THE PHYSICAL PROPERTIES OF WATER

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Major Concept

• The many unique properties of water are a result of the two elements involved, hydrogen and oxygen– their ratios, – the bond type, and – the physical architecture of the polar water

molecule

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Water Molecule- Polar Molecule- Covalant Bond

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WATER• In 1783 the English scientist Henry Cavendish

determined that water was made up of hydrogen and oxygen.

• Hydrogen and oxygen are bound together by sharing electrons, a type of bond called a covalent bond.

• Because of the shared electrons and the asymmetry of the bond, water molecules have a definite shape, a slightly negative end, and a slightly positive end. This is called a polar molecule.

• Due to its polarity, water can make weak bonds with itself (called hydrogen bonds) that means that even liquid water is slightly structured or ordered.

• This bonding also creates surface tension and enables water to be somewhat of a universal solvent.

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Major Concept

• Water exists in three forms or states in the range of temperature and pressure we find at Earth’s surface.

• It exists as a solid (the mineral ice), as a liquid, and as a gas (water vapor).

• To change temperatures in any one state requires heat energy input or export, and changing states from one form to another often requires large energy exchanges

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Water Properties• Tremendous amounts of heat (energy) are removed and

liberated in different atmospheric processes around the globe.

• In some cases it is possible to:– cool water below 0°C and remain liquid,– boil water at temperatures below 100°C, and– change the state of water from solid directly to gas in a process

called sublimation.

• Sublimation occurs when snow or ice evaporates under very cold, dry conditions.

• The properties of water (its behavior) are altered with the addition of trace amounts of other compounds. These usually serve to lower the freezing temperature and elevate the evaporation or boiling point of water

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Water Properties- Heat Capacity

• heat capacity of water is very high compared to the land or the atmosphere

• Heat capacity is a measure of how much heat a substance can absorb or release for a given change in temperature.

• Substances with a high heat capacity require the transfer of large amounts of heat energy for small changes in temperature.

• Water has a very high heat capacity compared to other naturally occurring earth materials. The heat capacity of water is much higher than most other liquids because of the hydrogen bonding between water molecules.

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Heat Capacity• Temperature variations on land range from a high of about

50°C in the Libyan desert in summer to about -50°C in the Antarctic for a range of 100°C.

• Temperature variations in the ocean range from a high of about 28°C in equatorial regions to a low of about -2°C in Antarctic waters for a total range of only 30°C.

• People who live by large bodies of water have a good understanding of the effect of different heat capacities of land and water.

• In Milwaukee it is common to hear weather predictions of daytime high temperatures for the city with the additional comment “cooler by the lake” in the summer and “warmer by the lake” in the winter.

• The high heat capacity of water and its ability to hold heat makes it possible for surface currents to redistribute excess heat in equatorial regions to higher latitudes as they flow away from the equator on the western sides of ocean basins.

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What does it mean?

• When ice forms heat is lost to a heat-deficient atmosphere

• When ice melts the energy is gained by water• This prevents the higher latitudes from becoming

much colder than -1.8oC, the freezing temperature of ocean water

• This prevents colder regions from getting colder and equatorial regions from getting warmer

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Major Concept

• The density of water is extremely sensitive to changes in temperature, salinity, and pressure.

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Density

• The density of seawater is greater than the density of freshwater because seawater contains dissolved salts.

• The density of pure water at 3.98°C, or approximately 4°C, is 1.0 g/cm3. The densest it can be.

• The density of seawater of average salinity is about 1.0278 g/cm3. Because of this, fresh water will float on ocean water.

• Pressure has relatively little effect on density compared to salinity and temperature, however, the density will increase with increasing pressure (or depth) and becomes important in the deep sea where salinity and temperature remain nearly constant.

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Pressure

• Pressure increases with depth in the oceans. Pressure increases by about 1 atmosphere, for each increase of 10 m (33 ft) in depth.

• The pressure in the deepest ocean trench at 11,000 m (36,000 ft) below sea level is about 1100 atmospheres.

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Density increases as salinity increases

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Universal Solvent

• Water has been called the universal solvent because of its exceptional ability to dissolve a wide variety of materials.

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Universal Solvent

• The polar nature of the water molecule and its hydrogen bonding allow water to dissolve salts and many other compounds easily.

• Polar water molecules are able to attract and surround ions in seawater.

• Salts are supplied to the oceans by underwater volcanism and runoff from land.

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Energy

• The water on, in, and around the Earth transmits energy in many forms, including light, heat, and sound.

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Color• Coastal water varies in color. It may be green, yellow, brown,

or red depending on the nature of the material in the water; silt from rivers, microscopic organisms, or even dissolved organic substances.

• Open-ocean water is often clear and blue.• The color of an object is due to the particular wavelength of

light that reflects off of the object and can be seen by our eyes. Objects in deep water often appear dark because only the blue light that penetrates to the greatest depth is illuminating them.– A lobster in shallow water is red but black in deep water

• Particles suspended and compounds dissolved in the water will absorb and scatter light, thus reducing its penetration below the surface. The depth of penetration will decrease as the amount of suspended and dissolved material in the water increases.

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Turbidity

• The attenuation of light with depth can be measured with a simple device called a Secchi disk. A Secchi disk is a white disk that is lowered into the water until it disappears from view.

• In water with large amounts of suspended sediment or marine organisms a Secchi disk may disappear at depths of 1–3 m (3-10 ft). In the open ocean it may still be visible at depths of 20–30 m (65-100 ft) and in one measurement in Antarctica's Weddell Sea a Secchi disk didn't disappear until it reached a depth of 79 m (260 ft).

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Sound

• Sound is transmitted very efficiently in seawater. The velocity of propagation of sound in seawater is roughly five times as great as in air due to its higher density.

• Sound propagating in the oceans can be used to locate organisms and objects either by listening for the sounds that they produce or by sending pulses of sound into the water to be reflected back from the target of interest.

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Traveling at an average velocity of 1500m/sec, a sound pulse leaves the ship, strikes the bottom and returns.In water 4500m deep sound requires 3 seconds to reach the bottom and 3 seconds to return

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Sound

• Sound is also used to locate objects underwater using a system called sonar. Sonar is an acronym that stands for “sound navigation and ranging.”

• Sound is also used extensively as a simple and accurate means of determining the depth of seawater. Precision depth recorders (PDR’s) transmit pulses of sound that reflect from the bottom and return to the surface. By knowing the average velocity of sound in water and the time required for the sound to travel downward and up again, the depth can be easily calculated.

• It is also possible to send communications large distances through the oceans with extremely low frequency (ELF) sound antennas.

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A chart recorder displaying a seismic reflection line

bottomSub-bottom

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Sound waves change velocity and refract as they travel at an angle through layers of different densities

The degree of refraction and change in velocity with depth must be known in order to determine the actual position of the target

Sound beams bend toward regions in which sound travels more slowly and away from faster regions

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Sound Applications

• In the 1950s and 1960s the United States Navy installed large arrays of hydrophones on the seafloor to monitor submarines.

• These arrays were known as sound surveillance systems (SOSUS).

• SOSUS arrays have been used by scientists to monitor sounds made by marine organisms and underwater volcanoes.

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Sea Ice and Fog

• Two states of water, solid and water vapor (or ice and fog), can produce particularly hazardous conditions.

• Sea ice forms at polar latitudes. As the water freezes it excludes the salt molecules from its crystalline structure lowering its density and allowing it to float. The salts that are left behind concentrate to increase the salinity of the near-surface water and slightly lower its freezing point.

• Sequences of freezing and thawing continue to exclude more and more salt and the salinity of the ice will decrease each time.

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ARCTIC

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ANTARCTIC

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Sea Ice

• With the first freezing of a thin layer of ice on the sea, waves and wind break it apart into small pieces called pancakes.

• Pancakes gradually freeze together to form larger pieces called floes, some of which float on the water and some of which are attached to land are called fast ice.

• Freshwater ice that enters the sea (icebergs) comes from glacial ice. These can be very hazardous to navigation as most of the iceberg is below the surface.

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Pancake Ice

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Pressure ridge generated by colliding floes

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Castle Berg

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Tabular Iceberg- 500m long

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Green Iceberg

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Ice shelf, seawater rich in organic matter freezes to underside, bergs calved capsize to expose green ice

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Fog

• Large accumulations of condensed water vapor (water droplets) close to the ground are called fog. (due to changes in temperature)

• The three types of fog encountered are:– advective fog, formed when air warmed by the

underlying water is saturated by water vapor and then moves over colder water to condense. (Grand Banks)

– sea smoke, dry, cold air moving over the sea from the land is warmed and picks up water vapor from the sea and rises to cool and condense.

– radiative fog, the result of warm days and cold nights – warm moisture laden air over the land cools at night and the water vapor condenses.

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Advective Fog over the Golden Gate Bridge

THE CHEMISTRY OF SEAWATER

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Buffering

• In a sequence of elegant and reversible interactions, carbon dioxide and water couple to form a stable buffering system that controls the pH of the world’s oceans.

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pH

• The water molecule can break apart to form a hydrogen ion, H+, and a hydroxide anion, OH-.

• In a pure water solution approximately one in every ten million water molecules will break apart. If the concentration of the two ions is equal the solution is said to be neutral.

• In solutions that are not pure water the concentration of the hydrogen and hydroxide ions may not be equal. If there are more hydrogen ions than hydroxide the solution is an acid. If there are more hydroxide ions than hydrogen ions then the solution is a base, or alkaline.

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pH

• The acidity or alkalinity of a solution is measure with the pH scale. The pH scale ranges from a low of 0 to a high of 14. pH less than 7 is acidic, pH of 7 is neutral, and pH greater than 7 is alkaline.

• pH = -Log10[H+]• pH= -Log10[10-7]= -(-7) = 7• The normal pH of rainwater is about 5.0-

5.6 (slightly acidic).

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Buffering

• When carbon dioxide dissolves in seawater, it readily enters into a sequence of reversible chemical reactions with carbon dioxide and H2O on one end, carbonic acid (H2CO3), bicarbonate (HCO3

-) and hydrogen ions (H+), and finally carbonate (CO3

2-) and two hydrogen ions (2H+).

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Buffering

• CO2 + H2O H2CO3 HCO3- + H+ CO3

2- + 2H+

• When CO2 buffers oceanic pH, it helps to maintain oceanic pH ranges from 7.5–8.5 (i.e., slightly basic or alkaline); the average ocean pH is about 7.8

• The pH of surface water can be as high as 8.5 if the water is warm and high rates of photosynthesis are extracting large amounts of CO2 from the water.

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Salinity

• The average waters of the world’s oceans contain 3.5% dissolved salts and 96.5% water. The content of dissolved salts in grams of salt per kilogram of seawater is called salinity. The average ocean salinity is about 35 g/kg.

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Salinity in the world’s Oceans

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Salinity

• Seawater salinity is typically reported in one of three different (but equivalent) ways: – 1) grams of salt per kilogram of water (g/kg), – 2) parts per thousand (ppt, or o/oo), or in – 3) Practical Salinity Units (PSU).

• The PSU scale is defined as the ratio of the conductivity of a seawater sample and a standard potassium chloride solution. The PSU scale and o/oo are numerically equal but the PSU scale doesn’t have any units because it is a ratio.

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Salinity

• Sea surface salinity is controlled largely by variations in precipitation and evaporation patterns

• These are generally a function of latitude. • Salinity is also influenced by proximity to

land and variations in freshwater runoff from the continents as well as seasonal freezing and thawing at middle and high latitudes.

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Average Sea Surface Salinities in the Northern Hemisphere Summer in ppt

Freshwater runoff

High salinity due to evaporation

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Salinity

• Salinity maxima occur approximately 20˚ to 30˚ north and south of the equator because of relatively dry climatic conditions that cause more evaporation than precipitation, thus concentrating the salts in the water.

• The salinity minima at about 5˚ north of the equator and about 45˚ north and south of the equator are caused by relatively wet climates where precipitation is greater than evaporation and rainfall dilutes salts in the water.

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Dissolved Salts

• When salts and many other particulates (minerals) are added to water, some amount of the crystals or solids dissolve.

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Sources of Salts

• The source of salts in the sea is from the weathering, erosion, and dissolution of the rocks that make up the earth’s crust, initially they are primarily from the igneous rocks of the crust as the crust and mantle degassed and differentiated.

• volcanic eruptions and gaseous emanations.• hydrothermal vents along mid ocean ridges.

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Major Sources and Inputs of Ions to seawater

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Salts

• The total amount of dissolved salts in the world oceans can be calculated at 5 x 10 22 grams (at 36 ppt).

• Each year we estimate that some 2.5 x 10 15 grams of salt are added to the oceans.

• Geological evidence and recent calculations indicate the salt content of the oceans has been very stable for 1.5 billion years or more.

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Residence Times• The average time that a substance remains in solution

in the ocean is called its residence time.• Aluminum and iron have very short residence times

because they react chemically with other substances to form insoluble particles that are added to the sediment.

• Sodium, chloride, potassium, and magnesium have very long residence times because they do not react chemically to form solids and they are not extracted from the water by biological processes.

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The salinometer determines salinity by measuring the

electrical conductivity of

seawater

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Gases

• Many gases move readily across the air-sea interface. T

• Three gases are particularly critical to life on earth and the physical and chemical balances within the ocean realm. T

• These are nitrogen (N2), oxygen (O2), and carbon dioxide (CO2). These gases comprise more than 99% of the gases in the atmosphere and the oceans.

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Gases• Dissolved gases are mixed to all depths in the oceans.• Gases dissolve more readily in cold water, water with low salinity,

and water under pressure. Conversely, warm waters (similar to warm sodas) can hold less gas. Very saline water has less gas.

• Deep, cold ocean water commonly contains relatively higher concentrations of dissolved gases.

• If we slowly add gas to a liquid we reach a point where we cannot add more gas without some of the gas being released from the solution simultaneously. The concentration level where the fluid will neither gain nor lose gas is the saturation value for that gas and is dependent on salinity, temperature, and pressure.

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The distributionof CO2 and O2 with

depthChanges in O2

concentration may exceed 400% from the surface to depthCO2 concentrations

change <15%

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CO2

• The annual ocean uptake of carbon (in the form of carbon dioxide) is calculated at 2.5 billion metric tons per year.

• Water temperature, pH, salinity, chemistry, and geological and biological processes control the rate at which the oceans absorb carbon dioxide.

• CO2 gets transferred from the surface to deep ocean waters and eventually is fixed in marine sediments as organisms remove calcium carbonate (CaCO3) to produce skeletal material.

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O2

• The oceans have played one of the critical roles in regulating the free oxygen concentration in the Earth’s atmosphere for nearly 3.5 billion years.

• Photosynthesis releases oxygen to the ocean and atmosphere.

• In excess of respiration, some 300 million metric tons per year are released that would eventually result in an increase in O2 concentrations in the oceans and atmosphere.

• However, the excess oxygen is consumed chemically during oxidation of exposed surface sediments and animal respiration, and decomposition of organic compounds.

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Nutrients• The nutrients needed for plant growth in the oceans include

nitrate (NO3-), phosphate (PO4

3-), and silicate (SiO4-).

• Silicate is used to form silica (SiO2), the material that forms the hard outer wall of single-celled plant-like organisms called diatoms as well as the skeletal parts of some protozoans.

• These nutrients are brought to the oceans by freshwater runoff from the continents.

• These nutrients are generally present in very low concentrations in seawater.

• The concentration of nutrients in seawater is cyclical. • Nutrient concentration decreases as increasing biological

productivity removes nutrients. • Concentrations increase when populations decline and

death and decay return nutrients to the water.

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Resources

• 30% of the world’s salt is extracted from seawater. • Salt can be concentrated by evaporation at low

latitudes, or by freezing at high latitudes.• 60% of the world’s magnesium comes from the sea,

along with 70% of the bromine.• Future technologies may be able to take advantage of

extremely valuable trace element concentrations in seawater that are not currently economical to recover.

• Two examples include gold and uranium that have estimated dissolved concentrations of 10 million tons and 4 billion tons in seawater respectively.