By Richard E. Bleil
Water is a fascinating compound. We don’t think about it because in our experience as human beings, it seems so common. The reality is that, while it is common to our experiences, it is extremely rare as a chemical.
As I write this, we are in the midst of what I am hoping will be the last winter storm of the year. We’ll see, but it is the inspiration for this blog. Let’s start with some chemical details which will help to explain some of what follows. See, in chemistry, there are two broad-stroke categories of forces; chemical and non-bonding intermolecular forces. Bonding forces includes ionic and covalent, and it is what holds the atoms together to create compounds. In water, there are two hydrogens, each are “covalently bound” to oxygen, meaning that they share electrons with oxygen. Non-bonding intermolecular forces are what hold molecules together to form the condensed states (liquids and solids). These forces give rise to properties like vapor pressure; the weaker the intermolecular forces, the easier to get the molecules into gaseous state.
Water has the strongest known intermolecular forces, unfortunately named “Hydrogen Bonding”. I say it is unfortunately named, because hydrogen bonds are actually non-bonding intermolecular forces. A little more technical details, this means that the partial positive hydrogen atoms are attracted to the lone pair electrons on the oxygen. Oxygen has a higher “electronegativity”, which means that while the electrons between oxygen and hydrogen are shared, they tend to spend more time near the oxygen. Because of the uneven sharing, the hydrogen has a partial positive characteristic, just as the oxygen has partial negative characteristic. Oxygen also has two pairs of non-bonding electrons near it, called “lone pairs”. These can be thought of as regions of space with high electron density, and because electrons are negatively charged, this means that they are regions of negative characteristic. The partial positive hydrogens are attracted to these negative regions of space, which is the “hydrogen bond”.
Every non-bonding intermolecular force known is weaker than bonding forces, but hydrogen bonds are right on the brink of chemical bonds. Electrons are not shared, but attracted. One of the great mysteries was how quickly the hydronium ion (H+ which is the ion responsible for acidity) can travel through water. As chemists were measuring diffusion rates of other ions like sodium (Na+), potassium (K+) and Calcium (Ca+2), hydronium just didn’t fit the trend, being way faster than predicted based on these experiments. As it turns out, the hydronium ion actually “traveled” by shifting bonds. Attracted to these lone-pair electrons, the hydronium would form a new bond, breaking one of the oxygen-hydrogen bond, thereby moving it from one location to another without actually moving at all. No other ion displays this kind of behavior, or, in fact, could.
This shows just how close these hydrogen bonds are to chemical bonds, demonstrating the strength of this intermolecular force compared with others. It’s because of this intermolecular force that causes water to expand on freezing at common conditions here on earth. What the reader may not know is that water is the only compound known to science that actually does expand on freezing (one element, Gallium, also expands on freezing). That means that ice is the only compound that will actually float on the liquid of the compound. For any other compound, the frozen form would sink through its liquid. This excessively rare trait of water is critical to the development of life. It’s because ice floats that plant life could develop on the ocean floor. If ice was more dense than water, a layer of ice would form on the ocean floor, and not have the time to melt through the summer months. The accumulation of this ice would prevent plant life, and life in general from developing beyond the microbe stage.
The force with which ice expands is well-known. I’ve seen astounding demonstrations of the explosive force of freezing water sealed in quarter-inch steel balls, but what happens if the water cannot expand? After all, it may be an incredible force, but it’s limited. As it turns out, there are multiple forms of ice. Just a couple of days ago, an article was published of what is called “Ice IX”. The common ice is Ice I, so there are as many as nine different forms of ice, although you will probably never seen any of them if you’re not a physicist.
Hopefully, this blog has given something to think about. Water (which I call “hydroxic acid”) is such a common substance here on earth, but it sure is unique. Much like we all are. You are as common as water. Isn’t that marvelous?