Water and Ocean Structure. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Key Concepts Water is a polar chemical compound composed of two hydrogen.

Water and OceanStructure

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

Key ConceptsKey Concepts

Water is a polar chemical compound composed of two hydrogen atoms and one oxygen atom. Its remarkable thermal properties result from the large number and relatively great strength of hydrogen bonds between water molecules.

Heat and temperature are not the same thing. Heat is energy produced by the random vibration of atoms or molecules. Heat is a measure of how many molecules are vibrating and how rapidly they are vibrating. Temperature records only how rapidly the molecules of a substance are vibrating. Temperature is an object's response to an input (or removal) of heat.

Without water's unique thermal properties, temperatures on Earth's surface would change dramatically with only minor changes in atmospheric transparency or solar output. Water acts as a "global thermostat.”

Water density is greatly influenced by changes in temperature and salinity. Water masses are usually layered by density, with the densest (coldest and saltiest) water on or near the ocean floor. Differences in the density of water masses power deep ocean circulation.

Light and sound are affected by the physical properties of water, with refraction and absorption effects playing important roles


The Water Molecule Is Held The Water Molecule Is Held Together by Chemical BondsTogether by Chemical Bonds

Compounds – substances that contain two or more different elements in fixed proportions

Element – a substance composed of identical particles that cannot be chemically broken down into simpler substances

Atoms – the particles that make up elements



A water molecule is composed of two hydrogen atoms and one oxygen atom. Water is a polar molecule, having a positive and a negative side.

A molecule is a group of atoms held together by chemical bonds.

Chemical bonds, the energy relationships between atoms that hold them together, are formed when electrons - tiny negatively charged particles found toward the outside of an atom - are shared between atoms or moved from one atom to another.

Fig. 6-1, p. 155

Two hydrogen atoms...

share their electrons with one oxygen atom...

to form a water molecule held together by covalent bonds...

which acts as if it has negative and positive ends.

Nucleus (+1 unit of charge)

Electron (–1 unit of charge)

Stepped Art

2 – 105°



What holds water molecules together?

Hydrogen bonds form when the positive end of one water molecule bonds to the negative end of another water molecule.

What are two important properties of water molecules?

Cohesion – the ability of water molecules to stick to each other, creating surface tension.

Adhesion – the tendency of water molecules to stick to other substances


Not All Substances Have the Same Not All Substances Have the Same Heat CapacityHeat Capacity

What is the difference between heat and temperature?

Heat is energy produced by the random vibrations of atoms or molecules.

Temperature is an object’s response to input or removal of heat.

Heat Capacity is a measure of the heat required to raise the temperature of 1g of a substance by 1C.

Water has a very high heat capacity, which means it resists changing temperature when heat is added or removed.


Water’s Temperature Affects Its Water’s Temperature Affects Its DensityDensity

The relationship of density and temperature for pure water.

Note that points C and D both represent 0°C (32°F) but different densities and thus different states of water. Ice floats because the density of ice is lower than the density of liquid water.


Water’s Temperature Affects Its Water’s Temperature Affects Its DensityDensity

The three common states of matter – solid, liquid, and gas. A gas is a substance that can expand to fill any empty container. Atoms or molecules of gas are in high-speed motion and move in random directions. A liquid is a substance that flows freely in response to unbalanced forces but has a free upper surface in container it does not fill. Atoms or molecules of a liquid move freely past one another as individuals or small groups. Liquids compress only slightly under pressure. Gases and liquids are classed as fluids because both substances flow easily. A solid is a substance that resists changes of shape or volume. A solid can typically withstand stresses without yielding permanently. A solid usually breaks suddenly. On Earth, water can occur in all three states: gas, liquid, and solid.


Water Becomes Less Dense When It Water Becomes Less Dense When It FreezesFreezes

The lattice structure of an ice crystal, showing its hexagonal arrangement at the molecular level.

The space taken by 24 water molecules in the solid lattice could be occupied by 27 water molecules in liquid state, so water expands about 9% as the crystal forms.

Because molecules of liquid water are packed less efficiently, ice is less dense than liquid water and floats.


Water Becomes Less Dense When It Water Becomes Less Dense When It FreezesFreezes

For water to evaporate, heat must be added to water in the liquid state. After water reaches 100C, an input of 540 cal/gram is required to break the hydrogen bonds and allow evaporation.

The amount of energy required to break the bonds is termed the latent heat of vaporization.

Water has the highest latent heat of vaporization of any known substance.


Water Removes Heat from Surfaces Water Removes Heat from Surfaces As It EvaporatesAs It Evaporates

The energy input or output associated with water in the three states of matter. We must add 80 calories of heat energy to change a gram of ice to liquid water. After the ice is melted, about 1 calorie of heat is needed to raise each gram of water by 1°C. But 540 calories must be added to each gram of water to vaporize it – to boil it away. The process is reversed for condensation and freezing.


Surface Water Moderates Global Surface Water Moderates Global TemperatureTemperature

San Francisco, California, and Norfolk, Virginia, are on the same line of latitude, yet San Francisco is warmer in the winter and cooler in the summer than Norfolk. Part of the reason is that wind tends to flow from west to east at this latitude. Thus, air in San Francisco has moved over the ocean while air in Norfolk has approached over land. Water doesn’t warm as much as land in the summer nor cool as much in winter – a demonstration of thermal inertia.


Ocean-Surface Conditions Depend on Ocean-Surface Conditions Depend on Latitude, Temperature, and SalinityLatitude, Temperature, and Salinity

Average surface temperature and salinity for the world ocean. As you would expect, temperatures are lowest in the polar regions and highest near the equator. Heavy rainfall in the equatorial regions “freshens” the ocean near the equator, whereas hot and dry conditions near the tropic lines (Tropic of Capricorn and Tropic of Cancer) result in higher surface salinity in those areas.



Sea-surface temperatures during Northern Hemisphere summer.



Sea-surface average salinities in parts per thousand (‰).


The Ocean Is Stratified by The Ocean Is Stratified by DensityDensity

The complex relationship between temperature, salinity and density of seawater.

Note that two samples of water can have the same density at different combinations of temperature and salinity.


The Ocean Is Stratified into Three Density The Ocean Is Stratified into Three Density Zones by Temperature and SalinityZones by Temperature and Salinity

The ocean is divided into three density zones:

• Surface zone – the upper layer of the ocean, containing the least dense water. The surface zone is only about 2% of total ocean volume.

• Pycnocline – a zone in which density increases with depth, containing about 18% of all ocean water

• Deep zone – contains about 80% of all ocean water. There is little change in density throughout this layer.


The Ocean Is Stratified into Three Density The Ocean Is Stratified into Three Density Zones by Temperature and SalinityZones by Temperature and Salinity

Density stratification in the ocean.

(a) In most of the ocean, a surface zone (or mixed layer) or relatively warm, low-density water overlies a layer called the pycnocline. Density increases rapidly with depth in the pycnocline. Below the pycnocline lies the deep zone of cold, dense water – about 80% of total ocean volume.

(b) The rapid density increase in the pycnocline is mainly due to a decrease in temperature with depth in this area – the thermocline.

(c) In some regions, especially in shallow water near rivers, a pycnocline may develop in which the density increase with depth is due to vertical variations in salinity. In this case, the pycnocline is a halocline.


The Ocean Is Stratified into Three The Ocean Is Stratified into Three Density Zones by Temperature and Density Zones by Temperature and

SalinitySalinity

Typical temperature profiles at polar, tropical, and middle (temperate) latitudes. Note that polar waters lack a thermocline.


Refraction Can Bend the Paths of Refraction Can Bend the Paths of Light and Sound through WaterLight and Sound through Water

Sound and light both travel in waves:

• Refraction is the bending of waves, which occurs when waves travel from one medium to another.

• The refractive index is a ratio that expresses how much light is refracted from one medium to another.

Sunlight does not travel well in the ocean. Scattering and absorption weaken light:

• Scattering occurs when light is bounced between air and water molecules, dust and other objects.

• Absorption occurs when light’s electromagnetic energy is converted to heat in the molecules of seawater.


Refraction Can Bend the Paths of Refraction Can Bend the Paths of Light and Sound through WaterLight and Sound through Water

An analogy for refraction. The ranks of marchers represent light or sound waves; the pavement and sand represent different media. The marchers can walk faster if they stay on the pavement than if they walk in the sand next to the highway.

(a) If the marchers head off the pavement at an angle other than 90o, their path will bend (refract) as they hit the sand because some will be walking more slowly than others.

(b) If they march straight off the pavement, the ranks will slow down but not bend as they hit the sand


Water Transmits Blue Light More Water Transmits Blue Light More Efficiently Than RedEfficiently Than Red

Only a thin film of seawater is illuminated by the sun. Except for light generated by living organisms, most of the ocean lies in complete blackness.(a) The table shows the percentage of light absorbed in the uppermost meter of the ocean and the depths at which only 1% of the light of each wavelength remains.(b) The bars show the depths of penetration of 1% of the light of each wavelength (as in the last column of the table)


Sound Travels Much Farther Than Sound Travels Much Farther Than Light in the OceanLight in the Ocean

The relationship between water depth and sound velocity.


Refraction Causes Sofar Layers Refraction Causes Sofar Layers and Shadow Zonesand Shadow Zones

The sofar layer, in which sound waves travel at minimum speed.

Sound transmission is particularly efficient - that is, sounds can be heard for great distances - because refraction tends to keep sound waves within the layer.


Sonar Systems Use Sound to Sonar Systems Use Sound to Detect Underwater ObjectsDetect Underwater Objects

The principle of active sonar.

Pulses of high-frequency sound are radiated from the sonar array of the sending vessel.

Some of the energy of this ping reflects from the submerged submarine and returns to the sending vessel.

The echo is analyzed to plot the position of the submarine.


Sonar Systems Use Sound to Sonar Systems Use Sound to Detect Underwater ObjectsDetect Underwater Objects

Side-scan sonar in action. Sound pulses leave the submerged towed array.


Chapter in PerspectiveChapter in Perspective

In this chapter you learned of the polar nature of the water molecule. This polarity and the hydrogen bonds that form between water molecules result in water’s unexpected thermal properties. You found that liquid water is remarkable resistant to temperature change with the addition or removal of heat; and ice, with its large latent heat of fusion and low density, melts and refreezes over large areas of the ocean to absorb or release heat with no change in temperature. These thermostatic effects, combined with the mass movement of water and water vapor, prevent large swings in Earth’s surface temperature. And, of course, you also learned that the words heat and temperature are not interchangeable.

Changes in temperature and salinity greatly influence water density. Ocean water is usually layered by density, with the densest water on or near the bottom.

The physical characteristics of the world ocean are largely determined by the physical properties of seawater. These properties include water’s heat capacity, density, salinity, and ability to transmit light and sound.

In the next chapter you will learn what happens when solids and gases dissolve in seawater. Most of the properties of seawater are different from those of pure water because of the substances dissolved in the seawater.

End of Chapter 6

Water and Ocean Structure. © 2006 Brooks/Cole, a division of Thomson Learning, Inc. Key Concepts Water is a polar chemical compound composed of two hydrogen.

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