Ocean Water Chemistry 4-3 Notes

Ocean Water Chemistry: Ocean water contains many different dissolved salts. When salts dissolve, they separate into particles called ions. Which ion is most common in ocean water? _________________________ The Salty Ocean: Salinity ● On average, one kilogram of ocean water contains about 35 grams of salts—that is, 35 parts per thousand or 3.5%. ● The total amount of dissolved salts in a sample of water is the salinity of that sample. ● The substance you know as table salt—sodium chloride—is the salt present in the greatest amount in ocean water. ● When sodium chloride dissolves in water, it separates into sodium and chloride particles called ions. Variations in Salinity ● In most parts of the ocean, the salinity is between 34 and 37 parts per thousand. ● But near the ocean’s surface, rain, snow, and melting ice add fresh water, lowering the salinity. Salinity is also lower near the mouths of large rivers such as the Amazon or Mississippi. These rivers empty great amounts of fresh water into the ocean. ● Evaporation, on the other hand, increases salinity, since the salt is left behind as the water evaporates. For example, in the Red Sea, where the climate is hot and dry, the salinity can be as high as 41 parts per thousand. ● Salinity can also be higher near the poles. As the surface water freezes into ice, the salt is left behind in the remaining water. These people are relaxing with the paper while floating in the water! The Dead Sea between Israel and Jordan is so salty that people float easily on its surface. How is the areas hot, dry climate related to the Dead Sea’s salinity? Effects of Salinity ● Salinity affects several properties of ocean water. ● For instance, ocean water does not freeze until the temperature drops to about –1.9°C. The salt acts as a kind of antifreeze by interfering with the formation of ice crystals. ● Salt water also has a higher density than fresh water. ● Because its density is greater, seawater has greater buoyancy. It lifts, or buoys up, less dense objects floating in it. ● This is why an egg floats higher in salt water than in fresh water, and why the people in the picture float so effortlessly in the Dead Sea. Other Ocean Properties Like temperatures on land, temperatures at the surface of the ocean vary with location and the seasons. Gases in ocean water vary as well. Temperature of Ocean Water ● The broad surface of the ocean absorbs energy from the sun. ● Near the equator, surface ocean temperatures often reach 25°C, about room temperature, and are warm. The temperature drops as you travel away from the equator. ● Warm water is less dense than cold water, warm water forms only a thin layer on the ocean surface. ● The deeper you descend into the ocean, the colder and more dense the water becomes. Gases in Ocean Water ● Just as land organisms use gases found in air, ocean organisms use gases found in ocean water. ● Two gases that ocean organisms use are carbon dioxide and oxygen. ● Carbon dioxide is more plentiful in the oceans than in the air. ● Algae need carbon dioxide for photosynthesis. ● Unlike carbon dioxide, oxygen is more scarce in seawater than in air. ● Oxygen is most plentiful in seawater near the surface and comes from the air and from algae in the ocean, as a product of photosynthesis. ● The amount of oxygen in seawater is affected by the water temperature. ● The cold waters in the polar regions contain more oxygen than warm, tropical waters. Changes with Depth Decreasing Temperature ● As you descend through the ocean, the water temperature decreases. ● There are three temperature zones in the water column. ● The surface zone is the warmest, at the top. ● The transition zone extends from the bottom of the surface zone to about 1 kilometer. Temperatures drop very quickly as you descend through the transition zone, to about 4°C. ● Below the transition zone is the deep zone. Average temperatures there are 3.5°C in most of the ocean. Increasing Pressure ● Water pressure is the force exerted by the weight of water. ● Pressure increases continuously with depth in the ocean. Because of the high pressure in the deep ocean, divers can descend safely only to about 40 meters. ● To observe the deep ocean, scientists must use a submersible, an underwater vehicle built of materials that resist pressure. Review Questions: 1. What is salinity and what is the average ocean salinity? - Salinity is the total amount of dissolved salts in ocean water; 3.5 % salt 2. Describe one factor that increases the salinity of ocean water and one that decreases the salinity. - Evaporation or ice formation increase salinity; rainfall, melting ice, or freshwater from rivers decreases salinity. 3. Where would you find the warmest ocean temperatures on Earth? - Found near the equator. 4. How does the temperature of ocean water affect oxygen levels in the water? - Cold water contains more oxygen than warm water does. 5. How do temperature and pressure change as you descend in the ocean? - Temperature decreases and pressure increases.