Part XX Water: What Makes it so Unique and so Important
“Water, water everywhere and all the boards did shrink,
Water, water everywhere, nor any drop to drink”
Those words learned so long ago from “The Rime of the Ancient Mariner” by Samuel Coleridge still ring in my ears like the day I memorized them in Miss Bennett’s 4th hour English IV class. I learned more from her about English grammar and literature than perhaps subject matter in any other high school class.
Water is the subject of this essay. I chose it because it (water) is so important in so many ways. It covers 75 percent of the earth’s surface, comprises 55-75 percent of our body depending on age and gender. It is necessary for life, and thus is one of the first considerations when looking for signs of life on other planets. But what makes it so important, what gives it its unique properties? Very few compounds have such an expansive temperature range in its liquid state. Very few compounds contract as they cool but then expand as they approach their freezing point as water does when it reaches 4 ͦC.
To find out the answers to those questions, we have to look at the structure of the water molecule in a way that some of you may never have done before. And, in the process we will visit chemistry and physics again.
Let’s start with some basic concepts of how chemical compounds are formed but first, a review of the three basic parts of an atom. Atoms have a nucleus in which are found one or more protons (+ charge) and one or more neutrons (except for hydrogen which has none and, therefore, is the lightest element) Neutrons have no net charge and, therefore, for our purpose are insignificant. Surrounding the nucleus are one or more energy levels containing electrons which have a negative charge. In terms of compound formation electros are the star players and everything else plays just a supporting role. The first energy level can hold a maximum of two electrons and the second level a maximum of eight. In some compounds like table salt (NaCl) sodium has 11 electrons (2, 8, 1) and chlorine has 17 electrons (2, 8, 7). Now since the goal is always to have the outer energy level filled it is much easier for the sodium atom to lose its one outer level electron than for chlorine to lose seven and so that is just what happens and sodium chloride (salt) is formed with totally new chemical and physical properties. But if you dissolve that salt in water it will dissociate into Na+ (ions) + Cl– (ions) and because it lost its negatively charged electron, it now still has 11 plus charges (protons) but only 10 negative charges. Chlorine, on the other hand, gained a – charge and now has 18 negative charges. A chemical bond was formed and we call such a bond resulting from a transfer of election(s) an ionic bond. Atoms that have gained or lost one or more electrons are called ions.
Water, on the other hand forms from a sharing of electrons. Written as H2O or H-O-H (the lines indicate chemical bonds). An important word about chemical bonds: they store energy, when bonds are formed energy is stored, when bonds are broken energy is released. Remember from essays VII and VIII, the former is an example of anabolism and the latter is an example of catabolism.
Hydrogen has one outer energy electron and oxygen has 6 but could hold 8. With two hydrogen atoms, each of which has one electron and the six from the oxygen atom we have the desired eight. But as I used to tell my students, the big bully oxygen atom agrees to share its six electrons but keeps the two hydrogen electrons most of the time thereby creating four areas of charges on the overall water molecule, 2 negatives around itself from the extra hydrogen atom’s electrons, plus the 6 from own atom, Meanwhile having partially lost their electrons the hydrogen atoms have a slightly + charge on their end. This unequal sharing of electrons is a covalent bond. Thus the water molecule is slightly polar which is super important. That is responsible for many of its physical properties. For instance, that creates an attraction to four other water molecules and is responsible for a high surface tension which enables some insects like water striders to walk on the surface, and kids (adults too) to “skip” stones on water. There is even a certain lizard (which I won’t give you the common name of) that can even do the same.
Credit to: Rutgers
That is also part of the reason water and drinks made from water as the solvean be sucked up in a straw and can rise to the top of very tall trees through microscopic tubes (xylem, the wood). There are, of course, other reasons for this too. But to catch up on terms here, the attraction of water molecules to each other is called cohesion, the attraction of water molecules to the sides of the straw, xylem tubes , or to glass, for that matter, is called adhesion, and the two forces together is called capillarity. If you’ve ever had two wet water glasses one inside the other get stuck, you know what I mean. And finally, the bond between water molecules is called a hydrogen bond (between a hydrogen atom of one molecule and an oxygen atom of another). During a review after we had studied this I would ask for the named of a fourth bond and, of course, no one knew. Then I would say “you mean you’ve never heard of James Bond”?
Water has a high heat of vaporization which means that it has a high boiling point. A calorie is defined as the amount of heat energy needed to raise the temperature of 1 gram of water 1 ֯ C. That’s a lot of energy compared to many other compounds. Why is this? It takes a lot of heat to break all those hydrogen bonds. Thus the saying “a watched pot never boils”. Not true, of course, it just seems that way. Changing 1 gram of liquid water to a gas requires an input of 540 calories. That’s huge and is called the heat of vaporization. So what does this mean and why is it significant? It means that water loses its heat very slowly, gains heat very slowly and freezes very slowly and at a very low temperature. The ramifications for living organisms internally and externally are almost endless. It also means that large bodies of water are the great equalizers of air temperature over solid land. Thus Seattle, Washington even through at a higher latitude than Omaha, Nebraska has more moderate temperatures year round and Chicago lakefront temperatures change less that the suburbs. This also allows animals to cool off by dunking themselves in bodies of water. It is also the reason why we feel cool when we step out of a pool or shower. We lose body heat as evaporation takes place on the surface of the skin. Note that it takes an input of 80 calories of heat to melt 1 gram of water which is also a lot of energy. Incidentally, the freezing and thawing of water has some serious implications for all drivers who have often experienced potholes in the temperate climate zones.
Water is called the universal solvent which means so many substances dissolve in it. That’s good since virtually all of the thousands of chemical reactions that must occur in our bodies need to be in solution. Exceptions include all lipids (fats, oils, etc.) which as you night guess are monopolar compounds. Finally unlike most substances, ice is less dense than liquid water which means that ice floats on water. Otherwise, bodies of water would freeze from the bottom up. Think about the consequences, especially in nature if it weren’t less dense. That happens because water unlike other liquids begins to expand at 4 ͦC. just before it freezes. Ever had to deal with frozen pipes?
Let’s review the important properties of water:
- Water expands as it freezes
- Ice is less dense than liquid water
- Water is the universal solvent
You may think I’m all wet for writing this essay but it is good scientific knowledge based on principles of physics and chemistry.
References
Mader, S. Biology, (2004 eighth edition), McGraw Hill, Boston, MA
I graduated from Northern Illinois University in 1966 with a Bachelor of Science degree in Education and earned a Master of Science degree in Education also from NIU in 1973. I taught in the Harlem School District (5 years), a Chicago suburb (1 year), and the Rockford, IL School District for 27 years (26 at East High School). I culminated my teaching career at Kishwaukee College (8 years) Two important events occurred in 1988: I married my wife Angie and I received a summer teacher’s research fellowship through the University of Illinois School of Medicine at Rockford. My primary responsibility was light microscopy and Scanning electron miscroscopy of rabbit renal arteries (effect of high cholesterol diet). For 14 years I was a citizen scientist for the Illinois Department of Natural Resources in their RiverWatch program (monitoring water quality) My hobbies and activities include gardening, golfing, bowling, downhill and cross country skiing, photography, including photomicroscopy and time lapse photography, spending time with my wife and our dog, and in the winter playing around in my small home biology & chemistry lab.
Beyond what I have written in past profiles, in the early 1980’s I was an EMT with the Boone Volunteer Ambulance & Rescue Squad (BVARS) which fit in nicely with my science training and teaching. I also enjoy public speaking and made frequent scholarship presentations to graduating seniors and outstanding middle school students through the former Belvidere Y’ Men’s Club. I also made power point presentations of the RiverWatch program. But I most enjoyed making presentations at my high school reunions. Thanks guys for allowing me to do this.