Introduction to Ocean Sciences

Chapter 7: Physical Properties of Water and Seawater

Guide to Reading and Learning

This chapter describes the anomalous nature of the physical properties of water and seawater and relates these properties to the many aspects of our daily life—and of ocean and atmospheric processes—that depend on these properties. Thus, Chapters 6 and 7 serve as a fundamental base on which to understand concepts in many other parts of the text, especially the processes by which the oceans and atmosphere participate in distributing heat and water on our planet and controlling our climate.

As we found in the previous chapter, water’s unique properties are intimately connected with every part of our lives. Chapter 7 examines water’s unique physical properties and their effects and influence on our weather and climate, the distribution of life on the planet, and many of the mundane, but valued, characteristics of our lives, such as ice-cold drinks and cooking.

As you read this chapter, try to think about how the properties of water affect your personal life. For example, if you live in a colder climate and go ice fishing or ice skating on lakes, the uniqueness of the freezing process for water will be of immediate interest and meaning to you. Similarly, if you live in the southeastern United States you will have an immediate and personal connection to the relationship between high latent heat of evaporation and hurricanes, while those who live in the Pacific Northwest may be more interested to learn that this same property is largely responsible for the large differences between the climates of the coastal and inland parts of your region.

The most important concept in an ocean science course is that large quantities of heat are effectively stored in the oceans and ice, and then transported through the oceans and atmosphere, as a result of water’s heat properties and phase changes. This is why Figure 7.2 occupies an entire page and shows the same information in a number of different ways. Be absolutely sure to understand and learn the information in this figure.
The discussion of the light transmission and sound transmission properties of seawater is somewhat separate from the rest of the chapter. However, it is these properties that make the ocean environment very different from the terrestrial environment, especially in the vast lightless volume of the deep ocean. Marine species, as well as scientists who study the oceans, have had to adapt to these differences.

Chapter 7 Essential to Know 

Critical Concepts used in this chapter

CC.1, CC.3, CC.5, CC.6, CC.14

7.1 Heat Properties of Water

  • The most important of water’s unique properties are its heat properties.

Heat Energy and Phase Changes

  • All substances can exist in three phases: solid, liquid, and gas.
  • In a solid, molecules are fixed in place with respect to each other. In a liquid, they have enough heat energy that they are free to move about but not to completely escape from the attractive forces of their neighbor. In a gas, they have enough energy to escape these attractive forces entirely.
  • If we add heat to a solid it will, at some point, have enough heat to become a liquid and then eventually, if we add more heat, a gas. If we remove heat the sequence occurs in reverse.
  • The attractive forces between molecules (van der Waals force) are different for different substances. The stronger the attractive forces, the higher the temperatures needed to melt the solid and to convert liquid to gas—the freezing and boiling points.

Freezing and Boiling Points

  • To melt ice to become water or boil water to become water vapor the molecules must have enough energy to overcome not only the van der Waals attractive force but also the much-stronger hydrogen bond.
  • As a result of the hydrogen bond, the melting point and boiling point temperatures of water are much higher than for other similar substances.
  • Without the hydrogen bond, ice would melt at about –90oC, water would boil at –70oC, and there would be no life as we know it on the Earth because all water would be gaseous.

Heat Capacity and Latent Heat

  • If we heat a solid that is at a temperature below its melting point, its temperature will increase until it reaches the melting point. If we continue to add heat, the temperature will not change any further until the solid is melted, and then the temperature will again rise until the liquid has reached its boiling point. If we continue to add heat, the temperature will not change again until all the liquid has become gas, after which adding more heat will again raise its temperature.
  • The amount of heat that is required to raise the temperature of a substance by 1oC without changing its phase is called its heat capacity. Heat that changes a substance’s temperature without changing its phase is called sensible heat.
  • Due to the hydrogen bond, the heat capacity of liquid water (4.2 J·g–1·oC–1) is higher than any other substance except ammonia (one of a very few compounds other than water that has a hydrogen bond, although it’s hydrogen bond is not as strong water’s). The heat capacity of ice and water vapor are also very high, although they are only about half that of liquid water.
  • The amount of heat that must be added to convert a solid to a liquid without changing its temperature, and to convert a liquid to a gas without changing its temperature, are called the latent heat of fusion and the latent heat of vaporization.
  • The latent heat of fusion of water (334 J·g-1) is higher than that of any other substance except ammonia, and the latent heat of vaporization of water (2,260 J·g-1) is higher than any other known substance.
  • When a gas is allowed to cool to become a liquid and eventually a solid, both the sensible heat and the latent heat of vaporization and fusion are released.

Implications of the High Heat Capacity and Latent Heats of Water

  • Water’s extremely high heat capacity, latent heat of fusion, and latent heat of vaporization are all important because they allow water to store and transport heat within the ocean–atmosphere system.
  • The high heat capacity of water allows the oceans to store large quantities of the sun’s heat energy with only a small change in the water temperature. Similarly, the oceans can release large amounts of heat to the atmosphere with only a very small temperature change. Thus, ocean water temperature changes much more slowly than land temperature with changes in solar heating or cooling, and this allows the oceans to modify coastal land climates to be much milder than locations in the interior of continents.
  • The high latent heat of fusion of water allows large amounts of heat to be added to an ice–water mixture without any change of temperature. In high latitudes, this allows heat to be stored by melting ice in summer and returned by freezing water to ice in the winter without any temperature change occurring in the polar ocean water. Thus, polar ocean water is always at or near the freezing point.
  • The high latent heat of vaporization of water is also important to the Earth’s climate (see “Evaporation”).

Evaporation

  • Liquids, including water, can be converted to the gaseous phase at temperatures below their boiling point by a process called evaporation. Evaporation occurs because the energy levels of individual molecules within a liquid vary, and some molecules may temporarily possess a high-enough energy level to escape the attractive bonds of other molecules and enter the gaseous phase.
  • To evaporate a molecule from liquid below its boiling point, the evaporated molecules require more heat than would be needed to evaporate the molecule if the liquid were at its boiling point. Thus, the latent heat of evaporation of water is higher than its latent heat of vaporization.
  • Water evaporated from the oceans carries the very large amount of heat associated with its evaporation with it when it enters the atmosphere. This heat is transported with the water vapor in the atmosphere until the water condenses to become rain or snow when the heat is released to the atmosphere.. This mechanism of heat transport is critical to the distribution of the sun’s heat in the Earth’s atmosphere, especially the transport of heat from low latitudes to higher latitudes and the transport of heat from the oceans onto the continents.  As a result, this mechanism is a critical factor in controlling climate and is also the source of energy for weather systems.

7.2 Effects of Pressure, Temperature, and Dissolved Salts on Seawater Density

  • Many of the movements of water masses in the oceans are driven by differences in water density. Higher-density water sinks, whereas lower-density water rises.

Pressure

  • Water density is increased by increasing pressure.
  • The effect of pressure on density is small compared to the effects of temperature and salinity. At the greatest depths of the oceans water density is increased only about 2% despite the more than 1,000 atmospheres pressure.

Temperature

  • Increasing temperature adds energy to molecules of solids and liquids so that they can move more easily with respect to neighboring molecules. As a result, the distance between molecules normally increases with increasing temperature, which decreases density. Thus, decreasing temperature normally increases density.
  • Throughout most of its temperature range, the density of water increases as temperature decreases. However, below 4oC, water behaves anomalously: from this temperature to the freezing point, reducing temperature actually decreases the density. This anomalous behavior is the result of the creation clusters of of water molecules in which molecules are further apart than they are in the normal arrangement. These form and reform and exist on average for less than one millionth of a second. However, they form more frequently as temperature decreases.  As a result, their effect  is to overcome the normal increase in density that occurs as temperature falls between 4oC and the freezing point and casue the density to decrease..

Dissolved Salts and Density

  • Salt dissolved in water increases water density because dissolved ions or molecules usually have higher densities than water molecules, but also because the dissolved substances reduce clustering of the water molecules.

Combined Effects of Salinity and Temperature

  • Seawater density is decreased by increases in salinity and by decreases in temperature.
  • The range of salinities found in most ocean water is small—99% of ocean water has a salinity between 33 and 37, and 75% has a salinity of between 34 and 35. As a result, the density of ocean water is generally affected more by changes in temperature than changes in salinity
  • Salinity changes are relatively more important than temperature changes in high-latitude regions because water temperatures are usually uniformly at or near the freezing point, and because salinity varies more due to river runoff and ice melt. Salinity is also more important than changes in temperature in determining the density of many estuarine waters.

7.3 Ice Formation

  • Two factors determine the behavior of ice and water during freezing. These are the relative density of ice and water and the variations of water density near the freezing point.

Dissolved Salts and Freezing

  • The freezing point of water decreases with increasing salinity. Seawater freezes at about –3 to –4oC. During freezing, the dissolved salts are excluded from the ice.
  • Pure water has a density maximum at 4oC. The density maximum is found at lower temperatures as salinity increases and disappears above salinity 24.7.
  • When lakes cool, the density of the water increases until the temperature reaches 4oC. However, below this temperature, freshwater density decreases as temperature decreases, so the surface layer continues to cool but does not sink. As cooling continues, ice forms at the surface while the water below the ice remains at 4oC.
  • When the oceans are cooled, the density of seawater increases with decreasing temperature until the freezing point is reached at about –2oC. Because cooled surface water continues to sink until ice is formed, the water column below sea ice is usually uniformly at a temperature of about –2oC.

Density of Ice

  • Ice has an open lattice structure in which the water molecules are further apart than in liquid water. As a result, ice has a substantially lower density than water and floats on both freshwater and seawater.
  • Most substances have a solid phase that is more dense than the liquid phase, so the open lattice structure of ice and its low density are anomalous. If ice did not float, it would sink to the bottom as it formed and the oceans and lakes would have a solid ice layer through most of their depth, with only a shallow water layer on top.

7.4 Surface Tension and Viscosity

Surface Tension

  • Because of the hydrogen bond, water has a high surface tension.
  • High surface tension enhances the formation of gas bubbles in the water column, and spray droplets in the air, when these are created by breaking waves. It also lengthens the time that these gas bubbles and spray droplets persist. Gas bubbles and spray enhance the rate of exchange of gases such as oxygen between ocean water and air.
  • High surface tension is responsible for reducing water surface roughness (very small waves) caused by light winds.
  • The high surface tension of water creates a surface microlayer within which certain species can anchor themselves or their eggs and larvae.

Viscosity

  • Water has a relatively high viscosity, or internal resistance to flow.
  • While water’s viscosity is not as high as that of liquids like honey or motor oil, its viscosity is high enough to make swimming or moving through water difficult for very small organisms. Many marine microscopic organisms make use of this viscosity to help them avoid sinking.
  • Viscosity is reduced when temperatures increases, so it is more difficult for microscopic organisms in tropical waters to avoid sinking than it is for species in higher latitudes.

7.5 Transmission of Light and Other Electromagnetic Radiation

  • Light is electromagnetic radiation emitted by the sun and stars. Visible light occupies only a very small part of the electromagnetic spectrum, which includes radiation ranging from very long wavelength radio waves to very short wavelength gamma and cosmic rays.

Absorption

  • Water strongly absorbs electromagnetic energy of most wavelengths.
  • Water absorbs light energy rapidly but not as effectively as it absorbs some other types of electromagnetic radiation, so light does penetrateinto ocean water but only for a short distance. Most light is totally absorbed in the upper few tens of meters of the ocean water column.
  • Water transparency is generally greatest in the blue-green wavelengths of visible light, but this varies with the concentration and composition of suspended particles.
  • Sunlight does not penetrate below the first 100 m below the surface and penetrates much less in high-turbidity coastal and estuarine waters.

Scattering and Reflection

  • As light passes through water, it is reflected by particles suspended in the water and by water molecules and molecules of dissolved substances. The light is reflected in all directions and is said to be scattered. The portion of light that is scattered back toward the ocean surface is said to be backscattered.
  • In clear ocean waters with few particles, red and yellow wavelengths of light are absorbed quickly but blue light penetrates to deeper depths. The blue light therefore has more opportunity to be backscattered, and it is this backscattered blue light that gives the open ocean its deep blue color.
  • In waters with more suspended particles the backscattered light is composed of a wider range of wavelengths, and coastal ocean waters appear either green (if many of the particles are green chlorophyll-containing phytoplankton) or brown (if the particles are primarily suspended mineral grains).

Refraction     

  • The speed of light is lower in seawater than in air. As a result, light rays are refracted (bent) as they pass between air and water. Refraction between air and water increases as salinity increases, and measurements of refraction are often used to measure approximate salinities.

7.6 Transmission of Sound

  • Sound is fundamentally different from electromagnetic energy. Sound is transmitted as a vibration in which adjacent molecules are compressed in sequence. Sound cannot travel through a vacuum but it can travel easily through water.

Sound Velocity

  • The speed of sound in water is about four times faster than in air.
  • The speed of sound increases with increased salinity, pressure, and temperature. However, changes caused by salinity differences are relatively small within the range of ocean water salinities.
  • Sound velocity generally decreases with depth in the upper layers of the oceans and then increase with depth in the deep layers.
  • Sound velocity can be precisely determined from the salinity, temperature, and pressure.

Sonar

  • Sound waves are used in various sonar systems to observe seafloor topography by sending a sound from the surface to be reflected off the seafloor and back to a receiver located near the source. Depth can be determined if the sound velocity structure of the water column is known and the time the sound takes to make the round trip to the seafloor is measured.
  • Sonar can also be used to locate submarines or fish schools by monitoring echoes from these objects.

Sound Refraction and the Sound Channel

  • Sound waves are transmitted through water with little loss of intensity but the sound waves are refracted as they move through water of varying density and they are reflected at interfaces between layers of different densities.
  • Because there is a sound velocity minimum at a depth of around 1 km in most parts of the oceans, sound is refracted in complex ways depending on the location of the sound source and the direction of the sound wave as it moves away from the source.
  • Refraction of sound emitted from a surface vessel can create a shadow zone at mid-depths into which no sound can be sent from that surface location. This shadow zone can hide submarines from the surface vessel sonar, and monitoring of this shadow zone by navies has provided ocean science with large amounts of data describing the variations of temperature and salinity with depth throughout the oceans
  • If the sound source is located at or near the depth of the sound velocity minimum, the emitted sound waves are refracted back and forth within a relatively narrow layer. Sound can travel great distances within this sound channel with very little loss in intensity.

Acoustic Thermometry

  • Because sound velocity in seawater depends on water temperature, changes in the average water temperature between two points in the oceans can be measured by monitoring for changes in the sound transmission time between the sites.
  • This monitoring technique, now called acoustic thermometry, is now used to monitor the average temperature of the ocean water at sound channel depth. Sound signals are routinely sent across thousands of kilometers of oceans to remote listening posts. The sensitivity of this technique is so high that it is expected to provide a much earlier indication of the direction and magnitude of future global temperature change than any other method as yet known.

Ocean Noise

  • Because sound is transmitted easily through seawater, many marine species have evolved means to use sound for communication and other purposes.
  • Sensitive listening devices are now used to study various marine species by isolating their sounds from the biological and vessel noise collected at ocean listening stations

 

Critical Concept Reminders:

CC.1 Density and Layering in Fluids (p. 154)

  • Water in the oceans is arranged in layers according to the water density. Many movements of water masses in the oceans are driven by differences in water density. To read CC1 go to page 2CC.

CC.3 Convection and Convection Cells (pp. 154, 157)

  • Fluids, including ocean water, that are cooled from above or heated from below, sink or rise because their density is increased or reduced respectively. This establishes convection processes that are a primary cause of vertical movements and the mixing of ocean waters. To read CC3 go to page 10CC.

CC.5 Transfer and Storage of Heat by Water (p. 153)

  • Water’s high heat capacity allows large amounts of heat to be stored in the oceans and released to the atmosphere without much change of ocean water temperature. Water’s high latent heat of vaporization allows large amounts of heat to be transferred to the atmosphere in water vapor and then transported elsewhere. Water’s high latent heat of fusion allows ice to act as a heat buffer reducing climate extremes in high latitude regions. To read CC5 go to page 15CC.

CC.6 Salinity, Temperature, Pressure, and Water Density (p. 154)

  • Sea water density is controlled by temperature, salinity, and to a lesser extent pressure. Density is higher at lower temperatures, higher salinities, and higher pressures. Movements of water below the ocean surface layer are driven primarily by density differences. To read CC6 go to page 16CC.

CC.14 Photosynthesis, Light, and Nutrients (p. 159)

  • Photosynthesis, the major process in the production of living matter, depends on the availability of light. Thus, the transmission of light through ocean water limits the depth to which photosynthesis may occur. To read CC14 go to page 46CC.

 

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