Seawater Chemistry Unit (4A-1) – page 1
Name:
Seawater Chemistry Unit II: Bonding
Section:
Surface Tension, Adhesion, and Cohesion
Water “beads up” more than other substances (e.g., alcohol), because the water molecules are
strongly attracted to one another – strongly bond with one another (“cohesion”) – so they hold
together instead of spreading out. Similarly, water molecules bond strongly with other substances
(“adhesion”). This is why drops of water remain on the window of your car after it rains or on
your body after a shower; the water molecules strongly bond with the glass molecules of the
window or the molecules of your skin, allowing them to resist gravity’s downward pull. Water
molecules “hold together” so strongly that small, dense objects (like a paper clip) will float on
the surface of the water if you place them on the water gently. (We call this effect “surface
tension.”) Once the object pierces the surface, though, the ordinary laws of density apply (it has
broken the bonds holding the water molecules together).
The strong bonds between the water molecules in a drop of water pull them tightly together; they
resist the Earth’s gravitational pull which is trying to pull them down and apart. However, the
weak bonds between the alcohol molecules in a drop of alcohol cannot hold them together; they
get pulled down and apart by gravity.
Water’s strong bonds play a role in its viscosity. Recall that the viscosity of a fluid relates to how
easily the fluid “flows.” For many plankton in the ocean, the bonds that neighboring water
molecules form with their bodies are so strong that the plankton do not sink (or sink much
slower) and have difficulty swimming (for them, it is somewhat like trying to swim in honey).
1. What is cohesion?
2. What is adhesion?
3. What is surface tension?
4. Which forms stronger bonds, molecules of water or molecules of alcohol?
Seawater Chemistry Unit (4A-1) – page 2
Dissolving (Solvents)
Water is often called the “universal solvent.” This refers to its ability to “break down” most other
substances. For example, soaking dirty dishes in water makes them easier to wash. The dried
food on the dishes is strongly bonded to the dish, but water molecules’ strong electrical attraction
pulls on the molecules of dried food (water molecules try to bond with them), pulling them off
the plate or at least weakening the bonds between the food molecules and the plate’s molecules
enough for you to easily rub the food off.
Recall, though, that the water’s hydrogen bonds are weaker than the bonds that hold most solids
together (e.g., the ionic bonds of salt crystals). How, then, can water dissolve them (break the
atoms or molecules apart)? A single water molecule is not attractive enough, but a group of
water molecules, all pulling together, is strong enough to overcome the electrically attraction
holding salt ions together.
Suppose two strong men are fighting. It is hard for one person – or even two – to pull them apart.
You need to have a group of people to separate them. In the same way, the bonds between the
atoms or molecules of solids are quite strong, so many water molecules – all pulling together –
are needed to separate them. (It is like a “tug-of-war.”)
When water “breaks down” substances like pollutants, it may separate their atoms (destroying
the pollutant molecules), or may separate the molecules and carry them around, spreading the
pollutants.
O
O
O
O
Cl-
Na+
O
O
O
Na+
Na+
O
Na+
O
O
Cl-
O
O
O
Cl-
O
Na+
O
O
O
Water can only dissolve so much (it can only “hold” so much salt), that it will eventually become
“saturated.” When all the water molecules are bonded to salt ions, they cannot break apart any
more salt crystals that are added to the water. The dense salt crystals will simply pile up on the
bottom of the container as we saw during the lab. Much like people, if the water molecules are
already “married” to salt ions, they are “unavailable;” they cannot go “bond” with other salt ions.
O
O
That would just be wrong.
+
Cl
Cl
Na
O
O
O
O
O
Na+
O
Cl-
Cl-
O
O
Na+
Na+
Cl-
O
O
Cl-
O
Groups of water molecules bond
(short, dark-red lines) with salt ions
(Na+ and Cl–), tearing them away
from the other salt ions.
O
O
Seawater Chemistry Unit (4A-1) – page 3
5. True or false? “Water can dissolve all substances.”
6. How is a dissolved salt atom different from a salt atom in a salt crystal (salt that has not
dissolved)?
7. Which bond is stronger, the bond between two water molecules in the ocean or the bond
between two salt atoms in a salt crystal?
8. Salt atoms have stronger bonds between them than water molecules. How, then, can
water molecules pull them away from one another (dissolve them)?
9. How or why does water become saturated and thus unable to dissolve any more salt?
Calculating Salinity
About 3.5% of seawater is “salt;” the other 96.5% is water. Salt refers to all of the substances
dissolved in ocean water which include gases like oxygen and nutrients like phosphate, not just
the components of ordinary salt, sodium and chlorine. Oceanographers typically describe salinity
in terms of “parts per thousandth” (‰ or ppt), not percentages, so oceanographers would say that
the ocean has an average salinity of 35 “parts per thousandth.”
Why do oceanographers do this? Scientists use the metric system (mathematically, it is a lot
easier, and doing science is hard enough without making the math harder). If they report their
measurements in parts per thousandth, then they can quickly determine how much salt is in the
water: 1 part per thousandth is about 1 gram of salt per liter of seawater. (A liter is half of a large
bottle of soda. A gallon is a little less than 4 liters.) So, if the ocean’s salinity is typically about
35‰, then there is about 35 grams of salt in each liter of seawater.
It is very easy to convert from percentages (“parts per hundredth”) to parts per thousandth: just
move the decimal point to the right by one place. So, 2.0% is 20‰, 3.1% is 31‰, 4.7% is 47‰,
and so on. This works because 2 out of 100 (2/100 = 0.02) is the same as 20 out of 1000
(20/1000 = 0.02). If you multiply both the top and bottom (the numerator and denominator) by
Seawater Chemistry Unit (4A-1) – page 4
10, then you are not actually changing the number, you are just changing the fraction from “out
of 100” to “out of 1000.”
Water is rarely “pure.” Good tasting tap water, for example, has a salinity below 0.6‰, and
premium bottled waters have salinities below 0.3‰. In coastal areas which get lots of “fresh”
water runoff, salinities may be as low as 10‰. We call this water “brackish.” In places with little
rainfall and lots of evaporation, the seawater salinity be over 40‰, and we say the water
“hypersaline.” The words “brine” or “briny” are also used to describe very salty water.
To calculate the salinity of a solution, just divide the amount of salt by the total amount of salt
water that is made. For example, suppose that we mixed 2.5 grams of salt with 100 grams of
water. The total amount of salt water would be 102.5 grams. So,
2.5 grams of salt / 102.5 grams of salt water = 0.0243
To make this into a percentage, we need to multiply by 100, so we’d get a salinity of 2.43%.
To make this into parts per thousandth, we need to multiply by 1000, so we’d get a salinity of
24.3‰.
To make it easier to measure ocean salinity, in 1978 oceanographers changed from using “parts
per thousandth” to “practical salinity units” (psu). 1 psu is about 1‰, so I am not going to worry
about the difference, and will use parts per thousandth. The change was made because
oceanographers typically determine seawater’s salinity by measuring its conductivity (salts are
electrically charged atoms – ions – so they conduct electricity). Psu is determined by comparing
the conductivity of a sample of seawater to the conductivity of a specially made salty solution.
10. What is the average salinity of the ocean? Give the percentage.
11. What is the average salinity of the ocean in parts per thousandth?
12. Suppose that some water has a salinity of 12 parts per thousandth.
What percent of the water sample is salt?
13. Suppose some water has a salinity of 6%. What is its salinity in parts per thousandth?
Seawater Chemistry Unit (4A-1) – page 5
pH: Acids and Bases
When a substance that we call an “acid” enters ocean water, its molecules often split, giving off
hydrogen nuclei, H+ (single protons). The water then becomes “acidic:” the wandering “acid”
H+ will try to bond with other substances, pulling them apart and breaking them down much like
a group of water molecules. However, since H+ forms ionic bonds, it is much stronger and more
violent than water molecules, which can be very damaging to living tissues (e.g., rip them apart).
pH is often said to stand for “potential of hydrogen.” The “p” is really an abbreviation for the
mathematical operation “log10” (a logarithm). pH = 7 indicates a neutral solution (acids and
bases balance one another), and solutions with pH < 7 are acidic and those with pH > 7 are basic
(alkaline). Differences in pH represent exponential differences (because of the logarithm), so
small changes in pH represent large differences in the amount of acids and bases in the solution.
For example, pH 6 means there are 10× more acids than bases in the solution. pH 5 means there
are 100× more acids than bases in the solution.
14. What numerical value of pH indicates that a fluid is neutral?
15. What numerical value of pH indicates that a fluid is acidic?
16. What numerical value of pH indicates that a fuild is basic (alkaline)?
17. If the pH of a fluid decreases (gets lower, gets smaller), is the fluid becoming more acidic
or more basic?
pH of the Ocean and Ocean Acidification
Today the average pH of the ocean is about 8.1. So, the ocean is a little basic (alkaline), but close
to neutral.
Fortunately for living things, there are carbon substances dissolved in ocean water which help
keep the ocean close to neutral (not too much “acid” H+ or “base” OH–, the opposite of “acid”).
When carbon dioxide from the atmosphere enters ocean water, it bonds with water molecules to
Seawater Chemistry Unit (4A-1) – page 6
become carbonic acid. Rain weathers the rocks of the land (breaks them down) and washes the
resulting bicarbonate and carbonate into the ocean. If the ocean is too basic, carbonic acid will
release acid into the ocean, neutralizing the base, and thus become bicarbonate. If the ocean is
still too basic, bicarbonate will release more acid into the ocean and become carbonate.
Similarly, if the ocean is too acidic, carbonate and bicarbonate will absorb acid out of the water,
making it more neutral. In other words, these substances (carbonic acid, bicarbonate, and
carbonate) release acid when the ocean is too basic and absorb acid when the ocean is too
acidic, keeping the ocean close to neutral pH (remember: the ocean is actually a little basic, on
average). We call this process “buffering.”
The fairly neutral pH of the ocean makes it better for ocean life. If ocean water were too acidic or
basic, it would begin to dissolve (“break down”) their bodies. Unfortunately, much of our carbon
dioxide pollution from burning fossil fuels (like gasoline or coal) leaks from the atmosphere into
the ocean (about half of it). Scientists have observed the resulting decrease in ocean pH, and are
particularly concerned about organisms that make their shells out of calcium carbonate. As was
discussed in the previous paragraph, if the ocean becomes too acidic, carbonate absorbs acid,
which causes the shells of ocean organisms to dissolve (the carbonate breaks its bond with the
calcium, and bonds with the acid instead). Even if this does not kill the organisms, acidic water
makes it harder for them to grow, draining their energy supplies and therefore making it more
difficult for them to survive and reproduce. For example, one study has shown that forams (a
kind of zooplankton) have calcium-carbonate shells almost 40% smaller than those of fossilized
foram remains found in ocean sediments. The good news is that by dissolving, the ocean
organisms are “buffering” the ocean, keeping it neutral for the life that remains. (They are
“taking one for the team.”) The concern, though, is that fewer phytoplankton and zooplankton
will survive, so there will be less food for animals higher up the food chain – including us.
18. Is ocean water strongly acidic, slightly acidic, neutral, slightly basic, or strongly basic?
19. How are humans changing the pH of ocean water? In other words, what are we doing?
20. Is the pH of ocean water increasing or decreasing?
Does this mean that ocean water is becoming more acidic or more basic?
21. What ocean organisms are suffering most directly due to the change in the pH of ocean
water? What is happening to them?