© 2006 Brooks/Cole, a division of Thomson Learning, Inc.  Sea Water Chemistry

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. Key Concepts 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. Though most solids and gases are soluble in water, the ocean is in chemical equilibrium, and neither the proportion nor the amount of most dissolved substances changes significantly through time. Gases dissolve in water in proportions that vary with their physical properties. Nitrogen is the most abundant dissolved gas in seawater; oxygen is the second most abundant. Carbon dioxide is the most soluble gas, and one of many substances that affect the ocean's pH balance. Seawater acts as a buffer to prevent broad swings of pH when acids or bases are introduced.

© 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. Sea Water Chemistry Water as a solvent –Water can dissolve more substances than almost any other liquid –What are some examples? Salt Sugar Oil –Why is water such an unusual solvent? Polar molecule Positive and negative ends Hydrogen bonds The ability to hydrate

© 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. Sea Water Chemistry Chemical Mechanism Remember discussion about ions An atom or group of atoms that has an unbalanced electrical charge True because one of the atom gives up an electron and the other receives an electron Atoms are held together do to the electrical force of opposite electrical charges Called ionic bonding

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. Sodium Chloride as an example Sodium ion is positively charged Chlorine is negatively charged Impact of the solution of water and sodium chloride Due to polar nature of water H 2 0 water molecules pull ionic NaCl apart Water molecules surround Na + and Cl - and prevents them from reuniting

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. Atomic charge balance –All atoms contain same number of protons and electrons. Neutrons may vary Electron level shells –Level 1 maximum 2 electrons –level 2 and up 8 –Level 3 up to 18 –Level 4 up to 32 –Outer most shell maximum of 8 electrons

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. Review of the nature of bonds Nucleus –Protons –Electrons Electrons –Exist in a cloud –Arranged in energy levels Atomic Number –Protons plus neutrons –Periodic table –Isotopes

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. Bonding Octet rule Atoms combine in order to attain 8 electrons in the outer energy level Allows the atom to have stability like the noble gases which naturally have inert outer shells In order to satisfy the octet rule an atom can gain, lose or share electrons with other atoms The electrons form an electronic glue which hold atoms together

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. Valence electrons –The electrons present in the outer energy level that are available for atomic bonding –The number of valence electrons will determine the number of bonds it can form –For example: Si has 4 valance electrons so it tends to form four bonds to obtain a stable 8 electrons in the highest energy level Oxygen has 6 electrons forms two bonds Hydrogen has one electron forms one bond

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. Ionic bonds –One or more electrons are transferred to another atom One atom becomes stable by giving up an electron and the other becomes stable by gain an electron The result is that oppositely charged ions attracting one another to produce an electrically charged state –Sodium Chloride (NaCl) Na has atomic number of 11 – Electrons configuration of – Thus Na needs to lose and electron in its out level

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. Chlorine (Cl) has an atomic number of 17 Electron configuration There are seven electrons in the out level and needs 1 to be stable Sodium and Chlorine form an ionic bond Sodium gives up an electron and becomes positively charged (cation) Chlorine gains and electron and becomes negatively charged (anion)

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. Covalent Bonds Share electrons Water is an example of covalent bonds

© 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. Gas Concentrations Vary with Depth How concentrations of oxygen and carbon dioxide vary with depth. Oxygen is abundant near the surface because of the photosynthetic activity of marine plants. Oxygen concentration decreases below the sunlit layer because of the respiration of marine animals and bacteria, and because of the oxygen consumed by the decay of tiny dead organisms slowly sinking through the area. In contrast, because plants use carbon dioxide during photosynthesis, surface levels of CO2 are low. Because photosynthesis cannot take place in the dark, CO2 given off by animals and bacteria tends to build up at depths below the sunlit layer. CO2 also increases with depth because its solubility increases as pressure increases and temperature decreases.

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth What are acids and bases? An acid is a substance that releases a hydrogen ion in solution. A base is a substance that combines with a hydrogen ion in solution. A solution containing a base is called an alkaline solution. Acidity or alkalinity is measured on the pH scale.

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth (right) The pH scale. A solution at pH 7 is neutral; higher numbers represent bases, and lower numbers represent acids.

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depth Carbon dioxide (CO 2 ) combines readily with seawater to form carbonic acid (H 2 CO 3 ). Carbonic acid can then lose a H+ ion to become a bicarbonate ion (HCO 3- ), or two H+ ions to become a carbonate ion (CO 3 2- ). Some bicarbonate ions dissociate to form carbonate ions, which combine with calcium ions in seawater to form calcium carbonate (CaCO3), used by some organisms to form hard shells and skeletons. When their builders die, these structures may fall to the seabed as carbonate sediments, eventually to be redissolved. As the double arrows indicate, all these reactions may move in either direction.

© 2006 Brooks/Cole, a division of Thomson Learning, Inc. Chapter 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. Most gases in the air dissolve readily in seawater at the ocean’s surface. Plants and animals living in the ocean require these dissolved gases to survive. In order of their relative abundance, the major gases found in seawater are nitrogen, oxygen, and carbon dioxide. The proportions of dissolved gases in the ocean are very different from the proportions of the same gases in the atmosphere because of differences in their solubility in water and air. In the next chapter you will learn how atmosphere and ocean work together to shape Earth’s weather, climate, and natural history. Out past discussions of heat, temperature, convection, dissolved gases, and geological cycles will be put to use. You’ll find surprising oceanic connections among storms, tropical organisms, deserts, and balloon races!