Solids in Seawater

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. Look For The Following Key Ideas In Chapter 7 The polar nature of the water molecule produces some unexpected chemical properties. One of the most important is water's remarkable ability to dissolve more substances than any other natural solvent. The most abundant ions dissolved in seawater are chloride, sodium, sulfate. The quantity of dissolved inorganic solids in water is its salinity. The proportion of ions in seawater is not the same as the proportion in concentrated river water, which indicates that ongoing geological and chemical processes affect the ocean's salinity.

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. Water Is a Powerful Solvent A simplified hydrologic cycle. Water moves from ocean to air, onto land, to lakes and streams and groundwater, back to the sky and ocean, in a continuous cycle. The numbers indicate the approximate volumes of water in cubic kilometers per year (km 3 /yr). Water is also stored in the ocean, ice, groundwater, lakes, and the atmosphere.

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. Water Is a Powerful Solvent What are solutions and mixtures? A solution is made of two components, with uniform molecular properties throughout: The solvent, which is usually a liquid, and is the more abundant component. The solute, often a solid or gas, is the less abundant component. A mixture is different from a solution. In a mixture the components retain separate identities, so it is NOT uniform throughout.

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. Water Is a Powerful Solvent Salt in solution. When a salt such as NaCl is put in water, the positively charged hydrogen end of the polar water molecule is attracted to the negatively charged Cl - ion, and the negatively charged oxygen end is attracted to the positively charged Na + ion. The ions are surrounded by water molecules that are attracted to them and become solute ions in the solvent.

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. Salinity Salinity is the total quantity of dissolved inorganic solids in water. Water’s colligative properties are: The heat capacity of water decreases with increasing salinity As salinity increases, freezing point decreases As salinity increases, evaporation slows Osmotic pressure increases as salinity increases

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. A Few Ions Account for Most of the Ocean’s Salinity A representation of the most abundant components of a kilogram of seawater at 35‰ salinity. Note that the specific ions are represented in grams per kilogram, equivalent to parts per thousand (‰).

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. The Components of Ocean Salinity Came from, and Have Been Modified by, Earth’s Crust Processes that regulate the major constituents in seawater. Ions are added to seawater by rivers running off crustal rocks, volcanic activity, groundwater, hydrothermal vents and cold springs, and the decay of once-living organisms. Ions are removed from the ocean by chemical entrapment as water percolates through the mid-ocean ridge systems and seamounts, sea spray, uptake by living organisms, incorporation into sediments, and ultimately by subduction.

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. The Ratio of Dissolved Solids in the Ocean is Constant Forchhammer’s principle, also known as the principle of constant proportions states that although the salinity of various samples of seawater may vary, the ratio of major salts is constant. How do scientists determine the salinity of seawater? Salinity can be determined by measuring the chlorinity of the sample. Since the chlorinity is easy to measure, and the principle of constant proportions applies to all seawater, scientists can use the following formula to determine salinity: Salinity in ‰ =  Chlorinity in ‰

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. The Ocean Is in Chemical Equilibrium Is the ocean becoming progressively saltier with age? No, the ocean is in chemical equilibrium. The proportion and amounts of dissolved solids remain constant. This concept is known as the “steady state ocean.“ Ions are being added to and removed from the ocean at the same rate. Residence time is the average length of time an element spends in the ocean. Residence time can be calculated by the equation: Residence Time = ___Amount of element in the ocean___ The rate at which the element is added to (or removed from) the ocean

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. Seawater’s Constituents May Be Conservative or Nonconservative Conservative constituents of seawater are those constituents that occur in constant proportions. Conservative elements have long residence times and are the most abundant dissolved material in the ocean. Nonconservative constituents have short residence times, and are usually associated with seasonal, biological or short geological cycles.

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. Lecture In Perspective In this chapter you learned that water has the remarkable ability to dissolve more substances than any other natural solvent. Though most solids and gases are soluble in water, the ocean is in chemical equilibrium, and neither the proportion nor amount of most dissolved substances changes significantly through time. Most of the properties of seawater are different from those of pure water because of the substances dissolved in the seawater. About 3.5% (35‰) of seawater consists of dissolved substances. These almost always exist as ions – “salts” don’t exist in the ocean. The most abundant ions dissolved in seawater are chloride, sodium, and sulfate. Seawater is not concentrated river water or rainwater – its chemical composition has been altered by circulation through the crust at oceanic spreading centers and by other chemical and biological processes.

End of Chapter 7