8/8/2019 6 Water- estructura y funcion a nivel 1 molecular

1/1

THE STRUCTURE OF LiVING MATTER

is exhibited on the macroscopic scale inthe form of snowflakes. Each moiecuieis surrouned by four nearest neighbors.so that the group has one molecule at thecenter and the other four at the cornersof a tetrahedron. The moiecuies andgroups of moiecuies are joined together

by hydrogen bonds.

The forces of attracton between. themoiecuies in ice or water prodtice astrong inward pressure. As we shall see.this accounts for some of water's peculiar

properties. In the fonn of ice, its operstracture resembles a bridge arcfa under heavy dowmvard stress. When thetemperatura of the ice rises to zerocentigrade. the thermai agitation of themolecules is sufficient to cause the icestructure to collapse, and the water be-comes fluid. It is well known that theapplication of pressure from outside willraake ice melt at a lower tempsraure;e\dently this reinforces the interna!

pressure within the ice and assists itscoliapse. CoDtrariwise, we can assumethat if the intemal pressure is reduced insome way, the melting point of ice willrise. Galculations indcate that if this

pressure were entirelv eliminated. icewould noi melt until its temperaturereached 15 desrees or more centigrade(59 degrees 'Fahrenheit).

According to X-ray determinations,the average distance between the cen ter of one oxygen atom and the center of the riext in the ice crvstal is 2.72Angstrom units (an. Ansstrom being onehundred-niillionth of a centmeter), ^ ;henice mels to'liquid water, the hy- cli -ogen

bonds are stretched and the moleculesmove farther apart: the dis tance betweenoxygens is increased to about '2.9Anssiroms on the averaee. Thisstretching would open the strucrurefurther and make water less dense wereit not for the fact that in the fluid themolecules crowd together in more com-

pact groups. Each molecule is now sur-rounded by fve or inore nsishborsinstead of oalv four.

The chaotic disorder in \vhidh water moiecuies esist in the liquid state is difE-cult to picrure. Theii" arrangement siitscontinually. The ani? between tiie twohydrosen atoms in the water moiecuieno lonser remains :;ed near 3. risntangle but becomes variable, so that themoiecuie is flexible. Each oxvgen atornnow attracts by electrcal torces not nvcextra hvdrosen atoms as in ice. bu: three or more. Thus we rnav fina an oxv- s.en aiomsurrounded bv \'e or si:-; hvdrogens and aivrosen tcm sur- rounded b'~ as man-as hree c.:_.'gen. In the cise!'.- knit.flexible structure the

ICE consist of water molecules in this arrangement. The top drawing shows a model of ice seenrom one direction. The bottoni dra-wing shows the same model sen as if the reader liad turnedthe top drawing forward on a horizontal axis in the plae of the page. Some hyd-ogens have.been omitted from the molecules which touch the grid. Each hydrogen in each molecuie is

joined to an oxygen in a neighboring molecule by a hydrosen bond { r o ds i . In actuality themolecules of ice are packed more closely together: here they have been pnlied apart to show thestructure. In a similar model of liquid water the molecules would be much more looseiyoreanized, farther apart and jo ined by more hydrogen bonds.