Key Concept Review (Answers to in-text “Concept Checks”)
Chapter 7
1. In the polar water molecule, opposites attract: Its positive end attracts particles
having a negative charge, and its negative end attracts particles having a positive
charge.
2. 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.
3. The ions of sodium and chloride in NaCl are held together by ionic bonds,
electrostatic attraction that exists between ions that have opposite charge. When
NaCl dissolves in water, the polarity of water reduces the electrostatic attraction
(ionic bonding) between the Na+ and Cl- ions and the two ions separate.
4. A solution is made of two components: The solvent, usually a liquid, is always the
more abundant constituent; the solute, often a dissolved solid or gas, is the less
abundant. In a true solution, the molecules of the solute are homogeneously
dispersed among the molecules of solvent -- the solution has uniform properties
throughout. In a mixture, different substances are closely intermingled but retain
separate identities.
5. Perfume fragrance leaving warm skin (where its concentration is high) and
moving into the surrounding air (where its concentration is low) is an example of
diffusion. Diffusion in the ocean is important for many reasons, among them the
transfer of gases across the sea-air interface.
6. 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.
Oceanographers use parts-per-thousand notation (not percent, which is parts-perhundred), so 3.3% = 33o/oo.
7. Because colligative properties are the properties of solutions, the more
concentrated (saline) water is, the more important these properties become. For
seawater, these properties include a decrease in heat capacity, lowering of
freezing point, and other factors. (Because it is not a solution, pure water has no
colligative properties.)
8. Other than hydrogen and oxygen, the most abundant ions in seawater are chloride,
sodium, sulfate, magnesium, and calcium.
9. 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.
10. 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.
11. 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.
12. 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.
13. Unlike solids, gases dissolve most readily in cold water.
14. In order of their relative abundance, the major gases found in seawater are
nitrogen, oxygen, and carbon dioxide.
15. Since gases dissolve most easily in cold water, the dissolved oxygen
concentrations in parts of the tropical ocean may be so low that animals may be
unable to survive.
16. There is about 60 times as much CO2 dissolved in the ocean as in the atmosphere.
17. 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.
18. 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.
19. 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.
20. 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.
21. Lowering of oceanic pH (increasing acidity) might make the deposition of hard
calcium-containing structures by marine organisms difficult or impossible. Coral
animals would have difficulty forming reefs, and organisms like the
coccolithophores and foraminifera you will encounter in Chapter 14 may have
weaker shells.