Oceanography Lecture Notes Outline

Physical and Chemical Properties of Water


I. Contents -  Topics Covered

The Chemical Structure of Water

The Three Physical States of Water

The Heat Capacity of Water

The Surface Tension of Water

The Viscosity of Water

Compressive Nature of Water

Water Density

Water – the Universal Solvent

Water Transparency

          Water and Sound


II. The Molecular Structure of Water

         A.  The Special Properties of Water are Due to its

                Chemical Structure

                  1. A water molecule is composed of two hydrogen atoms

                           covalently bonded to one central oxygen atom

· Water molecules have a bent “L” shape, with

         the hydrogen atoms bound to one side of the

         oxygen atom – this produces special properties


                  2. The hydrogen-oxygen bond of water is polar, which

                       means that the hydrogen has a positive electrical

                       charge and the oxygen has a negative charge


                  3. The positive and negative charges attract, forming a

                     hydrogen bond between water molecules that are close


·          The hydrogen bond is like an ionic bond


                  4. Hydrogen bonds are weak compared to the covalent

                      bonds that connect the hydrogens and oxygen of the

                      water molecule. However, they are much stronger than

                      the bonds that form between most other molecules.


         B. Water Exists in Three Physical States on the Earth:

                     Solid (Ice), Liquid, and Gas (Water Vapor)


              1. These changes of state represent changes in the relations

                  between water molecules and the motion of the molecules

·          In water vapor, the molecules are moving rapidly

           and are not associated with one another.


·           In liquid water, molecular motion is slower and

       transient 3-dimensional associations of molecules,

       connected by hydrogen bonds, occur.


·             In solid ice, molecular motion is slower still, and

      each molecule has a fixed place within a crystal

    structure, which is held together by hydrogen bonds.


·          Because of the hydrogen bonds between water

                                    molecules, it takes an unusual amount of energy

                                     to melt ice or evaporate liquid water.


         C. Latent Heat of Vaporization

1.  When water is heated to 100oC, additional heat must be applied in order to cause the water to evaporate (form steam). This added heat is called the latent heat of vaporization.

·      This latent heat must also be removed when water vapor condenses.


                  2. The latent heat of vaporization is very important in

                      moderating Earth's climate.

·      There is a global pattern of greater evaporation of water form the oceans at low latitudes, transport of water vapor toward the poles, and greater precipitation at high latitudes.


·   This results in removal of heat (latent heat of vaporization) from low latitudes and release of this heat at high latitudes as the water vapor condenses


D. Latent Heat of Fusion

1. When water is cooled to 0°C, an additional amount of heat must be removed in order to form ice. This additional heat is called the latent heat of fusion.


·      This latent heat must also be added when ice melts.


                  2. The latent heat of fusion is very important in moderating

                      Earth's climate.

·      During periods of global warming, melting of the ice caps acts as a large thermal “heat sink” for excess heat coming from the equator


·      This results in a global thermal buffer effect.


F. Dissolved Substances Change Water’s Boiling and Freezing Points

1.  The salt in seawater raises its boiling point and

                    decreases its freezing point, to about -2oC. The

                           change increases with the amount of salt in water.


2.  Note that any dissolved substance affects freezing and boiling point (antifreeze, ethylene glycol, works on this principle). This is because the dissolved substance affects the association between

                            water molecules.


III. The Heat Capacity of Water

         A.  Water Has an Unusually High Heat Capacity

                  1. Defined as the amount of heat required to raise the

                            temperature on water one degree centigrade


                2. An increase in temperature equals an increase in the

                    motion of water molecules. It takes lots of heat to make

                    water molecules move faster, because of hydrogen bonds


                  3. The high heat capacity of water is also important in

                    moderating climate. Coastal areas have less variable

                   temperatures than inland areas, because water

                   temperature changes slowly in response to changes in the

                   amount of heat from the sun.


IV. The Surface Tension of Water

         A.  Water Has High Surface Tension

                  1.  Defined: surface tension is resistance to penetration or

                       stretching of the surface


                  2. Surface tension is high because of the cohesion of the

                      water molecules due to hydrogen bonds. 


3. Water surfaces can support small objects that are denser than water and would otherwise sink.

·      These include some aquatic animals. (Water striders, freshwater insects, are the most familiar example.)


                  4. Surface tension damps (tends to decrease) capillary

                           waves, the small waves that are the first to form as

                             wind blows over a water surface.


V. The Viscocity of Water

         A.  Water Has a High Viscocity

                  1. Defined: viscosity is the resistance of a liquid to motion.


2. The viscosity of water is high compared to that of chemically similar liquids (for example, alcohol)


3. The viscosity of water is low compared to that of          chemically dissimilar liquids (for example, oil)


                  4. The viscosity of water is important to marine organisms.

·      It helps to prevent tiny plants and animals (plankton)

       from sinking

·      It also causes resistance to the motion of larger,

         swimming animals.


VI. The Compressive Nature of Water

         A.  Water is Not Very Compressive

                  1. Even at the very high pressures of the deep sea

·      About 500 times the pressure at the sea surface


                  2. This property is also important to marine organisms.

·      If  they lack air-filled cavities (like lungs or swim bladders), they can often tolerate large changes in depth and pressure without major ill effects.


VII. Water Density

         A.  Water Density Decreases with Increasing Temperature

                  1. Warm water is less dense than cold water


         B. Water Density Increases with Increasing Salt Content

                  1. Saltwater is denser than freshwater


         C. Water Density Profoundly Affects Ocean Circulation

1. Increases in seawater density with decreasing temperature (especially) and with increasing salt content cause the vertical circulation of ocean water.

·      This is essential to the ventilation, or supply of oxygen, to the deeper parts of the ocean.


2. Seawater (in contrast to pure or freshwater) has its greatest density at its freezing point.


         D. Water Density Profoundly Affects Sea Life

1. Decreased seawater density with increasing temperature creates a warm surface layer which suspends ocean plant life (phytoplankton) near the surface where they can get enough sunlight to grow.


         F. Ice is Less Dense than Water, so Ice Floats

              1.  Ice and its snow cover insulates bodies of water and

                   helps keep the underlying water from freezing.


VIII. Water as a Solvent

         A.  Water is a Very Good Solvent

                  1. Defined: a solvent is fluid that can dissolve substances


                  2. Water dissolves rocks (very slowly)

·      The dissolved minerals from rocks is a primary

                                         source of many salts found in seawater

                  3. Water also dissolves and makes available substances

                      essential to marine organisms

·      like fertilizers and carbon dioxide (for plants) and oxygen (for animals).


IX. The Transparency of Water

         A.  Water is Transparent to Visible Light

                  1.  This allows plant growth to substantial depths.


                  2. Water does gradually absorb light.

·      This prevents plant growth below about 200 m depth even in the clearest ocean water.


         B.  Water is NOT Transparent to Ultraviolet light or to Infrared

                  1. Absorption of UV light protects sensitive organisms.


                  2. Absorption of UV and Infrared acts to heat up water.


X. Water and Sound

         A.  Water Transmits Sound Well

                  1.  Many organisms rely on sound for echolocation or for



2. Many oceanographic instruments rely on sound transmission by seawater – for example:

·      Echo sounders to measure depth


·      Bioacoustics to measure animal abundance and location