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cheungcheung

water

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because the molecules in ice are very nicely and very compactly packed and are not spread out as they are in liquid water. there for, ice weights dramatically less then water.

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Err you've got that slightly confused. Ice is more ordered, but the bond lengths between molecules is larger, so they are more spread out.

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Well if the hot water rises and the cold water sinks, then the cold water is the cooled warmer water.Because that cold water has to come from somewhere.

Mmm convection.

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Cos the air is colder than the water so it'll freeze on top, then as ice is lighter than water it'll float.

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15 degree water is warmer than 5 degree water. Just because warmer water is at the top doesnt mean it is actually warm. Hot and cold are relative.

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I recently read "H2O: A Biography of Water" by Philip Ball. A very interesting book about a fascinating topic. The following is condensed from this book.

 

The structure of liquid water is determined more because of the attractive forces between the hydrogen bonds rather than the repulsive forces between molecules. Because the attractive forces are much stronger than the repulsive forces, the packing constraints are considerably lower for water than for other liquids, so the molecules pack more densely. The hydrogen-bonded network in liquid water is dynamic and disordered. When water freezes the hydrogen-bonded network locks into a rigid crystal lattice, and the upshot is that the space between adjacent molecules increases. So the density of ice is less than that of liquid water, and so ice floats. There are 13 different ice structures that have been identified to date, synthesized by varying the temperature and pressure during freezing. Ice-VII remains solid at temperatures above 100 C and has a density about 2 times greater than ice-I (normal ice).

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It has to do with the density of the water/ice, not their temperatures (in reply to #2-#6).

 

Like Sky said, ice has a less efficient bonding structure than water, which is why the same mass of water is smaller in a liquid state than it is in a solid state.

 

Increased volume with fixed mass = lower density.

 

Lower density water floats on higher density water.

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It has to do with the density of the water/ice' date=' not their temperatures (in reply to #2-#6).

 

Like Sky said, ice has a less efficient bonding structure than water, which is why the same mass of water is smaller in a liquid state than it is in a solid state.

 

Increased volume with fixed mass = lower density.

 

Lower density water floats on higher density water.[/quote']

 

Plus water's density does have a temperature dependence, so the assumption that the coldest water sinks isn't valid - water at 4 C is densest. (for fresh water, anyway.)

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I think most people know ice is less dense than water but the original question had to do with why the cold water is at the top to form the ice in the first place.

Are you saying the ice is formed at the bottom and floats to the top?

Or is it the fact that the water at the top is warmer than the water at the bottom but it is still below freezing?(like i said)

Or maybe its both?

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About the question of how the oceans freeze, this is from "H2O: A Biography of Water" by Philip Ball.

 

All the oceans are interconected and their water is constantly passing in concerted masses from one ocean to another both vertically and horizontally. Wind only drives the ocean surface currents to depths of about 100 meters. Deep circulation of the oceans (between 1-5 kilometers) is driven largely by difference in water temperature and forms a conveyor-belt flow that redistributes heat around the planet and links all 3 of the oceans via the Southern Ocean. Water that has been warmed in the tropics flows poleward, and as it does it becomes cooler and the surface becomes saltier due to surface evaporation (increased density). This dense cool water (called North Atlantic Deep Water) eventually becomes heavier than the water below and sinks before starting the return flow southward. A portion of this cool dense water will freeze as sea ice, leaving the remaining water even saltier. This denser water sinks even deeper and is the densest water in the oceans (called Antartic Bottom Water).

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You are also forgetting the pressure, down in the water the pressure increses and is harder to make ice and either to reduce temperature.

 

There is less presure up (the division between iced sea and sky(ground level)) than below where the pascals increase while going down.

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