Salt Water Earths Oceans and Seas

The vast layer of salty water that covers so much of the planet is in actuality one large connected body and is generally constant in composition. For this reason, oceanographers speak of a world ocean rather than of the separate oceans we learn about in geography books. 

Seawater is often referred to as saline water. The salinity of seawater is the mass in grams of dry salts present in 1 kg of seawater. In the world ocean, salinity averages about 35. To put it another way, seawater contains about 3.5% dissolved salts by mass. The list of elements present in seawater is very long. Most, however, are present only in very low concentrations. T Table 18.5 lists the 11 ionic species most abundant in seawater. 

The sea is so vast that if the concentration of a substance in seawater is 1 part per billion (1 X 10 g/kg of water), there is 1 X 10 kg of the substance in the world ocean. Nevertheless, because of high extracting costs, only three substances are obtained from seawater in commercially important amounts sodium chloride, bromine (from bromide salts), and magnesium (from its salts). 

Absorption of CO2 by the ocean plays a large role in global climate. Because carbon dioxide and water form carbonic acid, the H2CO3 concentration in the ocean increases as the water absorbs atmospheric CO2. Most of the carbon in the ocean, however, is in the form of HC03 and CO ions, which form a buffer system that maintains the ocean s pH between 8.0 and 8.3. The pH of the ocean is predicted to decrease as the concentration of CO2 in the atmosphere increases, as discussed in the Chemistry and Life box on ocean acidification in Section 17.5. 

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