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Waterby Arthur M. Buswsll and WoriH. Rodebush

Aprill956

Although we take its prapeties for granted, they are mostunusual. As an example, it has the rare property of being

lighter as a solid than a liquid. If it were nor, lakes wouldfreeze frorn the bottom up!

Water is the only common liquid on our planet.Next to air it is the substance with which weare most intimately acquainted. Because it is sofamiliar, we are apt to overlook the fact thatwater is an altogether peculiar substance. Its

properties and behavior are quite unlike thoseof any other liquid. To take just one example,water has the rare property of being denser asa liquid than as a solid, and it is probably theonly substance that attains its greatest densityat a few degrees above the freezing point(four degrees centigrade). The consequencesof this behavior are of great importance to lifeon our planet. When ice forms on a lake, forexample, its lower density (only nine tenthsthat of liquid water) keeps it on top and it actsas an insulating blanket to retard cooling of

the underlying water. As a result lakes in thetemperate zones do not freeze solid to the

bottom but leave a zone for the, wintersurvival of aquatic life. On the other hand thesame peculiar property of water has fatalconsequences for living cells. When water inthe cells freezes and becomes less dense, itsexpansion damages or breaks up the cells.

Even the elements of which water is

composedoxygen and hydrogenare

chemically exceptional. Both are unusuallyreactive. Oxygen is our chief source ofenergy, being responsible for the respirationof living organisms and the combustion offuels. Hydrogen, unique in the fact that ithas no enclosing; shell but only a singleelectron, is able to attraction of otheratorns not only by means of its electrn (avalence bond) but also by virtue or theattraction or its unoccupied. positivelvcharsecl sicie fcr an electrn in a secondatcm. Ths attschment is known as thehydrogen

bond. In water the two hvdrosen at.omatached to eaoh oxygen atom can 'be-comlinked to other atoms as well by means othese so-6lled hydrogen bonds. As consequence the H

2O mole-cules are joine

tosether. so tha water should be considereor a collection o seprate moecules bu unied asso-ciation. In effec:: the whole masof water in a vessel is a single molecule.

The best method of detctela hydrogebonds is to studv water with he in-frarespectrograph. We have found tha thydrogen bond absorbs radiaion rnostronglv. at -a wavelength of about thremicrons, which is in hanear infra-red regiof hea radiarani.e.

: cise o he visibl

light speotram. Liauid'water absorbs th

radiaion so powerfully tha i our eyes wersensitive to he in-rared. water woulappear jet black. There is sorae absorptioeven in the visible spectrurn at the red eneThe fac

MOLECULE o water consist; of one oxygen atom (black} ana two hyrogen atoms (whiie'i.

The distancs betwesn the center of he osygen atc-ms and the centsr o each ai the liyrosen

atoms is .9 Angstrom un: on e Angstrom unit: .00000001 centimeter't. The augie formad by the

two hydrogen atonis is 105 degrees. These dimensioas are Sted to-asther ir. the drawing at leit. In

the more schematic drawing at risht the size of the atoros ha= Leen reuced. Thii representation o

i.=e moiecuie :s aeed in ti:^ leOWiisE drav.-ingi..

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tha water absorbs- red light accouns f or ischiacieristic blue-green color,

Water's 33 Cornponents

One of the shocks o our familiar no- ionsabou water is ha its formula is not simplv

H:O, Nor is i a single sub stance. The

beginning of this disiihi-sionment carne in1934 when Harold rey discovered "heav\-water." Urev found that he puresi watercontained besides hydrogen and oxygenanoher substance like hydrogen but with anatomic weisht of two. or raice that ofhvdrogen, This substance, which is now

called deuterium. combines with osygen tform the compound D

2O. By now. we know

of course, that there is a third iso- tope ohydrogen. called tritium. and three isotopeof osygen: 0-16. 0-17 and -lS, Thus th

purest water that can be prepared in thlaboraron-' i made up