Answers to STUDY BREAK Questions
Essentials 5th
Chapter 6
WATER
1. How are atoms different from molecules?
Atoms cannot be broken into simpler substances by chemical means. A molecule is a
group of atoms held together by chemical bonds.
2. What holds molecules together?
Molecules are held together by chemical bonds. Chemical bonds, the energy
relationships between atoms that hold them together, are formed when electrons are shared
between atoms or moved from one atom to another.
3. Why is water a polar molecule? What properties of water derive from its polar nature?
The water molecule has a positive end that attracts particles that have a negative
charge, and a negative end (or pole) that attracts particles with a positive charge. When water
comes into contact with compounds whose elements are held together by the attraction of
opposite electrical charges (most salts, for example), the polar water molecule will separate
that compound’s component elements from each other.
4. Why does water look blue?
Hydrogen bonds selectively remove red light (changing its energy into heat). The
remaining blue light travels farther through the water, and may strike objects to reflect back
through the surface to your eyes.
5. How is heat different from temperature?
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.
6. What is meant by heat capacity? Why is the heat capacity of water unique?
Heat capacity is a measure of the heat required to raise the temperature of 1 gram of a
substance by 1°C. Not all substances respond to identical inputs of heat by rising in
temperature the same number of degrees. Water’s heat capacity is exceptionally high.
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Essentials 5th
7. What factors affect the density of water? Why does cold air or water tend to sink? What
role does salinity play?
Temperature and salinity affect water density. Cold, salty water tends to sink.
8. How is water’s density affected by freezing? Why does ice float?
Ice is less dense than liquid water—and thus floats—because the molecules are
packed less efficiently.
9. What is the difference between sensible and nonsensible heat?
Sensible heat can be sensed by a temperature change measurable on a thermometer.
Nonsensible heat is known as “latent.” (See next item.)
10. What’s the latent heat of fusion of water? The latent heat of vaporization? Why do we use
the term latent?
Removing a calorie of heat from freezing water at 0°C (32°F) won’t change its
temperature at all; 80 calories of heat energy must be removed per gram of pure water at 0°C
(32°F) to form ice. This heat is called the latent heat of fusion.
At 540 calories per gram at 20°C (68°F), water has the highest latent heat of
vaporization of any known substance.
The term latent applies to heat input that does not cause a temperature change but
does produce a change of state (as from a liquid to a gas, or vice versa).
11. What is thermal inertia?
The tendency of a substance to resist a change in temperature with the gain or loss of
heat energy is called thermal inertia.
12. Why is the fact that ice floats important to Earth’s generally moderate climate?
More 12 million square miles of surface, thaws and refreezes in the polar ocean each
year. Removing a calorie of heat from freezing pure water at 0°C (32°F) won’t change its
temperature at all—80 calories of heat energy must be removed per gram of liquid water to
form ice. Incoming solar heat melts ice in the local polar summer, but the ice melts and the
ocean’s temperature doesn’t change. The situation reverses in winter – the water freezes and
again the temperature doesn’t change.
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Essentials 5th
13. How is heat transported from tropical regions to polar regions?
Masses of moving air account for about two-thirds of the poleward transfer of heat;
ocean currents move the other third.
14. How has the ocean’s density structure thought to have changed perhaps because of
accelerating global warming?
Global warming appears to be causing the tropical ocean shallower than 1,000 meters
to become warmer and saltier, while water in the far north and south has become fresher.
15. How is seawater’s salinity expressed?
The total quantity (or concentration) of dissolved inorganic solids in water is its
salinity. The ocean’s salinity varies from about 3.3% to 3.7% by mass.
16. Other than hydrogen and oxygen, what are the most abundant ions in seawater?
Other than hydrogen and oxygen, the most abundant ions in seawater are chloride,
sodium, sulfate, magnesium, and calcium.
17. What are the sources of the ocean’s dissolved solids?
The ocean’s dissolved solids are derived from erosion and weathering of ocean basins
and the contact of seawater with hot minerals at mid-ocean rifts.
18. What is the principle of constant proportions?
The Principle of Constant Proportions states the ratio of various salts in seawater is
the same in samples from many places, regardless of how salty the water is.
19. How is salinity determined?
The chloride ion is abundant and comparatively easy to measure, and it always
accounts for the same proportion of dissolved solids (55.04%), so marine chemists devised
the concept of chlorinity to simplify measurement of salinity.
20. What is meant by “residence time”? Does seawater itself have a residence time?
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Essentials 5th
Residence time is the average length of time an atom of an element spends in the
ocean. Seawater itself has a residence time of about 4,100 years.
21. Which dissolves more gas per unit volume – cold seawater or warm seawater?
Unlike solids, gases dissolve most readily in cold water.
22. What happens when carbon dioxide dissolves in seawater?
Initially, carbon dioxide and water combine to form carbonic acid. Carbonic acid
then rapidly dissociates into a bicarbonate ion and a hydrogen ion. Most of the carbon
dioxide dissolved in seawater ends up as bicarbonate. Some of the bicarbonate and hydrogen
ions will then combine to form carbonate ions and two hydrogen ions.
23. How do concentrations of oxygen and carbon dioxide vary with ocean depth?
In areas of rapid photosynthesis pH will rise because carbon dioxide is used by plants
and plant-like organisms. Surface pH in warm productive water is usually around 8.5. At
middle depths and in deep water, more carbon dioxide may be present than at the surface.
With cold temperatures, high pressure, and no photosynthetic plants to remove it, this CO2
will lower the pH of water, making it more acid with depth.
24. How is pH expressed? What’s neutral?
The acidity or alkalinity of a solution is measured in terms of the pH scale, which
measures the concentration of hydrogen ions in a solution. An excess of hydrogen ions in a
solution makes that solution acidic. An excess of hydroxide ions makes a solution alkaline.
A pH of 7 is neutral.
25. What is a buffer? How might seawater’s ability to act as a buffer be important?
A buffer is a group of substances that tends to resist change in the pH of a solution.
Buffering prevents broad swings of pH when acids or bases are introduced into the ocean.
26. How is the ocean stratified by density? What names are given to the ocean’s density
zones?
Seawater’s density increases with increasing salinity, increasing pressure, and
decreasing temperature. The three strata (density zones) are the surface zone (or mixed
layer), the pycnocline, and the deep zone.
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Essentials 5th
27. What, generally, are the water characteristics of the surface zone? Do these conditions
differ significantly between the polar regions and the tropics?
The surface zone consists of water in contact with the atmosphere and exposed to
sunlight. It contains the ocean’s least dense water and accounts for only about 2% of total
ocean volume. Conditions in the surface zone differ greatly with latitude.
28. What, generally, are the water characteristics of the deep zone? Do these conditions
differ significantly between the polar regions and the tropics?
The deep zone lies below the pycnocline at depths below about 1,000 meters in midlatitudes (40°S to 40°N). Conditions are consistent at all latitudes. This deep zone contains
about 80% of all ocean water. Deep zone conditions are very similar everywhere in the
ocean.
29. How is the pycnocline related to the thermocline and halocline?
The pycnocline is a zone in which density increases with increasing depth. It is a
combination of the thermocline and halocline, zones in which density increases with depth as
temperature falls and salinity rises.
30. How is a water mass defined?
A water mass is a body of water with characteristic temperature and salinity and
therefore density.
31. How does the ocean’s density stratification limit the vertical movement of seawater?
The great difference in temperature, and therefore density, between surface water and
deep water in the tropics makes the water column very stable and prevents an exchange of
surface and deep water.
32. What factors influence the intensity and color of light in the sea?
Scattering occurs as light is bounced between air or water molecules, dust particles,
water droplets, or other objects before being absorbed. The absorption of light is governed by
the structure of the water molecules it happens to strike. When light is absorbed, molecules
vibrate and the light’s electromagnetic energy is converted to heat. In the ocean, red light is
converted to heat more efficiently than blue.
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Essentials 5th
33. Intensities being equal, which color of light moves farthest through seawater. Least far?
What happens to the energy of light when light is absorbed in seawater?
The top meter of ocean absorbs 71% of red light. The dimming light becomes bluer
with depth because the red, yellow, and orange wavelengths are being absorbed. By 300
meters even the blue light has been converted into heat.
34. What factors affect the depth of the photic zone? Could there be a photocline in the
ocean?
Even the clearest seawater is not perfectly transparent. In the very clearest tropical
waters the photic zone may extend to a depth of 600 meters, but a more typical value for the
open ocean is 100 meters. In contrast, light typically penetrates the coastal waters in which
we swim only to about 40 meters. “Photocline” would refer to a rapid decrease in light with
increasing depth. Local increases in turbidity could cause a photocline.
35. Which moves faster through the ocean—light or sound?
Light travels much faster than sound in both air and water.
36. How much faster is the speed of sound in water than in air?
The speed of sound is almost five times faster in water than in air, about 1,500 meters
per second.
37. Is the speed of sound the same at all ocean depths?
The speed of sound in seawater increases as temperature and pressure increase. Sound
travels faster at the warm ocean surface than it does in deeper, cooler water. Its speed
decreases with depth, eventually reaching a minimum at about 1,000 meters. Below that
depth, however, the effect of increasing pressure offsets the effect of decreasing temperature;
so speed increases again.
38. What’s a sofar layer?
Sound waves bend toward layers of lower sound velocity and so tend to stay within
this minimum-velocity zone. Therefore, loud noises made at this depth can be heard for
thousands of kilometers. The minimum-velocity layer has come to be known as the sofar
layer.
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Essentials 5th
39. How does sonar work? What kinds of sonar systems are used in oceanographic
research?
Active sonar, the projection and return through water of short pulses (pings) of highfrequency sound to search for objects in the ocean, is used to search for objects or ocean
conditions of interest to researchers. Side-scan solar is among the most useful active sonar
devices
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