Why do lakes freeze from top to bottom?
by 
sunil gupta
ANSWERS: 9
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There are two reasons why any body of water freezes from top to bottom. The first is that air cools faster than water does. Therefore the water body is cooled from the top down. The second reason is because ice is less dense than liquid water. Therefore, ice floats. These two factors mean that ice forms at the top of water bodies and does not sink to the bottom. Another thing about water is that ice acts as a pretty good insulator. These are all things for which we should be immensely grateful. If this were not the case, then in most high latitudes, lakes would freeze up completely in winter and they would only partially melt in the summer.
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H2O is an amazing compound. It becomes more dense as it's temperature is reduced. At about 4 degrees above freezing (C degrees) it starts to become less dense. This is the fact that explain why the somewhat solid form of H2O, ice, floats on top of water. This phenomenon is caused by the angle between the two hydrogen molecules being 105 degrees. When the crystals are forming they lock together geometrically thus forming a less dense physical formation. If you dissolve salt into water you lower the freezing point and raise the boiling point. But, that is another subject for discussion.
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Hey ...as we know that pressure is directly proportional to temperature....as pressure increases going down wards there the temperature also increases....so it takes much time to cool.
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OK, if you don't want the real reason, try this. The molecules in ice move around slower than the molecules in water, so the slower ones get pushed to the top by the faster ones.
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But density IS the reason ice on water. Density and displacement. Here's an explanation of density: Rocks, for example, sink in water because rocks are more dense than water. The action of the rocks sinking is also known as displacement. Ice, on the other hand, is less dense than water - about 9% less dense. The reason water becomes less dense as it freezes is due to a chemical process called hydrogen bonding. Ice weighs less than water. If you filled up a gallon (US) jug with water, the water would weigh roughly 8.34 pounds (US). After the water froze, however, the gallon of ice would weigh around 7.5 pounds. http://wiki.answers.com/.../...llon_of_ice_weigh In a glass of ice and water, the heavier water displaces the lighter ice and sinks, like the rocks did in the first paragraph.
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I would have figured that the earth was warmer than the atmosphere and it would therefore freeze from the top. It doesn't really matter to me though... I still have to chop through the ice if I'm going to get the little fishies.
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http://www.answerbag.com/q_view/11244
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Like many other substances, when water is cooling, its density grows, so cold water sinks at the bottom. However, when water temperature reaches 4°C, its density starts to diminish. So water under 4°C comes to the top, the cooler, the higher. In the end it reaches 0°C and the water at 0°C goes to the surface, where it turns to ice. Further information: http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/waterdens.html http://www.gi.alaska.edu/ScienceForum/ASF13/1362.html http://www.usatoday.com/weather/tg/wicetop/wicetop.htm http://en.wikipedia.org/wiki/Water_%28molecule%29#Density_of_water_and_ice
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Hydro-thermal-stratification.
Glenn Blaylock 
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