Properties of Seawater
Seawater is ordinary water (H2O) containing dissolved mineral salts in an average concentration of 3.5 per cent by weight. Seawater also contains numerous organic and inorganic particles in suspension (undissolved). The mineral salts come from eroded rocks. Much of the rock debris resulting from erosion is ultimately carried to the sea by rivers, glaciers, and winds, and thus several billion tons of salt are added to the sea each year.
About 68 per cent of the salt in seawater is sodium chloride, or ordinary table salt. There are lesser amounts of magnesium chloride (14 per cent), sodium sulfate (11 per cent), calcium chloride (3 per cent), and other salts.
The amount of dissolved salt in seawater, expressed in parts per thousand by weight, is referred to as its salinity. The average salinity of seawater is 35. The freezing point of seawater having this salinity is 28.6° F. (-1.9° C.) compared to 32° F. (0° C.) for freshwater.
The density of seawater varies somewhat with salinity, temperature, and depth. At normal atmospheric pressure, seawater has a density that is typically about 3 per cent greater than that of freshwater. This greater density makes it easier to float and swim in seawater than in freshwater.
A person should not try to drink seawater, because the dissolved salts will increase, rather than decrease, thirst. Since the body can excrete only a small excess of salt, most of the salt accumulates in the body, withdrawing water from the blood and tissues. Large quantities can make a person violently ill and can cause death through dehydration (drying out of the body tissues).
Rainwater, which comes primarily from the ocean surface by evaporation, contains no dissolved salts because the salts do not evaporate; they remain in the ocean.
Seawater samples are obtained using various devices, such as the Nansen bottle, a cylindrical container with a lid at each end. Both lids are open during the time the bottle is being lowered on a cable, so that the seawater flows through the bottle. When the Nansen bottle has reached the desired depth, a weight is sent down the cable. When this weight reaches the bottle, it strikes a tripping mechanism that closes the lids and thus traps the seawater inside. Some types of sampling containers have lids that are closed by remote control. An apparatus called a rosette has several such sampling containers; each container can sample water at a different depth. A rosette also has sensors for measuring salinity, depth, and temperature. Seawater samples are analyzed for their free-oxygen content, the species of phyto-plankton present, and various other characteristics.
The pressure exerted by seawater increases with depth. At the ocean's surface, the pressure equals the pressure of the atmosphere above it, normally 14.7 pounds per square inch (101 kPa). The pressure increases by about this amount with every 33 feet (10 meters) of depth. At a depth of 66 feet (20 meters), for example, the total pressure is 14.7 + 29.4, or 44.1, pounds per square inch (304 kPa).
Experienced divers have descended unaided to depths of about 350 feet (107 m), and to about 435 feet (133 m) with tanks of compressed air strapped to their backs. The record for a diver in a helmeted diving suit is 728 feet (222 m). In 1960, the U.S. Navy bathyscaphe Trieste, with two men inside, reached a record depth of 35,802 feet (10,912 m), where the pressure was about 8 tons per square inch (110,000 kPa).
