Chapter 3 Properties of Ocean Water

Chapter 3 Properties of Ocean Water
Ocean Studies Introduction to Oceanography American Meteorological Society Chapter 3 Properties of Ocean Water © AMS

Case in Point Since antiquity, people have used sea salt to meet their nutritional needs and, prior to the age of refrigeration, to preserve food for storage and transport The Dead Sea, located on the Israeli-Jordanian border, has long been a source of salt In the Middle Ages, salted cod was an important commodity for members of the Hanseatic League, a trading group of merchants in northern Germany and the Baltic Sea coast, and later an important food source for American colonists Today, salt is still used as a seasoning or preservative in meatpacking, fish curing, and food processing © AMS

Case in Point Most commercial salt used to be evaporated from seawater or natural brines in hot dry climates using the sun Many different salts precipitate from saltwater in addition to common household table salt (sodium chloride), including, for example, calcium carbonate, calcium sulfate, and magnesium sulfate Salt is also mined from deposits of rock salt © AMS

Properties of Ocean Water
Driving Question: How do the properties of water and dissolved salts affect the physical and chemical properties of seawater? © AMS

Properties of Ocean Water
In this chapter, we examine: Water’s unique properties, the fundamental reasons for those properties, and some of the implications for the functioning of the ocean in the Earth system The structure of the water molecule and how this structure is the principal reason for water’s unique physical and chemical properties Chemical properties of seawater, emphasizing the types, sources, and cycling of dissolved salts and gases © AMS

The Water Molecule and Hydrogen Bonding
A water molecule consists of two hydrogen (H) atoms bonded to an oxygen (O) atom Bonding between hydrogen and oxygen atoms involves sharing of electrons, one from each hydrogen atom and two from the oxygen atom An electron is a negatively charged subatomic particle © AMS

The electrons spend more time near the oxygen so that the oxygen acquires a small negative charge and the hydrogen is left with a small positive charge Covalent bonding Molecules having a separation of positive and negative charges are described as polar The positively charged (hydrogen) pole of one water molecule attracts the negatively charged (oxygen) pole of another water molecule; this attractive force constitutes hydrogen bonding Hydrogen bonding inhibits changes in water’s internal energy so that it absorbs or releases unusually great quantities of heat energy when changing phase © AMS

WATER AS ICE, LIQUID, AND VAPOR Ice has a regular internal three-dimensional framework consisting of a repeated pattern of molecules Hydrogen bonding is responsible for the ordered arrangement of water molecules in the crystal lattice and the hexagonal structure of ice crystals Because ice’s internal framework is an open network of water molecules, the molecules in ice crystals are not as closely packed as a similar number of molecules in liquid water © AMS

A physical model of the crystal lattice of ice. Each water molecule is bound tightly to its neighbors but intermolecular bonds are elastic so that molecules vibrate about fixed locations in the lattice. © AMS

WATER AS ICE, LIQUID, AND VAPOR Water molecules exhibit much greater activity in the liquid than solid phase In the liquid phase, water molecules undergo vibrational, rotational, and translational motions Greater freedom of movement explains why liquid water takes the shape of its container When liquid water changes to vapor, essentially all hydrogen bonds are broken Gas molecules exhibit vibrational, rotational, and translational motion and exert a force as they bombard a solid or liquid surface © AMS

When water changes phase from ice to liquid, many water molecules remain linked by hydrogen bonds as transient cluster of molecules surrounded by non-bonded water molecules. © AMS

WATER AS ICE, LIQUID, AND VAPOR Melting, evaporation, and sublimation are phase changes that absorb heat Phase changes that release heat to the surroundings are freezing, condensation, and deposition © AMS

TEMPERATURE AND HEAT All matter is composed of atoms or molecules that are in continual vibrational, rotational, and/or translational motion The energy represented by this motion is referred to as kinetic molecular energy or just kinetic energy, the energy of motion Temperature is directly proportional to the average kinetic energy of atoms or molecules composing a substance © AMS

TEMPERATURE AND HEAT Internal energy encompasses all the energy in a substance, that is, the kinetic energy of atoms and molecules plus the potential energy arising from forces between atoms or molecules If two objects have different temperatures and are brought into contact, energy will be transferred between objects; we call this energy in transit, heat Heat transferred to or from water brings about a change in temperature or a change in phase Heat energy is always transferred from a warmer object to a colder object © AMS

TEMPERATURE AND HEAT A convenient unit of heat energy is the calorie, defined as the amount of heat needed to raise the temperature of one gram of water by 1°C © AMS

CHANGES IN PHASE OF WATER When water freezes, latent heat is released to the environment For ice to melt an equivalent amount of latent heat is absorbed from the environment This heat energy is used only to change the phase of water and not the temperature of the water Whether freezing or melting is taking place, the latent heat involved is commonly called the latent heat of fusion © AMS

CHANGES IN PHASE OF WATER Evaporation: If more water molecules enter the atmosphere as vapor than return as liquid Condensation: If more water molecules return to the water surface as a liquid than escape as vapor Heat absorbed from the environment during evaporation and heat released to the environment during condensation are known as the latent heat of vaporization and the latent heat of condensation © AMS

CHANGES IN PHASE OF WATER Sublimation is the process whereby ice or snow becomes vapor without first becoming a liquid. Deposition is the process whereby water vapor becomes ice without first becoming a liquid. Heat is absorbed from the environment during sublimation and heat is released to the environment during deposition. Sublimation requires the latent heats of fusion plus vaporization, known as the latent heat of sublimation. Deposition releases to the environment an equivalent amount of latent heat, that is, the latent heat of deposition. © AMS

SPECIFIC HEAT OF WATER The amount of heat that is needed to raise the temperature of 1 gram of a substance by 1°C is defined as the specific heat of that substance. The specific heat of all substances is measured relative to that of liquid water, which is defined as 1 calorie per g per Celsius degree (at 15 ºC). Because of hydrogen bonding, water has an unusually high specific heat, in fact the highest specific heat of any naturally occurring liquid or solid. © AMS

Heating a 1-gram ice cube causes a rise in temperature plus phase changes, initially to liquid and then to vapor. © AMS

MARITIME INFLUENCE ON CLIMATE A large body of water (such as the ocean or Great Lakes) can significantly influence the climate of downwind localities Compared to an adjacent landmass, a body of water does not warm as much during the day (or in summer) and does not cool as much at night (or in winter) In other words, a large body of water exhibits a greater resistance to temperature change, called thermal inertia, than does a landmass Air over a large body of water tends to take on similar temperature characteristics as the surface water Places immediately downwind of the ocean experience much less contrast between average winter and summer temperatures (maritime climate) Places at the same latitude but well inland experience a much greater temperature contrast between winter and summer (continental climate) © AMS

Chemical Properties of Seawater
WATER AS A SOLVENT The polar nature of the water molecule favors the solution (i.e., dissolving) of both ionic and non-ionic substances. Many inorganic materials (primarily salts) are bonded ionically, whereas many organic chemicals have non-ionic bonds. River water, groundwater, and ocean water dissolve some of the rock or sediment (both organic and inorganic) that water contacts and some Earth materials dissolve in water more readily than other Earth materials. © AMS

SEA SALTS Seawater is a salt solution of nearly uniform composition; only the relative amount of water in the solution varies. Salinity is a measure of the amount of salt dissolved in seawater. On average, seawater is 96.5% water and 3.5% dissolved salts. © AMS

Common household table salt, sodium chloride (NaCl), dissolves in water. (A) In its cubic crystalline form, ionic bonds hold together the positively charged sodium ions (Na+) and the negatively charged chloride (Cl-) ions. (B) Once salt enters the water, however, hydrogen-bonded complexes of water molecules greatly reduce the force of attraction between oppositely charged sodium and chloride ions. The compound readily dissociates into sodium and chloride ions with the sodium ions attracted to the negatively charged pole of the water molecule and chloride ions attracted to the positively charged pole of the water molecule. © AMS

SEA SALTS The major constituents of seawater occur in the same relative concentrations throughout the ocean, a characteristic of seawater described as the principle of constant proportions. Concentrations of the major dissolved constituents of ocean water, such as chloride (Cl-1) and sodium (Na+1) ions, are conservative properties of seawater; these ions occur in constant proportions and change concentration very slowly by mixing or diffusion. Constituents that participate in biogeochemical or seasonal cycles have variable concentrations and are described as non-conservative properties. © AMS

SEA SALTS The principal source for salts dissolved in seawater is weathering and erosion of rock and sediment on land and transport by rivers and streams to the ocean. Although more than 70 different ions are dissolved in seawater, six make up more than 99% of all sea salts: chloride, sodium, sulfate, magnesium, calcium, and potassium. © AMS

SEA SALTS Difference in the chemical makeup of dissolved solids in river water versus seawater due to: Marine organisms extract calcium and silica from seawater to build their shells and skeletons. Differences in solubility and rates of physical-chemical reactions among ions also play a role by limiting the concentration of certain substances in seawater or causing some chemicals to precipitate from solution. © AMS

SEA SALTS Hydrothermal vents and chemical reactions between seawater and recently formed oceanic crust contribute to the salinity of ocean water. The rate of addition of new salts to the ocean balances the rate of removal. Salt ions are removed from the ocean by sea spray that is blown ashore and by isolation of arms of the sea from the ocean followed by evaporation to produce salt deposits. © AMS

VARIATIONS IN SALINITY Most dissolved substances are left behind when seawater evaporates or freezes, increasing the surface salinity locally. Precipitation, runoff from rivers, and melting ice add fresh water and decrease the local surface salinity. Where large rivers enter the ocean, salinity is reduced as fresh water and seawater mix. Salinity is also reduced where rainfall is heavy. Seawater salinity tends to be more variable in coastal areas than the open ocean. Average sea-surface salinity is generally lowest near the equator and in polar areas where annual precipitation is greater than the rate of evaporation. © AMS

DISSOLVED GASES Dissolved gases are present in ocean water; these include carbon dioxide, nitrogen, and oxygen. Gases are exchanged between the atmosphere and ocean at the ocean surface. The saturation (maximum) concentration of a gas in water depends primarily on temperature in freshwater bodies and a combination of temperature and salinity in seawater. Almost all gases are more soluble in cold water than in warm water. © AMS

DISSOLVED GASES As the temperature or salinity of seawater increases, water holds less gas at saturation. When water is saturated with a gas, the rate at which the gas dissolves in water equals the rate at which the gas escapes to the atmosphere. Waves on the ocean surface facilitate the transfer of gases between the atmosphere and ocean. Another important mechanism in the transfer of gases at the air/sea interface is bubble injection whereby breaking waves introduce a foam composed of small bubbles below the surface greatly enhancing exchange rates. © AMS

Global pattern of average annual sea-surface salinity shows the highest values in the subtropics of the Northern and Southern Hemispheres. Salinity values are in parts per thousand. © AMS

The saturation value of dissolved oxygen in fresh water decreases with rising temperature. © AMS

DISSOLVED GASES Below the ocean-atmosphere interface, biochemical processes play important roles in controlling the proportions of certain dissolved gases. Within the photic zone, the sunlit upper layer of the ocean where photosynthesis takes place, dissolved oxygen is enhanced relative to carbon dioxide. In surface ocean waters, the principal dissolved gases are nitrogen (48%), oxygen (36%), and carbon dioxide (15%). For the ocean as a whole, CO2 is the most abundant dissolved gas, accounting for 83% of the total. © AMS

DISSOLVED GASES If a dissolved gas does not participate in any biochemical process such as photosynthesis or cellular respiration, its concentration in a parcel of seawater remains unchanged except by the relatively slow movements of gas molecules (diffusion) through the water or the mixing of water masses containing different amounts of dissolved gas. Nitrogen and inert (chemically non-reactive) gases such as argon and neon behave in this way and their concentrations are described as conservative properties. Biochemical processes influence the concentration of some gases dissolved in seawater, primarily oxygen and carbon dioxide; the concentrations of these dissolved gases are examples of non-conservative properties. © AMS

SEAWATER pH An acid is a hydrogen containing compound that releases hydrogen ions (H+1) when dissolved in water. An alkaline substance releases hydroxyl ions (OH-1) when dissolved in water and may be weak or strong. The acidity of water (or any other substance) is expressed as pH, a measure of the hydrogen ion concentration. pH increases from 0 to 14 as the hydrogen ion concentration decreases. Pure water has a pH of 7, which is considered neutral. A pH above 7 is increasingly alkaline whereas a pH below 7 is increasingly acidic. © AMS

SEAWATER pH The pH of pristine seawater ranges between 8.0 and 8.3 (seawater is slightly alkaline). Carbon dioxide plays a key role in controlling the pH of seawater. Known as a buffer Atmospheric CO2 dissolves in ocean water producing carbonic acid (H2CO3) that dissociates into hydrogen (H+1), carbonate (CO3-2), and bicarbonate (HCO3-1) ions. © AMS

The acidity of water is expressed as pH, a measure of the hydrogen ion concentration. On this scale, pH increases from 0 to 14 as the hydrogen ion concentration decreases. Pure water has a pH of 7, which is considered neutral; a pH above 7 is increasingly alkaline and a pH below 7 is increasingly acidic. © AMS

OCEAN ACIDIFICATION CO2 that is absorbed by the ocean participates in chemical reactions that increase the acidity (lowers the pH) of ocean waters. ocean acidification The current trend in the flux of CO2 into the ocean will result in a 50% reduction in the ocean’s carbonate concentration by the end of the century. Carbonate ions are needed by certain marine organisms to build their shells and other hard parts. Marine organisms that are particularly vulnerable to ocean acidification are coccolithophorids, foraminifera (phytoplanktonic organisms), and pteropods (small marine snails). Also vulnerable are corals which filter plankton from ocean water and secrete calcium carbonate. © AMS

Physical Properties of Seawater
Adding salts to water changes its temperature of initial freezing and the temperature at which it reaches maximum density. Since salts are excluded from the ice structure as seawater freezes, the remaining unfrozen water becomes saltier and therefore freezes at still lower temperatures. Known as brine rejection © AMS

WATER DENSITY AND TEMPERATURE Density is defined as mass per unit volume. Freshwater density varies primarily with temperature whereas seawater density varies chiefly with temperature and salinity. Most substances contract when cooled and expand when heated; their density increases with falling temperature and decreases with rising temperature. As the average kinetic molecular energy decreases (i.e., as the temperature falls), the same number of molecules occupies a progressively smaller volume. © AMS

Water’s temperature of maximum density and initial freezing point temperature decrease at different rates as the salinity increases. The density of fresh water reaches a maximum at about 4 °C (39.2 °F) and, with additional cooling (below 4 °C), liquid water expands until it freezes at 0 °C. © AMS

WATER DENSITY AND TEMPERATURE As the temperature drops, water molecules have less kinetic energy and move closer together so that the number of hydrogen bonds increases resulting in more ice-like clusters. Whereas most liquids contract when they solidify, as water freezes, its molecules bond into an open hexagonal structure so that the density of ice is about 92% of liquid water. The density of seawater increases with increasing salinity because the atomic mass of dissolved salts is greater than that of water molecules. Less dense fresh water floats on more dense seawater. © AMS

WATER DENSITY AND TEMPERATURE Seawater density always increases with falling temperature and decreases with rising temperature. Differences in seawater density, caused by variations in temperature and salinity, are important controls of the vertical circulation of ocean water. The density of seawater varies with temperature and salinity. © AMS

PRESSURE Standard atmospheric pressure is the average air pressure at sea level at 45 degrees latitude and an air temperature of 15 °C (59 °F). Water is much denser than air so that a column of equivalent height produces much greater pressure. A column of fresh water with a height of m (33.9 ft) exerts a pressure at its base that approximately equals one standard atmosphere. The pressure at any point in a water column is directly related to depth and the relationship is linear; doubling the depth doubles the pressure. Ocean scientists commonly use the bar and its derivatives as a standard unit of pressure. The water pressure expressed in decibars (0.1 bar) is numerically equivalent to the water depth expressed in meters. © AMS

SEA ICE When seawater is chilled below its freezing point, microscopic ice crystals form, later growing into hexagonal needles. Eventually ice crystals begin to grow downward and form a thin, flexible, plastic-like ice layer, honeycombed with small cells that fill with seawater. The salt content of newly formed sea ice depends on temperature. © AMS

SEA ICE At lower temperatures, ice forms more rapidly and traps seawater. This sea ice contains more salt but is less salty than the seawater from which it formed. At temperatures near freezing, sea ice forms slowly, which allows brines to flow out leaving little seawater in the cells. An ice layer up to 1 m (3 ft) or so thick can form in one winter; this is called first-year ice. Dominates the Southern Ocean around Antarctica In the Arctic pack ice of the central basin, sea ice melts little during summer and multi-year ice dominates. © AMS

B A Sea ice extent in December 2007, based on satellite passive microwave data for (A) the Arctic where it was winter and (B) the Southern Ocean/Antarctica where it was summer. Total sea ice extent was 12.4 million square km in the Arctic and 12.6 million square km in the Antarctic. © AMS

SOUND TRANSMISSION Sound propagates through some medium (e.g., air or water) as compression waves with the speed of propagation dependent on the properties of the medium. The speed of sound in ocean water (about 1500 m per sec or 5000 ft per sec) is more than four times the average speed of sound in air because water is less compressible than air. Knowing the speed of sound in seawater is the principle behind an echo sounder used by most seagoing vessels to determine the depth of water beneath the ship. SONAR (SOund NAvigation and Ranging) is similar to an echo sounder except that the operator of the instrument can alter the direction of the sound signal. Pulses of sound are sent out to locate targets such as submarines and the return echoes are displayed electronically on a monitor. © AMS

SOUND TRANSMISSION The speed of sound in seawater is influenced by temperature, pressure, and salinity. Variation of the speed of sound due to temperature/salinity differences in the ocean gives rise to the SOFAR channel, a zone centered at an ocean depth of about 1000 m (3300 ft) where the speed of sound is at a minimum value. Sound waves are vertically trapped in the SOFAR channel and can travel thousands of kilometers horizontally with little loss of energy. At greater depths, the water temperature tends to be uniformly low and variations in sound speed depend chiefly on pressure change with depth. With increasing depth and pressure, sound speed increases. © AMS

Refraction of sound waves as they travel through ocean waters gives rise to the SOFAR channel, a zone centered at a depth of about 1000 m (3300 ft) where the speed of sound is at a minimum value. © AMS

Conclusions Water has unusual physical and chemical properties resulting from the water molecule’s unique polar structure that gives rise to hydrogen bonding. Water is a powerful solvent, dissolving both salts and gases, and is an effective heat-storage and transporting medium. Temperature, salinity, and pressure affect most physical properties of seawater such as density and sound velocity. © AMS

Chapter 3 Properties of Ocean Water

Similar presentations

Presentation on theme: "Chapter 3 Properties of Ocean Water"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

Chapter 3 Properties of Ocean Water

Similar presentations

Presentation on theme: "Chapter 3 Properties of Ocean Water"— Presentation transcript:

Similar presentations

About project

Feedback