The UN resolution introduced the annual World Water Day, which is celebrated in different states on March 22. On this day, events are held whose purpose is the conservation and development of water resources that are in rivers, lakes, canals, storages, seas and oceans, in underground sources in the form of soil moisture, ice of polar and mountain or vapor in the atmosphere. All resources include underground and surface sources and can be used or are used for human needs. Their total volume (in liquid, gaseous or solid state) on the Earth is 1390 million cubic meters, and only less than 3% of all resources are fresh reserves.
The oceans and seas contain the largest amount of water (approximately 98%) of all surface sources. Its chemistry varies depending on the terrain and time of year. Sea water contains much more dissolved ions than any kind of fresh. It also differs sharply in the ratio of substances dissolved in it. For example, the seas and oceans contain about 2.8 times more bicarbonates than in rivers, but their molar fraction in relation to all dissolved ions is much lower (0.14%) than in river sources (48%). This is due to the different residence times of the solutes. Sodium and chlorine ions are in a dissolved state, while calcium precipitates as carbonate. According to the content in a kilogram of sea water, the most common ions are:
- chlorine (0.546 moles);
- sodium (0.469 moles);
- magnesium (0.0528 moles);
- sulfate (0.0282 moles);
- calcium (0.0103 moles);
- potassium (0.0102 moles).
One of the most important indicators that characterize sea water is salinity. It is measured in ppm (‰) and is different for all seas and oceans. The average salinity is 35 ‰, that is, in 1 liter in a dissolved state there are 35 g of salts, mainly sodium chloride. At medium salinity, the elemental composition of water is as follows:
- oxygen 85.84%;
- hydrogen 10.82%;
- chlorine 1.94%;
- magnesium 1.1292%;
- sodium 1.08%;
- sulfur 0.091%;
- calcium 0.04%;
- potassium 0.04%;
- bromine 0.0067%;
- carbon 0.0028%.
Sea water on the surface at 25 ° C has an average density of 1,025 g / cm3 - this is higher than that of clean water, whose density at 25 ° C is 1.003 g / cm3. Density - a physical quantity that has a direct dependence on the concentration of salts and the inverse of temperature. The more salts are dissolved, the higher the density. The lower the temperature of the sea water, the greater the density. The temperature at the highest density is minus 3.52 ° C for marine and plus 3.98 ° C for clean. But these values vary not only from source to source, but also depend on the depth at which they are measured. At great depths, the density can reach 1.050 g / cm3 and even higher. The change in the qualitative and quantitative composition is also well traced by the pH value, which varies from 7.5-8.4 (slightly alkaline medium) and is due to the nature of the dissolved ions and their concentration. At the surface, the pH is higher than at depth. Neutral and slightly acidic may be the environment of desalinated areas. The highest pH value (the lowest concentration of hydrogen ions) is always observed at the sea surface.
The freezing temperature of sea water decreases with increasing salinity (concentration of dissolved salts). At moderate salinity, it freezes at a temperature close to minus 2C. In 2010, the lowest temperature in the stream under the Antarctic glacier was recorded throughout history, when minus 2.6C was measured. There are other characteristics that show the difference between the sources, as well as their variability depending on other parameters. For example, the speed of sound at 0C. It passes through the marine environment in 1 second 1450 m, and through the clear - 1407 m. Sea water at 25 ° C has a dynamic viscosity of 9.02 millipoise. For a clean environment, the same characteristic at the same temperature is 8.09 millipoise.
Sea water is not used for drinking due to the high concentration of mineral salts. When using it, you need to drink a lot more clean liquid to remove all the salts that came with it from the body. Therefore, they drink only fresh water in which not more than 0.001 g / cm3 of salts are dissolved. Due to its shortage, plants are built and special methods are used to reduce the concentration of salts. These include freezing, distillation, electrolysis, ion exchange, reverse osmosis, separation (hydrodynamic separation). For example, in Kazakhstan, the city of Aktau (formerly called Shevchenko) was built in 1972 and has been operating for almost 30 years, the world's only atomic water desalination plant.