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

© 2006 Brooks/Cole, a division of Thomson Learning, Inc.

Sea Water Chemistry


Key ConceptsKey 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.


Water Is a Powerful SolventWater 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 (km3/yr). Water is also stored in the ocean, ice, groundwater, lakes, and the atmosphere.


Sea Water ChemistrySea 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



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.



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.


Sea Water ChemistrySea 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


• 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 H20 water molecules pull ionic NaCl apart

• Water molecules surround Na+ and Cl- and prevents them from reuniting


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


Review of the nature of bondsReview 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


BondingBonding

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


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


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 2-8-1– Thus Na needs to lose and electron in its out level


• Chlorine (Cl) has an atomic number of 17• Electron configuration 2-8-7• 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)


Covalent Bonds• Share electrons• Water is an example of covalent bonds


SalinitySalinity

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


A Few Ions Account for Most of A Few Ions Account for Most of the Ocean’s Salinitythe 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 (‰).


The Components of Ocean Salinity Came The Components of Ocean Salinity Came from, and Have Been Modified by, from, and Have Been Modified by,

Earth’s CrustEarth’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.


The Ratio of Dissolved Solids in The Ratio of Dissolved Solids in the Ocean is Constantthe 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 ‰ = 1.80655 Chlorinity in ‰


The Ocean Is in Chemical The Ocean Is in Chemical EquilibriumEquilibrium

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


Seawater’s Constituents May Be Seawater’s Constituents May Be Conservative or NonconservativeConservative 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.


Gas Concentrations Vary with Gas Concentrations Vary with DepthDepth

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.


The Ocean’s Acid-Base Balance Varies The Ocean’s Acid-Base Balance Varies with Dissolved Components and Depthwith 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.



(right) The pH scale.

A solution at pH 7 is neutral; higher numbers represent bases, and lower numbers represent acids.



Carbon dioxide (CO2) combines readily with seawater to form carbonic acid (H2CO3). Carbonic acid can then lose a H+ ion to become a bicarbonate ion (HCO3-), or two H+ ions to become a carbonate ion (CO3

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.


Chapter In PerspectiveChapter 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!

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