Chapter 6: Water and Ocean Structure Insert: Textbook cover photo

The Water Molecule Is Held Together by Chemical Bonds Water Has Unusual Thermal Characteristics Surface Water Moderates Global Temperature The Ocean Is Stratified by Density Refraction Can Bend the Paths of Light and Sound through Water Light Does Not Travel Far through the Ocean Sound Travels Much Farther than Light through the Ocean

Main Concepts Heat is not the same as temperature. Temperature is an object’s response to an input (or removal) of heat. Not all substances respond in the same way. Water “resists” increasing in temperature as heat is added. Water gives off heat when it freezes, and absorbs heat as it thaws. These properties of liquid water moderate Earth’s surface temperatures. The ocean is density stratified. Dense cold and salty water underlies less dense warm and fresher water. Light is quickly extinguished by passage through water. Sound is not. Light and sound can be bent (refracted) by passage through water masses whose physical characteristics differ.

The Water Molecule Is Held Together by Chemical Bonds Compounds – substances that contain two or more different elements in fixed proportions Elements – a substance composed of identical particles that cannot be chemically broken down into simpler substances Atoms – the particles that make up elements

The Water Molecule Is Held Together by Chemical Bonds 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 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.

The Water Molecule Is Held Together by Chemical Bonds Hydrogen bonds hold water molecules together. They form when the positive end of one water molecule bonds to the negative end of another water molecule. 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

Water Has Unusual Thermal Characteristics Heat and Temperature Are Not the Same Thing 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 1C. Water has a very high heat capacity, which means it resists changing temperature when heat is added or removed.

Water’s Temperature Affects Its Density 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 Density The three common states of matter – solid, liquid, and gas. Gas: Atoms or molecules of gas are in high-speed motion and move in random directions. Liquid: Atoms or molecules of a liquid move freely past one another as individuals or small groups. Solid: A solid is a substance that resists changes of shape or volume.

Water Becomes Less Dense When It Freezes 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 Freezes For water to evaporate, heat must be added to water in the liquid state. After water reaches 100C, 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 As 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 Temperature 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 Latitude, Temperature, and Salinity Average surface temperature and salinity for the world ocean. Temperatures are lowest in the polar regions and highest near the equator. Heavy rainfall in the equatorial regions “freshens” the ocean near the equator. Hot and dry conditions near the tropic lines (Tropic of Capricorn and Tropic of Cancer) result in higher surface salinity in those areas.

The Ocean Is Stratified by Density 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 divided into three density zones The Ocean Is Stratified into Three Density Zones 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 Zones by Temperature and Salinity Density stratification in the ocean. (a) In most of the ocean, a surface zone overlies a layer called the pycnocline. Density increases rapidly with depth in the pycnocline. (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 Density Zones by Temperature and Salinity (LEFT) Typical temperature profiles at: polar, tropical, and middle latitudes. Where there is a thermocline, density stratification usually prevents vertical water movement.

Refraction Can Bend the Paths of Light 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.

Refraction Can Bend the Paths of Light 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 90°, their path will bend (refract) as they hit the sand. (b) If they march straight off the pavement, the ranks will slow down but not bend as they hit the sand

Light Does Not Travel Far Through the Ocean Only a thin film of seawater is illuminated by the sun. The thin film of lighted water at the top of the surface zone is called the photic zone. The ocean below the photic zone lies in blackness. This dark water beneath the photic zone is called the aphotic zone.

Water Transmits Blue Light More Efficiently Than Red From above, clear ocean water looks blue because blue light can travel through water far enough to be scattered back through the surface to our eyes. (LEFT) The bars show the depths of penetration of 1% of the light of each wavelength.

Sound Travels Much Farther Than Light in the Ocean The relationship between water depth and sound velocity.

Refraction Causes SOFAR Layers and Shadow Zones The SOFAR layer, in which sound waves travel at minimum speed. Sound transmission is particularly efficient because refraction tends to keep sound waves within the layer.

Sonar Systems Use Sound to Detect 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 Detect Underwater Objects Side-scan sonar. Sound pulses leave the submerged towed array, bounce off the bottom, and return to the device. Computers process the impulses into images.