I’ve been told that water is at its heaviest when its temperature is 4 degrees centigrade, and because of this, water at the bottom of the sea, lakes, rivers and so on is 4 degrees warm. If this is true (and correct me if it isn’t), I have a question.
Is water at 2 degrees centigrade the same temperature as water at 6 degrees centigrade? If it is, why doesn’t water at those temperatures end up at the same place and even out at 4 degrees centigrade? Basically, how does temperature affect the weight of water?
Water is most dense at 4 C. Most substances “shrink” (get more dense) as they cool, eventually there is a “state change” where it goes from liquid to solid. This is the molecules going from “jiggling and loose, but associated” to a tight rigid structure. There is a similar state change from gas (vapour) to liquid.
Water is unusually whereby the solid stucture is more open than the liquid, therefore ice is less dense than liquid water and, because of the bouyancy principle, it floats on top of the liquid. This re-organization into an open structure starts at 4 C, so at that point it is at it’s most dense.
Water freezes from the top down - as the water approaches 0 C. it starts to “percolate”, ie the coldest rises and mixes with the less cold. Eventually, the top layer stabilizes at 0 C. and then the state change can occur. This answers your question - the different temperatures mix and average out until everything is cold enough and you get 0 C and ice on the top, and slightly warmer liquid below. The densest liquid will be at 4 C and sinks to the bottom.
Isaac Asimov once postulated that, if not for water’s unusual characteristics, Earth would be a cold dead planet, with the oceans freezing from the bottom up, and only the top few feet liquid. There would be no algae/plankton rich layers at the bottom to support fish and mammals at the top.
Err, the algae/plankton rich layers are at the top; that’s where photosynthesis and primary prodution occur. They are often fed by nutrient rich waters from below, which might be a problem if they’re all froze up. Plus killing off all the microorganisms on lake/ocean/stream/slough beds too. Also ice on top of lakes and rivers gives the water below some insulation to prevent the rest from freezing solid. If ice started forming at the bottom, the liquid water would constantly be in contact with the freezing air and continue to freeze, possibly entirely (which would also be bad for aquatic creatures).