PolitiCook.net

What is water, anyway?

We all know that water is H2O, two hydrogen atoms to one oxygen atom. On a molar basis, water is two thirds hydrogen. On a mass basis, it is only 16/18ths hydrogen, since oxygen weighs 16 to hydrogen only one. But that is not the wonder of water.

Most compounds that have a molecular weight of 18 are gases. Nitrogen is 28, and boils at 77 kelvins, cold enough to freeze a banana into a hammer. Pretty much the same with oxygen, and argon. Carbon dioxide, with a molecular weight of 44, liquefies at higher temperatures.

The nearest relative to water is hydrogen sulfide, H2S, since sulfur in just under oxygen on the periodic table. But sulfur dioxide is a gas at room temperature, and water is a liquid. What gives?

What gives is the property of “hydrogen bonding” wherein small, very electronegative atoms have a peculiar affinity for hydrogen. Nitrogen, oxygen, and fluorine have these properties, and chlorine shows some weak tendencies for it, but those are very weak. It has to do with the electronic configuration of the atoms, and the propensity for them to attract electrons.

Nitrogen and oxygen have a Pauling electronegativity of over three, and fluorine four. They also are Row Two elements, meaning that they have very compact electron clouds. Chlorine has an electronegativity of 3.5, but a more diffuse electron cloud since it is a Row Three element.

The bottom line is that hydrogen, when subjected to the extreme electronegativity of nitrogen, oxygen, and fluorine experience a tremendous attractive force towards those atoms, and that stabilizes the collection of atoms. Water is the most extreme example, because it is relatively simple and has three sites for association: the two hydrogen atoms can attract two oxygen atoms from other water molecules, and the one oxygen atom can attract a hydrogen atom from still another molecule of water.

These energetic interactions are what makes water unique. Most substances contract as they cool, and attain maximum density upon freezing. Not water! It does contract until the temperature becomes 3.98 degrees C, and then begins to expand. This is because the molecules align themselves in a self organizing way, and become an organized structure that is more open than a random arrangement would be. Thus, water below 3.98 degrees C becomes more organized in a relatively open structure, and begins to rise over the more dense, warmer water.

Carried to the extreme, water now at zero degrees C begins to freeze, with an even more organized and open structure. The result is that ice is around 11% less dense that cold water. This is important, because, since ice floats on water and is also a relatively good insulator, lakes and ponds do not freeze from the bottom up quickly, but from the top down slowly. Life as we understand it would not otherwise be possible.

Water should boil at around 100 degrees below zero C, but the same hydrogen bonds keep it liquid very far beyond that, for the same reasons. Hydrogen bonding is very powerful.

Now a little fun with hydrogen bonds. Cotton clothes wrinkle because the cellulose that makes them hydrogen bonds with other, nearby cellulose residues. Ironing them, with steam, breaks the random hydrogen bonds and lays down more regular ones as assigned by the mass of the iron.

This is getting too long, so I will ask for comments, criticism, and good wishes. If I got anything wrong technically, please let me know right away. I never wish to mislead anyone. Warmest regards, Doc.