Atoms, Water Molecules, and the Electric Force

Oceanography 10, T. James Noyes, El Camino College
Water Chemistry
The Atomic Theory of Matter
For many centuries, people wondered what everything was made of. Some proposed that
everything was made of mixtures of 4 substances: earth, air, fire, and water. This is actually a
pretty good hypothesis, because it explained many of the observations they had at the time. For
example, mud is a combination of earth and water. Similarly, wood comes from plants which
contain earth and water that they absorb through their roots, and fire from the sun (everybody
knows plants need sunlight – “fire” – to grow). Therefore, plants burn – but not rock – because
they have an inner fire that can be released.
There are many things that the earth-air-fire-water hypothesis cannot not easily explain like the
existence of several “airs” (gases) that the early chemists began to isolate in the 18th century.
This led to the revival of an old idea (from 2500 years ago!) of some ancient Greeks that
everything is made of small particles that they called “atoms” (“uncuttable,” “unbreakable”), and
that the atoms’ motion and how they join or split determines the properties of a substance.
Today, we know that atoms are made of even smaller particles – and that some of these (protons
and neutrons) are made of even smaller objects called quarks. All of them may be composed of
“strings” of energy. In addition to the particles in atoms, there are a multitude of other particles
(muons, pions, kaons, omega particles, various “flavors” of neutrinos, and so on). Fortunately
for us, most of the particles that we encounter on the Earth are just plain, old atoms.
Interestingly, astronomers now estimate that less than 1/20th of the universe is made up of plain,
old atoms and the rest is made of more exotic stuff; we don’t know what this stuff is and call it
“dark matter” and “dark energy,” because we cannot see it directly using telescopes.
In this class we will study an old-fashioned 1 (early 20th century) version of the atomic theory of
matter, because we do not have time for much more. The basic idea is that everything (tables,
chairs, air, you, me, orange juice, and so on) is made of atoms, particles so small that there are
far more than a billion billion of them in a small cup of water. The properties of a substance
depend upon the kinds of atoms it is made of, how the atoms join together (bond), and how the
atoms are moving (fast/slow, wiggling in place/flying around)
Atoms are constantly entering and leaving objects. For example, you breathe in oxygen and
breathe out carbon dioxide. Plants extract carbon dioxide from the air and nutrients from the
soil, and use them to build their bodies. Animals like cows eat the plants and rearrange the
carbon and nutrients they eat to build their own bodies. We eat the cows, and breathe out the
carbon dioxide. The nutrients are excreted, and we can put them back in the soil to help plants
grow. (The great circle of life…) The key point here is that under most circumstance atoms are
not destroyed, but re-used again and again 2. You are literally made of different atoms than you
use to be. For example, your skin is replaced about every month (“exfoliate, exfoliate!”), and
your stomach lining is replaced every few weeks.
Basically, we’ll be leaving out quantum mechanics. 
Bigger atoms can be made by “fusing” (joining together) smaller atoms. This happens under the extraordinary
temperatures and pressures inside stars. Some larger atoms are “unstable” (radioactive) and spontaneously split.
Oceanography 10, T. James Noyes, El Camino College
Atoms, Water Molecules, and the Electric Force
The central core of an atom – called the nucleus –
(No Charge)
is occupied by particles known as protons and
neutrons. Protons and neutrons have just about the
same “weight,” but protons have a positive (or
“+”) electrical charge whereas neutrons are
electrically “neutral” (they do not get involved in
electrical interactions). Thus, the nucleus has a
positive electrical charge. Smaller, electrically
negative (or “–”) particles called electrons orbit
Electrons in Orbit
the nucleus.
(Negative Charge)
(Positive Charge)
The electric force holds the protons 3 and the
electrons making up atoms together. There are
two basic rules that the electrical force follows:
• two particles with different kinds of electrical charge are attracted to one another
(“opposites attract”) and two particles with the same kind of electrical charge are repelled
by (“pushed away from”) one another
• the closer together two charges are, the stronger the electrical attraction or repulsion is
Thus, the negatively-charged electrons of an atom orbit the positively-charged nucleus because
they are electrically attracted to it, similar to the way in which the Earth is attracted by the Sun’s
gravitational pull.
The pictures above show electric attraction and repulsion can be caused by “rubbing” objects.
We don’t see electrical attraction and repulsion very often in our everyday lives, because whenever
atoms or molecules are “unbalanced” (a little positive or a little negative), they quickly bond with
other atoms and molecules. The underlying attraction holding objects together can only be seen
when we rub off some of the electrons from atoms on the surface of objects.
The protons and neutrons are held together by the “strong nuclear force.”
Oceanography 10, T. James Noyes, El Camino College
Each kind of atom has a different number of protons, neutrons, and electrons. For example, the
hydrogen atom has 1 proton and 1 electron, the helium atom has 2 protons and 2 electrons, the
oxygen atom has 8 protons and 8 electrons, and so on. “Hydrogen” is just the name that we give
to atoms with 1 proton, and “oxygen” is the name we give to a larger atom with 8 protons.
You might think that atoms are happily electrically neutral, with equal numbers of positive
protons and negative electrons balancing one another out, but remember: electrons are on the
outside of the atom. As long as there is enough space for all the electrons in orbit to stay away
from one another (they all are negatively charged, so they are repulsed by one another), an extra
electron or two can squeeze into orbit, attracted by the positively-charged nucleus. This is what
often happens to a chlorine atom (Cl), making it overall a little electrically negative. In this case,
we call it a choride ion (Cl–). (An ion is an atom which has gained or lost a few electrons,
making it overall electrically negative or positive.) The opposite happens to sodium atoms (Na):
there is not enough space for all its electrons to fit
comfortably in orbit around it. One electron often gets
pushed away by the other electrons (like a puppy or
kitten that gets pushed aside by its brothers and sisters
when trying to drink their mother’s milk ), so the
sodium atom loses an electron, making it an
electrically-positive sodium ion (Na+). Therefore,
sodium ions Na+ are attracted to chloride Cl– ions
(“opposites attract”), and this attraction holds them
together in what we call an “ionic bond.” This bond is
so strong that they form salt, a solid (solids are harder
to break apart than liquids, because their atoms are held
Courtesy of Eyal Bairey (public domain)
together by strong bonds).
There are other kinds of bonds, though. As we noted above, the electrons
are on the outside of the atoms. Since electrons do not stay in one place,
the electrons of one atom sometimes leave a “hole,” revealing the
positively-charged nucleus of their atom to the electrons in orbit around
another atom. If there is enough space, the electrons of the two atoms
will try to orbit the nuclei of both atoms. In this case, the elections act
like a “glue” to hold both atoms together, and this leads to a strong bond
between the atoms called a “covalent bond.” The nuclei are both positive
and therefore repel one another, and the electrons also repel one another,
but as long as there is enough space for the electrons to spread out inbetween the two nuclei, the electrons can hold the nuclei together.
A hydrogen atom has 1 proton and 1 electron. Its electron is happy that it has a proton,
but the electron is also attracted to every other proton that comes by and would like
nothing better than to have two (hopefully your boyfriend or girlfriend is not like this).
Thus, the electron can bring two protons together to meet whether they really want to or
not. Think of a covalent bond as a bond in which the atoms “share” their electrons.
Apparently some particles are very open-minded when it comes to their relationships.
Oceanography 10, T. James Noyes, El Camino College
A molecule is formed when 2 or more atoms are
Salt is a compound, not a molecule,
bonded together. A water molecule consists of 2
because the sodium and chloride ions are
hydrogen atoms (H) and 1 oxygen atom (O), hence
attracted (bonded) to all their neighbors.
the formula H 2 O. The hydrogen atoms are not
In other words, salt is not made up of
symmetrically distributed about the central oxygen
sodium and chlorine pairs (molecules)
atom, but are offset to one side 4. The large oxygen
which are then bonded together.
nucleus (8 protons) exerts a stronger electrical pull
on the electrons than the hydrogen nuclei (1 proton each), so the
hydrogen atoms’ electrons spend more time near the oxygen nucleus
than their own nuclei. As a result, the water molecule has a positive
(“+”) side and a negative (“–”) side: negative by the oxygen nucleus
where there are “extra electrons,” and positive by the hydrogen nuclei,
where there is a lack of electrons to balance out the nuclei’s positive
charge. H 2 O is therefore a dipolar molecule (“two-sided” molecule),
which leads to the many of the special characteristics of water.
A water molecule.
Since water molecules are both positive and negative at the same time,
One side of the molecule
they are attracted to everything, whether positive or negative. (Their
positive side is even attracted to electrically-neutral atoms whose outside is positively charged,
and the other side is
is covered by electrically-negative electrons). Thus, they commonly
negatively charged.
form bonds with other substances, bonds we call “hydrogen bonds”
because many substances with hydrogen also exhibit this kind of bonding. Hydrogen bonds are
stronger than other kinds of bonds that form between molecules, but weaker than the bonds
between the atoms in molecules (i.e., hydrogen bonds are weaker than covalent and ionic bonds).
What makes water special is that it forms unusually strong bonds for a molecule , especially
for a small, light molecule. For example, oxygen molecules consist of two, covalently-bonded
oxygen atoms (O 2 ). At room temperature, water molecules form a liquid but oxygen molecules
form a gas, because water molecules are more
Bonds between Atoms
Very Strong
strongly bonded together by their hydrogen bonds
(making water more “solid”). On the other hand,
Bonds with
Unusually Strong,
salt is a solid and water is a liquid, because the
Water Molecules
Not Very Weak
ionic bonds between salt atoms are stronger than
Bonds between
the hydrogen bonds between water molecules.
Non-Water Molecules
Do you have difficulty distinguishing between
bonds, atoms, and molecules? Think about bonds between members of a family. Each person
represents an atom. A child and its parents are bonded together strongly forming a family of
atoms (a molecule). Another family of atoms (molecule) might share a family bond with the first
family if the parents are related: for example, if the fathers in both families are brothers, then a
family bond connects them. Typically we consider the family bond between the brothers weaker
(between two families = molecules) than their bonds with their wives and children (their atoms).
The hydrogen atoms are on one side together because of the amount of space available in orbit around
the oxygen atom. There is enough space for “4” electrons. No matter how you arrange 2 items out of 4
items symmetrically around a sphere, they are always next to one another. (Technically there is room in
orbit for 8 electrons because 2 electrons with opposite “spin” can occupy the same orbit, but we don’t
have the time or space to explain how that reduces to 4.)
Oceanography 10, T. James Noyes, El Camino College
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).
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 syrup or honey).
The strong bonds between water molecules pull
them tightly together, but the weak bonds between
alcohol molecules cannot hold them together; they
get pulled down and apart by gravity.
Water Drops. Courtesy of (public domain)
Floating Paperclip. Even though the paperclip is made of iron,
which is more dense than water, the paperclip is able to float
because the water molecules are strongly attracted to one another
and do not want to separate and let the paperclip sink.
Water Strider. Courtesy of Markus Gayda (CC BY-SA 3.0).
Oceanography 10, T. James Noyes, El Camino College
Dissolving (Solvents)
Water is often called the “universal solvent.” This refers to its ability to “break down” 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 5 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
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.
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. That would just be wrong. ☺
Groups of water molecules bond (short, dark-red lines) with salt
ions (Na+ and Cl–), tearing them away from the other salt ions.
Adding salt to water.
Eventually the water becomes
clear as the salt dissolves.
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.
This makes the water look “dirty,” because now molecules of the dried food are mixed in with the water.
Oceanography 10, T. James Noyes, El Camino College
Calculating Salinity
About 3.5% of seawater is “salt;” the other 96.5% is water. (Remember: “salt” means all 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 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
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 just like your
textbook does. 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.
Oceanography 10, T. James Noyes, El Camino College
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).
Fortunately, there are carbon
pH is often said to stand for “potential of hydrogen.” The “p” is
substances dissolved in ocean water
really an abbreviation for the mathematical operation “log10” (a
which help keep the ocean neutral
logarithm). pH = 7 indicates a neutral solution (acids and bases
(not too much “acid” H+ or “base”
balance one another), and solutions with pH < 7 are acidic and
those with pH > 7 are basic (alkaline). Differences in pH
OH , the opposite of “acid”). When
represent exponential differences (because of the logarithm), so
carbon dioxide from the atmosphere
small changes in pH represent large differences in the amount of
enters ocean water, it bonds with
acids and bases in the solution. For example, pH 6 means there
water molecules to become carbonic are 10× more acids than bases in the solution. pH 5 means there
acid. Rain “weathers” the rocks of
are 100× more acids than bases in the solution.
the land 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 (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 1/2). 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.
Oceanography 10, T. James Noyes, El Camino College
Solids, Liquids, Gases, and Temperature
The greater the distance between a pair of atoms or molecules, the weaker the electrical
attraction between them. If they are moving fast enough, their speed can overcome the
attraction, and they will fly apart 6. Thus, if atoms or molecules move too fast, their bonds will
weaken and/or break.
What would you do if I asked you to make water transform from a liquid to a gas (in other
words, to make the water molecules fly apart into the atmosphere)? You would probably put a
pot of water on a hot stove burner. Thus, temperature must be related to the speed of the atoms
or molecules: the higher the temperature of the substance, the faster its atoms or molecules are
A solid – like ice (or salt) – has slowly-moving
molecules with strong bonds. The molecules are
constantly moving, but they are wiggling in place;
they vibrate (oscillate) but don’t leave their spot
(like a child fidgeting in their stroller or car seat),
because strong bonds hold them in place. This is
why a solid is, well, “solid:” it has a fixed size and
shape, because all the molecules are firmly attached Red dye in warm water (right) spreads out faster than
blue dye in cold water (left) due to faster-moving molecules.
to their neighbors.
In gases like water vapor or oxygen, on the other hand, there are no bonds between the
molecules, because they are moving too fast. When the molecules bump into one another, the
electrical attraction they feel for one another increases, but it is not strong enough to hold them
together. Thus, a gas has no fixed size or shape; it will expand to fill whatever space is available.
Green arrows
show the speed
and direction of
the traveling
Dark red-brown
lines are bonds.
A liquid like ordinary water is, of course, somewhere in between a solid and a gas. The water
molecules are often moving fast enough to break their bonds. However, they immediately form
a new bond with their new neighbor. Thus, liquids can flow (change their shape), because bonds
are easily broken, but the molecules of the liquid do not fly apart, because the bonds are strong
Like a rocket moves very fast to overcome the gravitational attraction of the Earth. The speed needed to leave the
Earth and go off into space is called the “escape velocity.”
Oceanography 10, T. James Noyes, El Camino College
enough to hold the molecules together most of the time. This is somewhat like the story of
Goldilocks and the Three Bears: the bonds are not too strong and not too weak, and the speed of
the molecules is not too fast and not too slow.
Latent Heat
Latent heat is the amount of heat that must be added to make molecules break their bonds or the
amount of heat that must be removed to allow molecules to form new bonds. The added heat
makes the molecules move faster. The farther apart their wiggling carries them, the weaker their
electrical attraction becomes and the easier it is for them to fly apart. Removing heat slows the
molecules down. If they are going slow enough when they come close together, the electrical
attraction will increase enough to hold them together.
Latent heat is the amount of heat that must be added or removed to make a substance to
transform from one phase (solid, liquid, gas) to another phase. When sunlight makes ocean
water evaporate, we say that the water “gained latent heat,” and when water cools and condenses
in the atmosphere, we say that the water “lost latent heat” (cooling = losing heat) to the
neighboring air molecules.
Interestingly, adding heat will not increase the temperature of a substance when it is at the
melting point or the boiling point: all the heat added is “latent heat” that is breaking the bonds
holding the atoms and/or molecules of the substance together instead of making the atoms and/or
molecules move faster. This means that a pot filled with melting ice water will remain at 0oC
while the ice is melting, even if the stove burner is on “high”. Similarly, the temperature of
boiling water on a stove remains at 100oC even though more and more heat is being applied. 7
Compared to other substances, water has extremely large latent heats of melting and evaporation,
because of the unusually strong bonds (hydrogen bonds) between the water molecules. The
strong bonds keep the molecules together, making it harder for them to melt (turn into water) or
evaporate (turn into gas). Similarly, at temperatures where most small molecules 8 (e.g., oxygen,
carbon dioxide, nitrogen) fly apart into gases, water molecules form solid ice or a liquid.
The fact that water remains a liquid is crucial for life. Living things cannot exist without liquids.
The molecules of a gas do not stay together, but fly apart, so you cannot construct a body with a
gas. Solids, on the other hand, do not change, something living organisms must do all the time.
This is why the human body is mostly made of water.
It is a good thing that water
molecules do not hold onto one
another too strongly: remember,
most of our fresh water
evaporated from the ocean not
so long ago!
Melting Heat needed Boiling Heat needed
to melt it
Point to vaporize it
-114 C
104 kJ/kg
854 kJ/kg
1063 C
67 kJ/kg
1578 kJ/kg
23 kJ/kg
871 kJ/kg
-210 C
26 kJ/kg
199 kJ/kg
-218 C
14 kJ/kg
-183 C
213 kJ/kg
334 kJ/kg
100 C
2259 kJ/kg
0oC is the freezing point of fresh water at 1 atmosphere of pressure. 100oC is its boiling point.
It takes more heat to make a heavy atom or molecule move fast than it takes to make a small atom or molecule
move fast, just like it takes more energy to make a car move than it takes to make a toy car move.
Oceanography 10, T. James Noyes, El Camino College
Thermal Expansion, Contraction,
and the Freezing Point
When atoms or molecules of a substance are
heated, they move faster. They bump into their
neighbors more strongly, and this jostling causes
them to push away from one another; they spread
out a bit more. This not only increases the size of
the object, but it also lowers its density.
Cold Atoms
Warm Atoms
Similarly, cooling an object (removing heat)
Note: The molecules themselves do not “expand”
causes its atoms or molecules to move slower,
(get bigger) or “contract” (get smaller). They just
so they don’t push away from one another as
get farther apart or closer together.
strongly when they collide and their electrical
attraction (bonds) bring them closer together.
Thus, the object’s size decreases, and its density increases.
There is one exception to this rule: at the freezing point of water (and only at the freezing point),
cooling water decreases the water’s density. Unlike most other substances, the solid form of
water (ice) has a lower density than the liquid form (liquid water), so it floats. Ice has a lower
density than liquid water, because its molecules are more spread out in ice than when they are
liquid owing to the shape of water molecules. Water molecules have a “triangular” or “bent”
shape and can only bond in certain directions. In a liquid, water molecules do not have to bond
in these specific directions on all sides; they are constantly shifting and sliding past one another.
If you try to fit water molecules together in these directions, though, you find that you have a lot
of empty space: since each water molecule “bends” through an angle of about 105o, 6 water
molecules form a big circle with a hole in the middle. Thus, water crystals tend to have 6-sided
symmetries as you can see in pictures of snowflakes.
O Empty
Water molecules bonded
in an ice crystal.
Wilson Bentley (public domain)
Sea Ice. National Oceanic and
Atmospheric Administration,
Department of Commerce.
Salt ions cannot fit into the final crystal structure well, because of mismatches in size and
bonding directions. Thus, salt in seawater gets in the way of the freezing process, making it
harder for the water to freeze into ice until it is pushed out of the way. This lowers the freezing
point of seawater (by about 3.5oF), and also means that sea ice contains less salt that seawater.
(This was very useful to early Arctic explorers who learned to drink melted sea ice.)
Oceanographers say that the salt is “rejected” by the sea ice, and therefore the “rejected” salt
stays in the unfrozen ocean water, making it saltier (and thus harder to freeze). This happens
again and again during the winter. Eventually the cooling surface water becomes both cold and
salty enough to sink down deep into the ocean instead of freezing into ice.
Oceanography 10, T. James Noyes, El Camino College
Heat Capacity
Think about a hot afternoon at the
beach: Both the sand and the water get
the same amount of heat from the sun,
but which is warmer, the sand or the
Very Hot
Nice & Cool
Clearly, the sand is much warmer than the water. Oddly, this means water has a “high heat
capacity,” meaning that it holds more heat than another substance with the same temperature.
Think of “heat capacity” as a kind of thermal inertia: an object with a large inertia (mass or
“weight”) is much harder to move than an object with a small inertia. In the same way, it is hard
to change the temperature of a substance (like water) with a high heat capacity. So water does
not get as hot or as cold as the land 9; it takes more heat to raise its temperature, and it must lose
more heat to lower its temperature.
So, why do we call this property “heat capacity” if the hotter object (sand) has the lower heat
capacity? Think about nighttime when the situation is reversed: the ocean (high heat capacity) is
warmer than the land. This makes sense, right? So, however you define this property, one
situation or the other (day or night) is going to be confusing (seem “backwards”).
Why is this the best way to define heat capacity? Here’s a way to think about it: consider your
experiences biting into a hot piece of pizza. The crust may be warm, but the watery sauce and
cheese burn the roof your mouth, because they are much hotter. All of them (bread, sauce,
cheese) were warmed up to the same temperature of 400oF+ in the oven, but the watery sauce
and cheese have a higher heat capacity, so they had to absorb more heat to reach this
temperature. All of them lose heat as the pizza cools, but the crust cools down faster, because it
has less heat in it to begin with. The sauce and cheese have a lot more heat to give up, so it takes
longer for them to cool down, and they remain warm enough to burn you.
Why do water molecules have an unusually high heat capacity? Well, that is difficult to explain in
a simple way. As usual, it is related to water’s unusually strong bonds, but it also has to do with
how temperature is related to heat. As we know, heat makes atoms and molecules move faster,
but temperature is mainly related to the wiggling or movement of the entire molecule from place
to place. However, there are other motions that the heat can cause: rotation (spinning), vibrations
within the molecule, and so on. Heat that goes into these motions does not contribute to
temperature. So, the key to water’s higher heat capacity is that water molecules have more motion
“options” than other atoms and molecules.
As we learned in section 2A (“The Ocean Environment”), other factors can be important too, like how deep
sunlight penetrates, and mixing caused by waves and other factors. Even latent heat plays a role, since it is the
warmest molecules that evaporate, leaving the cooler molecules behind.
Oceanography 10, T. James Noyes, El Camino College
As we saw during the lab, atoms and molecules never stop moving: add a drop of dye to a
container of water and the dye will naturally spread out on its own without any stirring. The
water molecules bond with the dye molecules, and carry the dye molecules with them as they
move around the tank. Eventually the tank is one uniform color, because the water molecules
and dye are equally likely to be found anywhere since they moving around and bumping into one
another at random.
This random motion of liquid and gas molecules leads to “diffusion,” the process by which
substances that are concentrated in one place spread out until they are evenly distributed
everywhere (until the concentration is the same everywhere). This also happens to sugar (or salt)
in your beverages. Over time, it spreads out even without stirring 10, and it will never “un-mix”
and fall to the bottom. (In other words, it never becomes concentrated in one place again.)
Diffusion is very important for life in the ocean. Many microscopic organisms get substances
that they need (e.g., nutrients, carbon dioxide, oxygen) when the random motion of molecules
brings dissolved substances that they need into their bodies. (Notice the holes in their shells
below.) They then absorb the substances, reducing the concentration inside their bodies.
Similarly, wastes (e.g., oxygen, carbon dioxide) just drift out of their bodies naturally.
Courtesy of Dr. Markus
Geisen (Public Domain)
Diatoms (Phytoplankton).
(Public Domain)
Just after dye
was added.
Much later.
Like other molecules, randomly-moving water molecules tend to move from the place where
they are most concentrated to where they are less concentrated; in other words, their movement
“evens out” differences (“osmosis”). If ocean water is too salty for an organism (saltier than the
cells of its body), water molecules leave its body, moving outside and reducing the ocean’s
salinity11. The organism then suffers from dehydration (lack of water); this is why freshwater
fish cannot live in the ocean 12. Similarly, if the ocean is too fresh for an organism, extra water
molecules will enter their body, and cause the organism and its cells to bloat. Our bodies cannot
be so open to our environment (on land surrounded by air), because we would quickly lose most
of our water. However, water can move through our skin; the “wrinkles” that we get from being
in water too long (e.g., in the bathtub, washing dishes) are caused by our bodies absorbing extra
water. (Our bodies are much saltier than fresh water.) If you drink too much seawater, water
Stirring helps, of course, by helping the sugar molecules meet more unattached (un-bonded) water molecules.
It might be helpful to think of the water molecules moving out of the body to bond with the excess salt ions.
A freshwater fish dies of thirst in the ocean!
Oceanography 10, T. James Noyes, El Camino College
from you body’s tissues will move into your stomach (where the salt is), and then be lost from
you body when you “excrete.” As a result, you become dehydrated (lose too much water).
Unlike ordinary objects, atoms and molecules never stop moving, because there is no friction on the
atomic level. Friction results when two objects “rub” against one another, causing their atoms and
molecules to bump into one another. This causes the atoms and molecules to move faster – in other
words, it generates heat and warms them up – but the objects themselves slow down and are brought
to a stop. The “organized” energy of all the atoms and molecules moving forward together in their
objects (kinetic energy) is transformed into the “disordered” energy of all the atoms and molecules
wiggling randomly (heat). No energy is lost from the universe; it is merely transformed from one
kind of energy to another kind of energy. Physicists call this the “conservation of energy.”
Moving Forward
Comes to a Stop
Why Heat Flows from Hot Objects to Cold Objects
Faster ("Warmer")
Slowed Down
("Cooler") #1
O#2 ("Cooler")
We all know that heat flows from the hotter object to the cooler object. If you think about your
experiences with collisions, you can understand why. When two objects collide, typically the
faster-moving object gives its energy to the slower-moving object, causing the slower-moving
object to speed up 13. The faster-moving object loses energy, so it slows down. (Think of a car
getting rear-ended. The car behind will slow down, and the car in front will speed up.)
Similarly, atoms and molecules are constantly bumping into one another. When warmer, fastermoving molecules hit cooler, slower-moving molecules, the cooler molecules gain energy and
speed up (get warmer) while the warmer molecules lose energy and slow down (get cooler). The
warmer molecules tend to give the cooler molecules their energy via collisions until all the
molecules have about the same speed 14 – in other words, until they have reached the same
Speeded Up
This discussion oversimplifies molecular collisions. In particular, the objects are assumed to have of similar
masses. Both energy and momentum (translational and rotational) are exchanged in collisions, and in solids and
liquids potential energy must be considered as well.
Actually, there are a distribution of speeds, because less-likely collisions do occur in which energy is transferred
from “cooler” molecules to “warmer” ones. These kinds of collisions become more common when all the molecules
have similar speeds.
Oceanography 10, T. James Noyes, El Camino College
Answering Questions Using the Atomic Theory of Matter
This section of the class is often one of the more challenging ones for students. The most
common mistake on tests and assignments is that students discuss what happens at our scale
rather than what happens at the microscopic level of atoms and molecules. So, when answering
questions “using the Atomic Theory of Matter,” make sure that you discuss the behavior of the
atoms and/or molecules. In other words, if your answer discusses “water evaporating” or “salt
dissolving,” then you have not answered the question. Instead, you should discuss the speed of
the water molecules and salt atoms, as well as the bonds between the water molecules and salt
atoms. 15 To describe atoms and/or molecules behavior, discuss their motion and bonding
(electrical attraction): Are new bonds forming? Are bonds breaking? Getting stronger or
weaker? Why? Are the molecules moving fast or slow? Are they wiggling in place or traveling
(flying from place to place)? How do collisions with other molecules affect them?
Ions, technically.
Oceanography 10, T. James Noyes, El Camino College