Water heaviest at 4 degrees

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I know that water is the most dense at 4 degrees celsius. That's why water at the bottom of a sea has that temperature. But how did it get that temperatur in the first place and why does the energy not distribute itself in such a way that the temperature is the same throught? Is it the particles that have a temperature of 4 degrees that sinks to the bottom or is it the pressure at the bottom that makes the water more dense?

What if a sea is surrounded by constant weather condition and constant atmospheric temperature. How would the temperature distribution look like as a function of altitude under water? If the surrounding temperature is 20 degrees, would the water at the bottom if it is deep enought still be 4 degrees?
 

Answers and Replies

  • #2
jack action
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The reason water is denser at 4°C is because of the particular way molecules are disposed with respect to other at that temperature.

Molecular disposition of ice (voids are present):
spacefillWater.gif


Molecular disposition of water (voids are filled):
water_caption.gif


There's good reading on Wikipedia:
[...] When cooled from room temperature liquid water becomes increasingly dense, as with other substances, but at approximately 4 °C (39 °F), pure water reaches its maximum density. As it is cooled further, it expands to become less dense. This unusual negative thermal expansion is attributed to strong, orientation-dependent, intermolecular interactions and is also observed in molten silica.[21]

The solid form of most substances is denser than the liquid phase; thus, a block of most solids will sink in the liquid. However, a block of ice floats in liquid water because ice is less dense. Upon freezing, the density of water decreases by about 9%.[22] This is due to the 'cooling' of intermolecular vibrations allowing the molecules to form steady hydrogen bonds with their neighbors and thereby gradually locking into positions reminiscent of the hexagonal packing achieved upon freezing to ice Ih. Whereas the hydrogen bonds are shorter in the crystal than in the liquid, this locking effect reduces the average coordination number of molecules as the liquid approaches nucleation. Other substances that expand on freezing are acetic acid, silicon, gallium, germanium, antimony, bismuth, plutonium and also chemical compounds that form spacious crystal lattices with tetrahedral coordination.

[...]

In cold countries, when the temperature of fresh water reaches 4 °C, the layers of water near the top in contact with cold air continue to lose heat energy and their temperature falls below 4 °C. On cooling below 4 °C, these layers do not sink as fresh water has a maximum density at 4 °C. Due to this, the layer of water at 4 °C remains at the bottom and above this layers of water colder than 4 °C are formed. As water at 0 °C is the least dense it floats on the top and turns into ice as the water continues to cool. Ice growth continues on the bottom of the ice as heat is drawn away through the ice (the heat conductivity of ice is similar to glass). All the while the water further down below the ice is still 4 °C. As the ice layer shields the lake from the effect of the wind, water in the lake will no longer turn over. Although both water and ice are relatively good conductors of heat, a thick layer of ice and a thick layer of stratified water under the ice slow down further heat loss from the lake relative to when the lake was exposed. It is, therefore, unlikely that sufficiently deep lakes will freeze completely, unless stirred by strong currents that mix cooler and warmer water and accelerate the cooling. Thus, as long as the pond or lake does not freeze up completely, aquatic creatures are not exposed to freezing temperatures. In warming weather, chunks of ice float, rather than sink to the bottom where they might melt extremely slowly. These properties therefore allow aquatic life in the lake to survive during the winter.
Heat goes from hot to cold, no matter what, until equilibrium is reached. So in theory, in a constant environment, given enough time, water - or any other substance - will reach a uniform temperature.

But the ocean is far from being surrounded by a uniform temperature (top and bottom) and there are lots of currents adding variables to the mix.
 
  • #3
CWatters
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how did it get that temperatur in the first place and why does the energy not distribute itself in such a way that the temperature is the same throught?
The planet isn't the same temperature all over, it's colder near the poles and warmer at the equator. Google the Great Ocean Conveyor Belt.

http://oceanservice.noaa.gov/facts/conveyor.html

The ocean is not a still body of water. There is constant motion in the ocean in the form of a global ocean conveyor belt. This motion is caused by a combination of thermohaline currents (thermo = temperature; haline = salinity) in the deep ocean and wind-driven currents on the surface. Cold, salty water is dense and sinks to the bottom of the ocean while warm water is less dense and remains on the surface.

The ocean conveyor gets its “start” in the Norwegian Sea, where warm water from the Gulf Stream heats the atmosphere in the cold northern latitudes. This loss of heat to the atmosphere makes the water cooler and denser, causing it to sink to the bottom of the ocean. As more warm water is transported north, the cooler water sinks and moves south to make room for the incoming warm water. This cold bottom water flows south of the equator all the way down to Antarctica. Eventually, the cold bottom waters returm to the surface through mixing and wind-driven upwelling, continuing the conveyor belt that encircles the globe.
This effect works to chill the ocean floor faster than other mechanisms can heat it.

Edit: or perhaps I should say it chills the ocean floor at roughly the same rate as other mechanisms can heat it. If you ignore man made global warming then the system is roughly in equilibrium.
 

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