I Edlted by: MARY VIRGINIA ORNA, O.S.U. College of New Rochelle New Rochelle. NY 10801
Water, the Peculiar Molecule Mary Virginia Orna, O.S.U. College of New Rochelle New Rochelle, NY 10801
Water is a substance we take for granted. Phenomena involving water such as oceans, icehergs, raindrops, whistling teakettles, and salty and soapy solutions are part of our background consciousness. Yet, if water were not a very peculiar molecule, if it were not quite different from its surrounding analogs in the Periodic Table, none of these phenomena would exist. And if these phenomena did not exist, life as we know it would not exist on this planet. Let us take each phenomenon one a t a time and identify the physical property of water responsible for its existence. First of all. oceans exist because water is a liauid a t the mean temperatwe of the earth, i.e., around 15%. icebergs exist because the density of liquid water is greater than that of ice. Raindrops exist because of intermolecular forces of attraction due to hydrogen bonding in water. "A watched pot never boils" because water has an extremely high specific heat and a very high molar heat of vaporization. And finally, salty, soapy, and other kinds of solutions exist because water is a polar molecule that can dissolve polar and ionic substances. Why are these properties of water peculiar? First of all, the properties of water deviate markedly from those of its periodic analogs, CHa, NH3 and HF. Although the molecular weights of methane, ammonia, and hydrogen fluoride are all roughly the same as that of water, all of these compounds are gases a t room temperature. Secondly, even the group analogs of water, H A H&e and H?Te. with very much higher molecular wiightsthan wateriare also gases-at room temperature. I t is a very remarkable thing that water is even a liquid a t all. ~urthermore,water has a higher specific heat (except for liquid ammonia), a higher molar heat of vaporization and a larger liquid range (except for HF) than any of these compounds.
About the Editor hlnry V~rymiaOrna, 0 5 I ' i, P n ~ f c ~ s r.,fChrmisrry >r nt t h p d N c n R~,chclle.She rrrri\rd her dortnrarr in nnnlytieal chemistry from Fordham University in 1962 and taught at the Academy of Mt. St. Ursula in the Bronx before joining the New Rochelle faculty in 1966. Virginia is an active memher of many professional organiCdIege
zations including ACS, DIVCHED, HIST, Sigma Xi, the Inter-SocietvColor Council and the International Institute for the Conservation of Historic and Artistic Works. Her interest
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Now, why is life as we know it so dependent upon these particular properties of water? If we accept the idea of chemical evolution, that "living" molecules evolved from simpler molecular arrangements, it is also necessary to postulate the presence of a liquid medium in which numerous random molecular encounters can take place. A solid would not allow molecules the mobility necessary for many chemical reactions, and a gas would not provide the possibility of either high molar concentrations or the suspension of high molecular weight aggregates. Water, as a liquid, allows such encounters, and its other . nrooerties actuallv make it an ideal medium to . support chemical evolution, and, eventually, biological evolution. Water is a liauid over the temuerature range within S which proteins are stable. Its high speiific heat means that it E will heat up and cool down very slowly, thus providing a c built-in thermostat for primitive organisms. The fact that the densityof its liquid is greater than that of its solid means that 0 ice floats. Floating ice is susceptible to the warming temper- N atures of the sun in springtime. However, if ice, as it formed, n sank to the bottom ofthe ocean, it would never melt, the mean temperature of the ocean would he much lower than it actually A is. a i d nolar ice cans would cover a much larger orooortion of R the earth's surface. All of these factors wouliprovide a much y more hostile environment for evolving organisms. In addition, water's molar heat of vaporization is very high. Water evaporates with difficulty, resulting in the fact that the shallow S portions of oceans, lakes, ponds, rivers, etc., do not dry up each C summer. thus dlowina for the continued existence of the many LI " forms of life which must have evolved and thriven in such 0 environments. \\'i~terus a wlvenr is s l ~ l vt o pnwide the ionic l~itli~tice dnd 0 the nutrients which support lire. All uf these I~fe-wppurting I properties of water depend upon its molecular properties. Water is an angular molecule with an H-0-H hond angle of about 105' and an 0-H hond length of 0.95 .&. We may C think of a water molecule as a sphere roughly 2.8 A in diameter H with two positive and two negative charge centers distributed E in a tetrahedral arrangement on its surface. Because of its angular structure and the electronegativity difference between M oxygen and hydrogen, water has a permanent (and quite large) I dipole moment, thus providing its attractive properties for s ionic and polar molecules. Water molecules are also attracted to each other and oarticiuate in hvdrorren bonding. a fact which accounts f o r k s un&ually high melting poin
