Heavy ice

Europhysics News, Jul 2018

L.J.F.(Jo) Hermans

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Heavy ice

L.J.F. 0 Hermans 0 0 Leiden University , The Netherlands - - ater is a vital substance for life on It is interesting to speculate on what would happen if planet Earth. This is obvious for liq- water would behave like most substances, i.e., if it would uid water with its crucial role for any shrink upon freezing. The freezing process would be enspecies living on the globe. But also tirely different. To begin with, the very first pieces of ice solid water plays a more important role than one might that are formed at the surface would sink to the bottom, expect. For one thing, in the old days before people man- exposing a fresh layer of water ready to be frozen at the aged to build bridges across rivers,the formation of ice in top. The freezing top layer would now remain directly in winter time increased the mobility of mankind substan- contact with the cold air above, and the slowing down of tially.And – for the sporty among us – ice is great fun to the freezing process described above would not occur. skate on. This pleasant aspect of ice was touched upon in Instead, the pond would freeze rapidly from the bottom an earlier column (EPN 41/6; Physics in Daily Life p. 29). up. The poor fish would have no choice but to swim The freezing process itself has some remarkable as- higher and higher until the whole pond had frozen solid. pects. For example, in Poor fish! most of Europe, the ice It’s even worse. In cold layer on lakes and ponds winters, many lakes never reaches the bottom, out s i d e t h e t ropi c s which is good news for fish would behave precisely and other creatures living the same, and most of in the water. As physicists, the marine life would we realise that the basic be ruined. And that’s reason behind this is that not even the end of the the ice layer grows more story. Just think of the slowly as it gets thicker. glaciers in Greenland Remember that the ice or Antarctica, sliding layer grows at its bottom, into the ocean. Instead where the temperature is of forming floating iceconstant at 0 °C by defi- bergs as they do today, nition, and that the heat they would sink to the of solidification must be bottom, out of reach of transported through the the sunrays that would ice layer to the cold air melt them. Sooner or above. Let us assume for simplicity that the temperature later the deep sea would be filled with ice, instead of at the top of the ice layer is constant, so that the heat flow with water at 4 °C as it is today. That would completely though the ice layer of thickness h is proportional to 1/h. upset life in the deep ocean. Since the growth rate dh/dt is proportional to the heat Fortunately there is a bright side to the story, and things flow, we have dh/dt ~ 1/h, yielding h~√t. might not be as bad as sketched above. The density of In other words, if air temperature and wind speed are sea water is about 1.027 kg/l, and it would go up even constant, the ice layer thickness is proportional to the higher if the above scenario were to materialize. So, if square root of time. freezing water would shrink only a little bit, the oceans All this relies on a rather unique property of water: the fact would remain safe: only our ponds and lakes would that it expands when it goes solid.For most substances this freeze solid.And our favourite fish restaurant would no is not the case.For example,if we look at the elements,only longer serve pike-perch with champagne sauce, but just gallium,germanium,silicon and bismuth have that property. herring, salmon and cod. n

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L.J.F.(Jo) Hermans. Heavy ice, Europhysics News, 19-19, DOI: 10.1051/epn/2013101