Seawater Chemistry Unit (4A-1) – page 1
Name:
Seawater Chemistry Unit III: Motion
Section:
Breaking Bonds
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. Thus, if atoms or molecules move too fast, their bonds will weaken
and/or break. (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.”)
What would you do if I asked you to make water transform from a liquid to a gas (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 moving.
1. If two atoms or molecules move farther apart, will the electrical attraction between them
grow stronger or weaker?
2. Which are moving faster, hot water molecules or cold water molecules?
3. Which can break their bonds and fly apart more easily, hot water molecules or cold water
molecules?
Seawater Chemistry Unit (4A-1) – page 2
Solids, Liquids, and Gases
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 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.
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
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.
Note: We call solids, liquids, and gases different phases or states of matter. There is another
one, plasma, in which protons and electrons no longer stay together and fly apart.
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4. Which molecules have the strongest bonds between them, molecules of ice (solid water),
molecules of liquid water, or water vapor (a gas)?
5. Which molecules have no bonds between them, molecules of ice (solid water), molecules
of liquid water, or water vapor (a gas)?
6. Which molecules have weak bonds between them, molecules of ice (solid water),
molecules of liquid water, or water vapor (a gas)?
7. Which molecules are moving fastest, molecules of ice (solid water), molecules of liquid
water, or water vapor (a gas)?
8. Which molecules are moving slowest, molecules of ice (solid water), molecules of liquid
water, or water vapor (a gas)?
9. True or false? “The molecules of ice (solid water) are not moving at all. They are
stationary. They are still.”
10. Which molecules are traveling (moving from place to place, changing location),
molecules of ice (solid water), molecules of liquid water, or water vapor (a gas)?
11. Which molecules are vibrating (wiggling, oscillating), molecules of ice (solid water),
molecules of liquid water, or water vapor (a gas)?
Seawater Chemistry Unit (4A-1) – page 4
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 holding them together 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.
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.
Since latent heat is heat that breaks bonds or allows them to form, 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. Therefore, latent heat is being added when something evaporates
(transforms from a liquid into a gas) or melts (transforms from a solid into a liquid). Latent heat
is being lost when something condenses (transforms from a gas into a gas) or freezes (transforms
from a liquid into a gas).
One skill we want you to have is to be able to identify the direction heat is flowing based on
observed changes of phase. Check your understanding by answering the following questions.
12. What word do we use to describe a substance that is changing from a liquid to a gas?
In other words, do we say the substance is condensing, evaporating, freezing or melting?
13. If a substance transforms from a liquid to a gas, is it gaining latent heat or losing latent
heat?
14. What word do we use to describe a substance that is changing from a gas to a liquid?
In other words, do we say the substance is condensing, evaporating, freezing or melting?
15. If a substance transforms from a gas to a liquid, is it gaining latent heat or losing latent
heat?
16. What word do we use to describe a substance that is changing from a liquid to a solid?
In other words, do we say the substance is condensing, evaporating, freezing or melting?
Seawater Chemistry Unit (4A-1) – page 5
17. If a substance transforms from a liquid to a solid, is it gaining latent heat or losing latent
heat?
18. What word do we use to describe a substance that is changing from a solid to a liquid?
In other words, do we say the substance is condensing, evaporating, freezing or melting?
19. If a substance transforms from a solid to a liquid, is it gaining latent heat or losing latent
heat?
Water has an unusually high latent heat of evaporation
and an unusually high latent heat of melting
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.
(Technically, 0oC and 100oC are the freezing and boiling points, respectively, of fresh water at 1
atmosphere of pressure.)
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 (e.g., oxygen,
carbon dioxide, nitrogen) fly apart into gases, water molecules form solid ice or a liquid.
20. Does water have unusually high or low latent heats? Why?
21. If you want to make water evaporate, do you have to add more heat or less heat than you
would to make most other substances evaporate?
22. If you want to make water freeze, do you have to remove more heat or less heat than you
would to make most other substances freeze?
Seawater Chemistry Unit (4A-1) – page 6
Thermal Expansion and Contraction
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. Similarly,
Cold Atoms
Warm Atoms
cooling an object (removing heat) causes its atoms
or molecules to move slower, so they don’t push away from one another as strongly when they
collide and their electrical attraction (bonds) bring them closer together. Thus, the object’s size
decreases, and its density increases. Note: The molecules themselves do not “expand” (get
bigger) or “contract” (get smaller). They just get farther apart or closer together.
23. If you add heat to a substance, do it expand (get bigger) or contract (get smaller)?
24. If you add heat to a substance, do its atoms and/or molecules move faster or slower?
25. If you add heat to a substance, do it atoms and/or molecules get farther apart or closer
together?
26. If you remove heat from a substance, do it expand (get bigger) or contract (get smaller)?
27. If you remove heat from a substance, do its atoms and/or molecules move faster or
slower?
28. If you remove heat from a substance, do it atoms and/or molecules get farther apart or
closer together?
Seawater Chemistry Unit (4A-1) – page 7
The Freezing Point of Water and Seawater
There is one major exception to rule that heating things make them
expand and cooling things makes them contract: 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 6sided symmetries as you can see in pictures of snowflakes.
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.
29. True or false? “When liquid water freezes into ice, the water contracts (gets smaller). So,
the ice takes up less space than the liquid water.”
30. Does salt water have a higher freezing point or lower freezing point than fresh water?
Is it easier to freeze salt water or fresh water?
31. Which is saltier, sea ice (frozen ocean water) or ocean water?
32. When salty ocean water freezes into sea ice, does the water left behind in the ocean (the
nearby water which does not freeze) get fresher or saltier?
Seawater Chemistry Unit (4A-1) – page 8
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 water? Clearly, the sand is much warmer
than the water. 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; 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? 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.
Seawater Chemistry Unit (4A-1) – page 9
The Importance of Water’s High Latent Heat and High Heat Capacity
The fact that water remains a liquid over a wide range of temperatures and pressures 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. There is another reason it is a good thing that water molecules do not hold onto one
another too strongly: most of our fresh water evaporated from the ocean not so long ago!
33. If you add the same amount of heat to the land and the ocean, which one warms up more?
In other words, which one’s temperature increases more?
34. If you remove the same amount of heat from the land and the ocean, which one cools
down more? In other words, which one’s temperature decreases the more?
35. Which is warmer during the summer, the land or the ocean?
36. Which is cooler in the winter, the land or the ocean?
37. Does water have an unusually high heat capacity or an unusually low heat capacity?
38. If two things have the same temperature (e.g., sand and water), which one has more heat
in it, the one with a higher heat capacity or the one with the lower heat capacity?
Seawater Chemistry Unit (4A-1) – page 10
Diffusion
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, and it will never “un-mix” and
fall to the bottom. In other words, it never becomes concentrated in one place again. (Stirring
helps, of course, by helping the sugar molecules move around quickly and meet more unattached
(un-bonded) water molecules.)
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. 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.
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
salinity. (It might be helpful to think of the water molecules moving out of the body to bond with
the excess salt ions.) The organism then suffers from dehydration (lack of water); this is why
freshwater fish cannot live in the ocean. (A freshwater fish dies of thirst in the ocean!) 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. If you drink too much seawater, water from you body’s tissues
will move into your stomach (where the salt is), and then be lost from your 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
Seawater Chemistry Unit (4A-1) – page 11
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
39. What is diffusion?
40. How do microscopic organisms like phytoplankton get their nutrients? Do they grab the
nutrients out of the water with tentacle-like structures or do the nutrients randomly drift
to their bodies?
41. If an ocean animal or algae is in fresh water, does water enter (bloat) or leave (dehydrate)
its body?
42. If an ocean animal or algae is in water that is too salty for it, does water enter (bloat) or
leave (dehydrate) its body?
43. True or false? “Atoms and molecules never stop moving.”
Seawater Chemistry Unit (4A-1) – page 12
Why Heat Flows from Hot Objects to Cold Objects
Faster ("Warmer")
O
Slowed Down
("Cooler") #1
#1
Slower
O#2 ("Cooler")
O
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. 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, faster-moving
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 – in other words, until they have reached the same temperature.
O#2
Speeded Up
("Warmer")
44. Which move faster, hot molecules or cold molecules?
45. How does one atom or molecule transfer heat (i.e., motion) to another atom or molecule?
46. If a fast-moving molecule hits a slower-moving molecule, does the faster-moving
molecule speed up or slow down? Does this mean that the faster-moving molecule gets
“warmer” or “cooler”?
47. If a fast-moving molecule hits a slower-moving molecule, does the slower-moving
molecule speed up or slow down? Does this mean that slower-moving molecule gets
“warmer” or “cooler”?
Seawater Chemistry Unit (4A-1) – page 13
Why Heat Flows from Hot Objects to Cold Objects: A Few More Comments
The discussion on the previous page oversimplifies molecular collisions. In particular, the
objects are assumed to have of similar masses. Also, both energy and momentum (translational
and rotational) are exchanged in collisions, and in solids and liquids potential energy must be
considered as well. Moreover, there is 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.
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. 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?
48. If you are asked to discuss what is happening in a situation using the Atomic Theory of
Matter, which of the following might you want to discuss when describing what the
molecules are doing: bonding of the molecules, evaporation of the molecules, motion of
the molecules, melting of the molecules, and/or latent heat of the molecules?
Seawater Chemistry Unit (4A-1) – page 14
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