Salt Water. Copyright 2012 LessonSnips

Transcription

1 Salt Water Humans need salt in their diet to achieve stable body chemistry. As Americans who frequently eat in fast food restaurants, our problem is ingesting more salt than needed rather than getting enough salt. Centuries ago salt was an important trade commodity as salt that we consume comes from mines dug into the earth. Areas with salt mines possessed a valuable natural resource that could be traded in areas lacking salt. Salt has the chemical formula NaCl, one atom of sodium combined with one atom of chlorine, and chemically is called sodium chloride. However, nature is not that simple; salt from salt mines is naturally mixed with a variety of minerals that occur in the vicinity of the mine. The food grade salt that reaches our tables is refined to remove these impurities. Although sea salt, the salt in the ocean, is fundamentally sodium chloride, it has a different mix of minerals than salt that is mined from the earth. Sea salt also contains sulfate, magnesium, calcium, potassium, and bicarbonate and the percent of each of these components within sea salt is relatively stable across ocean water around the world. The salinity of ocean water, that is the proportion of ocean water that is accounted for by sea salt, ranges between 32 to 37 parts per thousand. The saltiest ocean waters occur in the Red Sea, the Persian Gulf, the middle of the North Atlantic Ocean along the Tropic of Cancer, and the Atlantic Ocean extending outward from the mouth of the Amazon River. While the ocean around the Polar Regions might be expected to be the least salty due to seasonal melting of the polar ice, its salinity is between 33 and 34 parts per thousand. The least salty areas of the ocean are found off the coast of Alaska and northern Canada, the Pacific coast of Central America, and the Pacific coast of Asia, extending westward to the coast of India. The ocean holds so much salt that if the ocean water entirely evaporated, what would remain would be about 150 feet of salt residue over the entire ocean basin. Formation of the Ocean When the earth was formed, there were no land masses or ocean. In a process called outgassing, gases exited the early earth s crust and mantle and eventually condensed into water vapor when they reached the atmosphere.

2 Outgassing continues to the present day and occurs in fissures at the junction of tectonic plates and in the chimneys of volcanoes. Since about 71 percent of the earth s surface is ocean, most outgassing activity today takes place under ocean waters and still contributes to the chemical mix of sea water. The early condensate on earth remained in the atmosphere because the molten earth was hot enough to create clouds of steam hovering over the planet. Additionally, scientists theorize that thousands of icy comets collided with earth; these comets melted due to the earth s high temperature, contributing additional water to the steamy environment. Beginning about 3.8 billion years ago the earth began to cool, causing the steam to become water vapor. This excess of water vapor caused continuous rains that lasted for centuries and formed a vast ocean. The ocean originally contained fresh water. So how did the ocean become salt water? Water is one of the best solvents in the world. A solvent, usually a liquid, holds other chemicals in suspension within itself. With the onslaught of continuous rains, the water began to dissolve materials in the earth s newly formed crust. Over time, measured in millennia, the immense expanse of water dissolved a significant amount of material from the crust. Meanwhile, outgassing continued to add minerals originating from the earth s mantle. Plus erupting undersea volcanoes added more gases and chemicals to sea water. The sum of all these factors created the current level of salinity. Scientists estimate that the salinity of the ocean reached its present level 200 to 300 million years ago. Maintaining Stable Salinity Why doesn t the ocean get saltier if outgassing and undersea volcanic eruptions continue to occur? This question is answered by exploring naturally occurring cycles and sinks. In this context, a sink is a place where the dissolved salt in the ocean descends and enters the sediment on the ocean floor. The residue of decaying organic matter from the life cycle of marine creatures eventually reaches the ocean floor. In its journey down the ocean column, this residue attracts some of the chemicals present in the ocean salt and brings them to the ocean floor. Also as sea water evaporates in shallow parts of the ocean, the water becomes saltier.

3 Hence, there are high salt concentrations along the coastlines of the warm Pacific Ocean. This excess salty material again sinks into the sediment. Over time the sediment itself sinks and forms sedimentary rock. In subduction zones along undersea tectonic plates, sedimentary rock returns to the earth s core and becomes molten material. Meanwhile, new chemicals and organic material constantly enter the ocean, carried by rivers and streams that eventually empty into the ocean. The Amazon River drains a huge portion of the South American continent before emptying into the Atlantic Ocean. The muddy Amazon water brings with it mineral-rich water which contributes to the high salinity of the Atlantic in that region of the world.

4 Circle True or False after analyzing each of the following statements. 1. True False Salt has the chemical formula NaCl, one atom of sodium combined with one atom of chlorine, and chemically is called sodium chloride. 2. True False Sea salt is identical to the salt found below the surface in salt mines on land. 3. True False Magnesium, calcium, and potassium are some of the major components of sea salt. 4. True False Some of the saltiest ocean waters of the world occur in the Red Sea and the Persian Gulf. 5. True False Since the Amazon River dumps huge volumes of fresh water into the Atlantic Ocean, the Atlantic Ocean extending outward from the mouth of the Amazon River has some of the least salty water in the world. 6. True False Due to annual melt-off of polar ice, the ocean waters around Antarctica are some of the least salty water in the world. 7. True False Scientists theorize that thousands of icy comets collided with earth in its ancient history; these comets melted due to the earth s high temperature, contributing additional water to the steamy environment. 8. True False Outgassing refers to the process whereby water turns to steam. 9. True False The salinity of the ocean has remained approximately constant for millions of years. 10. True False In subduction zones along undersea tectonic plates sedimentary rock returns to the earth s core and becomes molten material.

5 Answers 1. True 2. False 3. True 4. True 5. False 6. False 7. True 8. False 9. True 10. True