Understandably, the capacity of water to become denser on mixing has the most essential 

 effect on ocean dynamics and is of greatest importance in the polar regions. 



LITERATURE: 77. 



Section 7. The Properties of Sec Water of Higfi Salinity 



The average salinity of ocean water is 35 o/oo, but in seas where evaporation exceeds precip- 

 itation and influx of shore waters, the salinity is somewhat higher. 



The increase in salinity with evaporation is to be explained by the fact that in this process 

 only a very insignificant part of the salts dissolved in the sea water escapes into the atmosphere. A 

 second possibility of increased salinity of sea water is ice formation, since the salinity of ice is al- 

 ways considerably less than that of the sea water from which it is formed. 



Let us assume in first approximation that pure ice, free of all salts, is formed from sea water 

 and let us trace the phenomena which occur during the cooling of an isolated amount of sea water. 



After the water temperature, on cooling, reaches the freezing point, ice begins to separate 

 from it, which causes increased salinity of the remaining sea water. A new decrease in tempera- 

 ture is required for further ice formation Thus, the concept of a freezing point for sea water 

 differs from that usually taken for fresh water. Actually, if we keep any amount of fresh water at a 

 constant temperature slightly below zero for a sufficient interval of time, eventually it will freeze 

 completely without a residue If, however, we take sea water, we can keep it at a constant temper- 

 ature below the freezing point for as long as we like In this case only a very definite amount of 

 pure ice will be separated from it, which will raise the salinity of the remaining volume to the 

 extent that further ice formation at that temperature will be impossible. If the temperature is 

 lowered, the ice production and the salinity of the remaining solution will be increased. After pro- 

 longed cooling and ice formation, the salinity of the sea water may increase to the extent that 

 eutectic phenomena will begin in the solution. 



Let us assume that we have a solution of a single salt, e g. , table salt and subject this solu- 

 tion to cooling. At some temperature below zero (depending on the concentration of the solution) 

 pure ice will begin to form and subsequently the solution concentration will gradually begin to in- 

 crease, each temperature will have a corresponding definite solution concentration. The formation 

 of pure ice will continue until the temperature reaches -21. 9°, until the concentration of the solu- 

 tion becomes 22.4 o/oo. After this, with further cooling, the entire solution as a whole will harden 

 into a conglomerate representing a mixture of ice and salt crystals and called a cryohydrate or 

 eutectic mixture. 



If the concentration of table salt at the initial moment at a high temperature is greater than 

 22.4 o/oo*, on cooling the salt will begin to precipitate out from the solution again such that for 

 each temperature there will be a corresponding definite amount of salt saturating the solution at the 

 given temperature. This phenomenon would continue until the concentration of the solution became 

 22. 4 o/oo and the temperature -21. 9°, at which point the entire solution would harden as a cryohy- 

 drate. The solution concentration 22. 4 o/oo and temperature -21. 9° are the eutectics of table salt. 



The difference in the eutectic temperatures of the salt solutions forming the sea water com- 

 plicates the chemical processes during intense salinification caused by ice formation. 



*A saturated solution of table salt at 0° is about 27 g per 100 g water. 



16 



