Here is a large body of water. Beautiful, isn't it? Makes me feel calm.
For my first post, I want to talk to you about water. You may think water is pretty boring. It’s tasteless, colorless*, and odorless. You know you need to drink it, but otherwise it’s pretty uninteresting, right?
WRONG! Incredibly wrong.
Water is a fascinating substance that is extremely unique! In this post I’m going to try to get you to understand why I get all excited when I think about water, so hopefully after you read this, you will get excited about water, and your friends and family will look at you askance. But then you will show them this site, and they will understand.
A single water molecule, space-filling model. The red atom is oxygen. The gray atoms are hydrogen. The spheres represent the area the electrons travel through - you see they overlap. This is what is meant by "sharing electrons" in a covalent bond.
So let’s start with water’s chemical structure. You’re probably familiar with the chemical formula of water, H2O. That water’s chemical formula is H2O means that water is made of one oxygen atom and two hydrogen atoms. Now, we say that these atoms are covalently bonded together. All this means is that the atoms share electrons. And in order to share electrons, the atoms have to be close together – that’s where we get the “bond” part of “covalent bond.” If I share a blanket with you, we have to be close, right? So it’s just like that, but with atoms.
Please examine the picture of the water molecule here on the right. You see the two small hydrogen atoms connected to the single, large oxygen atom. This structure is important, because it is part of the reason water is considered a polar molecule, and it is the polar nature of water that makes it so special. Now, remember that the reason these atoms are so close together is that they share electrons. But here’s the thing: they don’t share evenly. In fact, the oxygen atom attracts electrons much more strongly than the hydrogen – we say it has a higher electronegativity. As a result of this, the electrons hang out next to the oxygen more than they do the hydrogens. Since electrons have a negative charge, and they cluster around the oxygen atom, this means that the oxygen side of the atom has a negative charge, while the hydrogens have a positive charge. This is what we mean when we say water is polar.
Here is a model of hydrogen bonds in water. You see the positive, gray hydrogens are attracted to the negative, red oxygens.
Why does this matter? Boy howdy, don’t get impatient, because I’m about to tell you.
It matters that water is polar because it means that water molecules are attracted to each other. The positive hydrogen atoms of one water molecule are attracted to the negative oxygen atom of another, just like the positive end of a magnet is attracted to the negative end of another magnet. And when a hydrogen gets lovesick for an oxygen atom, we call the bond that forms a hydrogen bond. Scientists are very creative people, you see. These hydrogen bonds form and break rapidly, but the point is that they exist, and they help the substance to stick together. Examine the figure to the left to see how these bonds look structurally.
So that’s it, right? Water has hydrogen bonds and now you’re supposed to be all excited. Yeah, right. We’re just getting started.
Those hydrogen bonds are the most important thing in the whole world! You wouldn’t be able to read this if it wasn’t for those hydrogen bonds! You wouldn’t even exist! Gosh! I am getting so worked up here!
Just look at those water droplets sticking to that glass. That's hydrogen bonding, baby!
Let me tell you why hydrogen bonds are really important, and why my heart rate increases just thinking about them. Like I just said, hydrogen bonds are why water sticks to itself. They are why if you throw a glass of water into the air, you see the water fly out in globs, rather than it turning into a fine mist. The molecules stick together! And they also stick to other substances, like glass. This is why when it rains a bit you might see water droplets on your window, just sitting there, not moving. They are stuck to the glass like a magnet to a refrigerator. If they get big enough, though, they will slide down, just like a magnet that you tried to force to hold up something too big for it, like, say, a list of all your regrets.
Okay, that’s it for now. I know you want more! I know now I’ve got you screaming, “More! Tell me more about water and how awesome it is!” I know. I know. More is coming, very very soon. And it only gets better.
-Neil
*Water actually has a very faint blue color, but this is impossible to see unless you have a large amount of water, as in, say, an ocean.
Dang, That's Cool! 
Hey Neil,
Water is really interesting! I have a few questions though:
1) Are there other polar molecules? Do they have hydrogen bonds or an equivalent?
2) How is a hydrogen bond different from a covalent bond? Are they the same strength or type of attraction? I notice that the circles in the diagram don’t overlap like they do for the covalent bonds that make up the whole molecule.
3) Do the ideas presented here apply to all the phases of water, or just liquid?
Thanks!
Hi Sam, great questions.
1) There are other polar molecules! In fact it is very important that this is the case, because it is only polar molecules which will dissolve in a polar solvent like water. This will be the subject of a coming post about water as the solvent of life. An example of a non water polar molecule would be glucose, C6H12O6. Glucose is a much larger molecule than water, as you can see by its chemical formula, and it is also polar in a slightly different way. Rather than the whole molecule having a slightly negative and a slightly positive side, as with water, the molecule has several polar components, called hydroxyl groups. Hydroxyl group is a fancy way of saying an oxygen bound to a hydrogen. As you have learned in the above article, when oxygen and hydrogen get together they don’t share electrons evenly, and this is true in hydroxyl groups as well as in water molecules. So these hydroxyl groups provide “hook points” for water molecules to latch on with hydrogen bonds, in the same way they latch on to one another. Having these polar “hook points” is what determines whether or not a molecule will dissolve in water. No polar parts, no dissolution. Lipids like vegetable oil, for instance, have uniform charge and no polar parts. As a result, they can’t form bonds with water and can’t dissolve.
In addition to big molecules with polar parts like glucose, there are also molecules more similar to water, made of fewer atoms. For instance consider Hydrogen Fluoride, or HF. It is far less ubiquitous and didn’t get a name until after the development of chemical theory, so we call it hydrogen fluoride instead of, I don’t know, Bikon. Anyway, HF is similar to water in that the fluorine atom has a stronger affinity for electrons (higher electronegativity) than the hydrogen, and as a result the atom is polar. So why don’t we have oceans of HF, and why isn’t HF the solvent of life that water is? Well, for one, it boils at 19.5°C, about room temperature, which means it won’t do well on a planet such as ours where temperatures can get up into the 50s C. Second, HF has a tendency to split apart into F- and H+ ions. These H+ ions are extremely reactive, and if there’s too many of them they tend to react with and change other molecules. In fact this is what we mean when we say something is acidic. It splits up and frees H+ ions to react with whatever they feel like! So when you get HF (also known as hydrofluoric acid) thrown in your face, the free hydrogen ions bind with all sorts of molecules in your skin and blood and that is where the terrible damage comes from. Water, while also dissociating into H+ and OH- ions, does this at a far lower rate (about 1 x 10^-7 moles of H+ per liter of water). Water is really the perfect molecule for life as we know it to live in.
2) A hydrogen bond is quite a bit different from a covalent bond. First, a hydrogen bond from a water molecule is about twenty times weaker than a covalent bond, and far less permanent. While covalent bonds often take a fair bit of energy to break, hydrogen bonds in liquid water break and form spontaneously, usually lasting no more than a trillionth of a second! Only at low temperatures, when very little kinetic energy is present in the solution, will hydrogen bonds maintain their grasp.
Hydrogen bonds are also different in the way they attract. Whereas covalent bonds are the result of the sharing of electrons, hydrogen bonds are a result of electrostatic attraction (like a balloon you rubbed on your head sticking to the wall) between partial negative and partial positive charges.
3) Hydrogen bonding applies to water in solid, liquid, and gaseous form, but it is affected significantly by temperature. Remember that temperature refers to the average kinetic energy of the molecules in a sample, and that it is the abundance or lack of this energy that determines whether hydrogen bonds hold (as in ice crystals) or break (as in liquid and gas).
Keep them questions coming!
What about a molecule that is polar, but doesn’t involve hydrogen? Does it have an equivalent of hyrdogen bonding?
Replying to the below post:
Any molecule with an atom that has a partial negative charge can be the recipient of a hydrogen bond. For instance, sulfur dioxide SO2. But no, the definition of a hydrogen bond is a bond between hydrogen and an electronegative atom with a partial negative charge.
Also, I thought the ocean was blue because of the way sunlight penetrated or broke apart with depth, or something. Or is it a combination of factors?
You are correct. The ocean and water are only blue because of what happens to light when it passes through them. Color, in fact, is not a property native to any material but is only our brains’ way of perceiving different wavelengths of light. Outside living things, color as we know it does not exist.
An explanation of the blue color of water requires a basic understanding of visible light itself. You may be familiar with this, but others may not be so I will explain it for their benefit.
Visible light consists of electromagnetic waves with a wavelength between 400 and 700 nanometers (or .0000004 – .0000007 meters). If electromagnetic waves are within this range, they can change the physical structure of photoreceptive chemicals in the rods and cones of your eyes, setting off a chain reaction that results in the transmission* of an electrical signal to visual processing regions of the brain. Only waves within this range affect these chemicals. Waves much bigger than visible light, like microwaves, cannot be perceived by the human eye, and waves much smaller, like ultraviolet or gamma rays, also cannot be perceived, and are also very very dangerous, because as a wave’s wavelength gets smaller, its energy increases!**
Anyway, that 300 nanometer range gives us a bit of a palette to work with, and what we call color is the brain representing different wavelengths and combinations of wavelengths differently. For instance, 400nm light is interpreted by the brain as blue, 500nm light is interpreted as green, and 700nm light is interpreted as red. When you add them all together, as in the emissions from the sun, which emits the full spectrum of electromagnetic radiation, you get white light. But if you have a substance that blocks or absorbs certain wavelengths and transmits others, you can get light of specific colors. This is the case with water.
Water primarily absorbs red light, which means when light passes through water some of the red light is taken out, while blue light is permitted to pass. As a result, white light (like sunlight) that passes through water emerges as blue light. The more water you have, the more red light is removed, and the bluer the resulting light is. So a glass of water appears colorless, because very little red light has been removed. The transmitted light is still perceived as white. But a swimming pool, pond, lake, or ocean all contain enough water that a perceptible amount of red light is removed, resulting in more light at the 400nm side of the spectrum than 700nm light. The end product is that blue stuff we all know, love, and couldn’t exist without.
-Neil
P.S. If you’re interested in a more in-depth and technical discussion of water’s light-absorbing properties, including WHY light absorbs certain spectra and transmits others, please check out the website of Martin Chaplin, here: http://www.lsbu.ac.uk/water/vibrat.html
*Actually, many photoreceptors in the eye only transmit signals when they are NOT being struck by light. This is referred to as “dark current.” In these cells, light hitting the photoreceptive molecule destabilizes an existing chain reaction and shuts it down. Somehow, though, the brain makes sense of all the incoming signals (or lack of signals) and provides you with the excellent visual field we humans rely on for survival.
**The relationship between wavelength of an electromagnetic wave and its energy is described by the equation
E = (h * c) / λ
Where E = the energy of the photon in joules
h = the Planck constant, 6.626 x 10^-34 joule seconds
c = the speed of light, 3 x 10^8 meters/second
and λ = the wavelength of the electromagnetic wave in meters
Since the product of our constants h and c are always divided by the wavelength, the smaller the wavelength is, the bigger E is. Let’s plug some numbers in for λ to confirm this.
For radio waves with a 200m wavelength (yes, radio waves are BIG!), the equation is
E= (6.626 x 10^-34 * 3.0 x 10^8) / 200
Which is 9.9 x 10^-28 joules. That’s a decimal point, then twenty-seven zeroes, then 99. A tiny tiny number! Of course you will rarely be hit with only one photon, but let’s work with just one for the sake of comparison.
For red light, 700 nm, the equation is
E = (6.626 x 10^-34 * 3.0 x 10^8) / .0000007
Which comes out to E = 2.8 x 10^-19 joules – not exactly fearsome, but get enough of it and you will be having a problem.
How about gamma rays, which have a wavelength of about 1 x 10^-10 meters?
E= (6.626 x 10^-34 * 3.0 x 10^8) / .0000000001
Gives us E = 2.0 x 10^-15 joules per photon. I know this is still a very small number, but get enough of these little guys together and you will be fried to a crisp very, very quickly.