Molecular Shape
Let's start at a molecular level for this. Many of water's special and unique attributes are derived the components of the water molecule. Water is composed of two hydrogen atoms covalently bonded to an oxygen atom. In addition, to the hydrogen atoms, the oxygen also has two lone pairs of electrons attached to it. So we could say the oxygen in a water molecule has four electron complexes attached to it (two hydrogen atoms and two lone pairs of electrons). Each of the electron complexes is "tied" to the oxygen, but repulses the other electron entities. Imagine the oxygen as a ball in the middle of the four electron complexes, with each complex at the end of a string tied to the ball--for each complex to be as far from each other complex as possible, it will form this shape, which we call a tetrahedron:
But, in this case, two of those complexes are lone pairs of electrons. We don't count lone pairs when looking at the shape of a molecule. The shape of a water molecule, then, is composed of the center oxygen with two hydrogens attached at an angle--it's a bent line. We call this molecule's shape "bent." [link to more on molecule shapes]
Molecular Attraction
Remember those lone pairs of electrons on the oxygen atom in a molecule of water? Think of that portion of the water molecule as the "top," and think of the portion with the hydrogens as the "bottom." The top portion of the molecule has an oxygen molecule in it, which is relatively electronegative, or attractive to electrons. [link to electronegativity] Oxygen is more electronegative than hydrogen, so it doesn't share the electrons quite equally with the hydrogen atoms--it hogs the electrons, you might say, sharing with the hydrogens only enough to mostly keep them attached. Due to this electron hogging and the presence of lone pairs of electrons, the top portion of the water molecule--where the oxygen is--is relatively more negative than the bottom portion of the molecule.
As we know, a positively charged object tends to attract a negatively charged object. Well, the ends of a water molecule have a positive and a negative change. So each end of a water molecule attracts the opposite end of nearby water molecules--the top of one is attracted to the bottom of the next, much like positive and negative ends of magnets. In the case of a water molecule there are two positive points sticking out at an angle (the hydrogen atoms), and one negative portion (the oxygen atom).
Hydrogen Bonding
The positive-negative attraction between the top and bottom of a pair of water molecules is called a "hydrogen bond" because it comes from the attraction of a hydrogen in one molecule to the oxygen in another. In liquid water, in which the molecules are moving quickly, hydrogens are constantly being formed and broken. But when water molecules are moving sufficiently slow [link to the relation between temperature and kinetic energy] the attraction between portions of water molecules becomes strong enough that more permanent hydrogen bonds can form between the molecules. This bond isn't as strong as a covalent bond (the bond within one water molecule, caused by the sharing of electrons between the oxygen and the hydrogen), but it is strong enough to cause a lattice to form when many different molecules are attracted together in this way. This is what happens when water turns into ice--the individual water molecules work together, using the attraction between a hydrogen in one molecule to the oxygen in another to form a lattice structure. Each hydrogen in one water molecule will attach to a different oxygen atom in a different water molecule, and each oxygen atom in the lattice would have several hydrogens attracted to it. Molecules are moving slow enough to begin formation of the lattice structure when the temperature of the water is below four degrees Celsius.
Here's an illustration of liquid water on a molecular level:
Note the random placement of the molecules. Hydrogen bonds are forming and being broken constantly in this picture as hydrogen molecules get close to oxygen molecules in the frenzy.
Here are a couple illustrations of the lattice structure:
Note that the lattice structure is not space efficient. When the movement of the individual water molecules becomes more erratic--as the water becomes warmer--the hydrogen bond is not strong enough to hold the lattice structure in place. Some molecules, for example, could flow right in the middle of the hexagonal structures in the lattice. When there are molecules in those holes in the lattice, the unit is more dense--it has more stuff in the same amount of space. If you pack more stuff in the same amount of space, then you've made that material more dense. In this case, that is the reason why ice floats--it is less dense than liquid water. When the temperature of water is low enough that the water molecules are moving slowly enough for the lattice structure to form, the low-density ice lattice is formed. But when the temperature is raised, making the individual molecules more erratic, the hydrogen bonding fails to hold the molecules in place causing the low density lattice to break down, melting the ice into higher density water.
What does hydrogen bonding mean to my life?
Without hydrogen bonding, water would not be a liquid at temperatures in which humans can survive. Even though the hydrogen bonds in liquid water are not strong enough to form ice above zero degrees Celsius, they still play an important role in the properties of liquid water. The attraction of the water molecules to each other (or cohesiveness) causes the molecules to be reluctant to separate into a gas. Without hydrogen bonding water would probably boil at a temperature similar to that of other substances of similar molecular weight. Carbon dioxide, which is significantly heavier than water--which means it would boil at a higher temperature than water--boils at about -72 degrees Celsius. Life on this Earth as we know it requires the use of liquid water. Without hydrogen bonding, the Earth would either be barren or life would be sustained by a means that doesn't require liquid water.
Without hydrogen bonding ice would also be totally different. I'll use a bucket of water to explain: Because colder water is more dense than warmer water, colder water sinks to the bottom of the bucket. If I left a bucket of water out on a winter day in Alaska, the molecules of water at the top would cool down, become more dense, and sink to the bottom, but wouldn't freeze because, at the bottom of the bucket the water is not exposed to the cold air. Other molecules would then be at the surface; these would also cool down and sink to the bottom without freezing. This process would occur over and over again, circulating the water until the whole bucket was four degrees Celsius. At this point, the water at the top, exposed to the cold air, would be further cooled below four degrees, causing it to begin to form the lattice structure. The surface water, then, because it is forming the lattice, would be less dense than the water below it, and as it cooled would stay at the top instead of sinking to the bottom, like it would do above four degrees. Those surface molecules would then form a thin layer of ice. The top layer of ice acts as an insulator from the cold for the water below it, but slowly the energy will drain from the water causing a thicker layer of ice to form (remember this energy being "drained" is just the molecules slowing down). At some point, if the bucket is big enough, a thick enough layer of ice will form that it insulates the water below, keeping it from freezing, even though the outside temperature may be very cold. This is how many fish survive in frozen lakes--if the lake is deep enough it won't freeze through, even in Alaska. There are other factors as well, keeping the deeper water liquid--for example pressure. The pressure on the water below from the water and ice above keeps it from going into a less dense state like ice.
Without hydrogen bonding, however, the low density lattice structure would never form. Accordingly, when cycle of the surface water cooling and then sinking would continue below four degrees Celsius. Eventually, as the molecules became sufficiently slow, the water would feel thicker to the touch, and as it got colder, it would get thicker still--much like oil. Eventually, the water would become thick enough that it would all be hard, but since there would be no lattice structure, the ice would not form from the top down, it would form relatively uniformly throughout--because of the constant circulation of the densest water going to the bottom.
See these helpful links for more in-depth information regarding the topics discussed in this blog:
All about the intracacies of the water molecule
Showing how strong the hydrogen bond can really be (a tribute to the power of the hydrogen bond!)
Wikipedia article on water
Why is water blue? (discussion of hydrogen bonding's role in determining the color of water)
Everything--seriously everything--about water (this is a great site covering a wide variety of topics concerning water in depth)
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