Physical properties of sea water

Water makes up more than 96% of the total mass of seawater, so it is appropriate to begin with a short discussion of the properties of water itself. The weather and climate on Earth are in several ways controlled by the physical properties of water.

Some of the important properties are, 

a. its expansion when freezing, 

b. its infrared absorption causes water vapor to be the most important greenhouse gas, 

c. its radiative properties cause water to be important in the radiation of heat away from Earth,

d. Variations from place to place in its isotopic composition provide insights into several aspects of Earth science,

e. The chemical property of water that is perhaps most important to the marine chemist is its high solvent power for polar substances and substances that form charged ions in solution etc., 

 Physical properties of water:

     The boiling point of water is unusually high. This very high boiling point suggests that the water molecules must be exceptionally attracted to each other, and this strong tendency for the molecules to hang together is also advanced to explain many of the other anomalous properties of water.

These unusual properties ultimately result from the shape of the molecule itself. In a water molecule, the two hydrogen atoms are arranged at an angle of about 104.5o to each other. There is a small tendency for the electrons to be concentrated towards the oxygen and away from the hydrogen atoms, so that the weighted average locations of negative and positive charges are separated, the molecule is a dipole, and the molecules orient in an electric field. This feature of the molecule also leads to the rather strong tendency for hydrogen bonds to form. This results in a relatively high freezing point, and in strong associations between the molecules in the liquid state as well.

 

Density:

    When ice is formed, the water molecules become oriented into a regular tetrahedral structure, shown in Figures b, which occupies even more volume than in the liquid state. Water is one of the very few substances that increase in volume upon freezing. The increase is about 9%, so ice floats. Were this not the case, the climate and other aspects of the physical nature of the Earth would be vastly different.

    A peculiarity of pure water is that as it is cooled the density reaches a maximum at 3.98° C, well above the freezing point, and then decreases slightly towards the freezing point.

 

    When a lake, for example, cools to this point the densest water fills the lake at this temperature from top to bottom. With further cooling the water becomes less dense and floats on the top. Ice is even less dense and floats above this. Deep lakes therefore cannot get colder than 3.98° C on the bottom, unless vigorously mixed by the wind.

    The presence of salt in the water must disrupt this tendency toward an ordered arrangement, the more salt the lower the temperature of maximum density. 

    This phenomenon is important in the ocean because it leads to the necessity for the entire water column to cool to the freezing point before freezing begins

 

Reference: An Introduction to the Chemistry of the Sea - MICHAEL E. Q. PILSON