soda or an ice berg in the Arctic Ocean.

Some carefree high-latitude humans frolic on the surface of a frozen lake. They wouldn't be able to do this if not for hydrogen bonds, because they would not exist. It's hard to frolic if you don't exist.

Some carefree high-latitude humans frolic on the surface of a frozen lake. They wouldn't be able to do this if not for hydrogen bonds, because they would not exist. It's hard to frolic if you don't exist.

Now the reason that THIS is important might not be obvious. Let me make it obvious for you. Ice floats. When it comes winter time, a lake starts to freeze. Where does it start to freeze? On the surface, of course! The surface starts to freeze, and slowly it freezes the water beneath it until you have a layer of ice floating above a large body of water. The freezing does not tend to go much further than a few meters because ice is a very good insulator, as anyone who has built an igloo can tell you. But what if water was denser as a solid than as a liquid?

It would sink. The surface of that lake would freeze, and the ice would sink to the bottom. Then the water on the surface would freeze, and sink to the bottom. More water would freeze, and sink. There would be no insulation, no protection, and eventually the entire lake would freeze along with every living thing in it. And the population of the lake would die a cold, possibly painful death. Pretty grim, isn’t it?

But thankfully we do not live in a universe where water gets denser as it freezes. We live in a world where water expands when it freezes! And that is a fact that should make you exultant, because if that wasn’t the case life as we know it could not exist! All the water would be frozen, and nothing could survive a year! And that’s all thanks to hydrogen bonds.

A big, tall chestnut tree. Look at the size of it! I wonder how water gets all the way up there...

A big, tall chestnut tree. Look at the size of it! I wonder how water gets all the way up there...

I’ve got one more example of why hydrogen bonds are awesome, and then I’m going to wrap up for now. But don’t worry, this is only the second in a series of posts about the wonderful substance we call water.

Look outside. There’s a good chance you’ll see a tree. Can you see a tree outside? Great. If you can’t, just look at this nice picture of a tree I’ve found for you here. See how big it is? Those nice green leaves way up at the top? Pretty impressive, isn’t it? I get excited about trees, too.

Now think about this: When you water a plant, what do you water? That’s right, you water the roots. Think about that. How does the water get from the roots all the way up to those leaves? Plants don’t have pumps like we do, and gravity is pretty good at preventing things from spontaneously flying into the air. So how does it work? How does water get from the roots to the leaves? How??? HOW!?!? TELL ME NOW!!!

Relax! The answer is simple. It’s hydrogen bonding.

Look at the leaves of that tree. They’re big and broad, and have a lot of surface area. This means there’s a lot of evaporation going on, so water is getting heated up by sunlight and ambient heat and is being ejected from the leaf upwards into the air. Actually, this can be a problem for plants, since if all their water evaporates out, they dehydrate and die! In fact, most leaves have a waxy coating on them to help keep water in and prevent them from losing it all to evaporation. This is why leaves tend to be shiny.

Now remember that water molecules are attracted to one another. If I move a water molecule to the left, the one next to it, the one hydrogen bonded to it, will follow it to the left. If I move it to the right, a molecule will follow it right. If it goes down, a molecule follows it down. And if I move it up… a molecule will follow it up.

So when that water evaporates from the leaf of the tree, it moves up into the air, and the molecule beneath it follows it upward slightly. And the molecule beneath that one moves up as well. And the one below that, and the one below that, and the one below that, all the way down through the twigs, the branches, the limbs, and the trunk, all the way down to the roots, just like a little train of water molecules, all linked together. And so evaporation drives the movement of water up the plant into the leaves and upper reaches.

If this wasn’t the case, you wouldn’t be able to eat nice leaf based foods like spinach. Nor would you have fruits! Nor would you have any tasty animals that feed on these! Your menu would be very bare indeed! But that is not the case, and the reason is because water molecules are attracted to one another, and the reason for that is hydrogen bonding.

Okay! We’re done. I hope you found this exciting. I know I have. Next in the series is Water, Part Three. The Solvent of Life. Ready yourself.

-Neil

*I’m realizing that I’m assuming some knowledge about temperature and its relation to molecular kinetic energy. Let me break it down for you quick and dirty. Molecules are always in motion, and the speed they’re moving around at depends on the quantity of movement energy they have, what we call kinetic energy. Now temperature is a measure of the average kinetic energy of a sample of something. The higher the temperature, the faster the average molecule is zipping around inside. This has important implications for all sorts of things, including the rate that chemical reactions take place, the amount of space substances take up, and the phase (solid, liquid, or gas) that they’re in. Maybe an article on temperature will come along soon…!

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27 09 2009
The Times They Are A-Changing « Dang, That's Cool!

[…] balance between photosynthesis and water loss that every plant must deal with. If you’ve read the article here on water and hydrogen bonding, you know that plants like trees transport water to their upper parts because the evaporation of […]

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