|
Grade
Level: Middle
| Time:
Four
45 min. class periods | Content Standard NSES Physical Science, properties and changes of properties in
matter | Ocean Literacy Principle 1e:
Most of Earth's water (97%) is in the ocean. Seawater has unique
properties: it is saline, its freezing point is slightly lower than
fresh water,
its density is slightly higher, its electrical conductivity is much higher, and
it is slightly basic.
Big Idea
Salt
causes water to freeze at a lower temperature.
The oceans do not freeze (except in extreme polar areas)
because of the salinity (i.e., concentration of
salts).
Key
Concepts
- Dissolving
any substance in pure water raises or lowers
the freezing
and boiling point.
- When
water freezes - goes from the liquid state
to the solid
state - its molecules go from a disorganized state to
an organized state.
- When
water freezes to a solid, molecular motion slows down
enough that the molecules become permanently fixed in
an orderly arrangement called a crystal.
- The
individual particles that make up salt (known as ions)
arrange themselves around the water
molecules. In doing
so, they shield the water molecules from interactions
among themselves, making it less likely that they will
find each other and form ice.
- The
water molecules have to be slowed down even
more in the
presence of salt in order to form a solid.
So you have to go to a lower temperature in
order to freeze
water that contains salt.
- Salt
is excluded in the formation of ice;
therefore ice made
from salt water is essentially
salt-free.
- Salt
water is denser than freshwater because of
its salt content,
and, as a result, an object is more buoyant
in the ocean
than in a freshwater lake. The density of
salt water depends
on its salinity and varies around the world.
Essential
Questions
- What
happens to water when it freezes?
- How
is salt used in our everyday lives?
- Why
don't the oceans freeze?
- If
oceans don't freeze, where do icebergs come
from?
- Are
icebergs salty? Is sea ice salty?
- Will
an object float in saltwater and freshwater
in the same
way?
Knowledge
and Skills
-
Demonstrate how salt affects ice by forming a
hypothesis,
experimenting with salt, ice, and water, and recording
the results.
-
Describe why salt water freezes at a lower temperature
than fresh water.
-
Conduct an experiment that demonstrates how
salt is left
behind as salt water freezes, and that this
process forms
brine.
-
Learn that ice is essentially salt-free whether it is
formed from fresh or salt water.
-
Explain the differences between sea ice and icebergs.
-
Demonstrate and explain why ice floats higher in salt
water than in fresh water.
Prior
Knowledge
- The
salt in seawater comes from the weathering of earth's
land surface.
- Salinity
is the measure of the amount of dissolved
salts in seawater
- Water
can be a liquid or a solid and can go back
and forth from
one form to the other.
- When
liquid water disappears, it turns into a gas
(vapor) in
the air and can reappear as a liquid when
cooled, or as
a solid if cooled below the freezing point of
water.
- Icebergs
are floating pieces of ice in the ocean that came from
a glacier.
Common
Preconceptions
- The
freezing point (and boiling point) of
substances is fixed
points and never varies.
- Ice
never gets colder than 0 °C.
- Substances
are always cold when they freeze.
- Salt
is dissolved in the water therefore must be
included in
the ice when it forms.
- Because
ice that melts in a drinking glass does not alter the
water level, when icebergs melt they don't raise
sea level.
Concept
Map
This lesson and activity relates to the branch
"Phases
of Water" from the comprehensive Aquarius
Concept Map:
Water and its
patterns on Earth's
Surface.
Activity
1 Background
Under
normal conditions, ordinary fresh water
freezes and melts
at 0°C, or 32°F. If water and ice are
at 0°C,
which is the melting point of ice and the freezing point
of water then molecules of ice are escaping
into the water
(melting), and molecules of water are being captured on
the surface of the ice (freezing).
When
the rate of freezing is the same as the rate of melting,
the amount of ice and the amount of water won't change.
The ice and water are said to be in
dynamic equilibrium
with each other, therefore there is no
net change
in either quantity. This balance will be
maintained as long
as the water stays at 0°C, or unless
something happens
to favor one of the processes over the other.
For example,
when the water temperature is dropped to
-10°C the molecules
of water are moving more slowly, i.e. they contain less
heat. These slower-moving water molecules are
more easily
captured by the ice, and freezing occurs
at a greater
rate than melting. Because there are more
water molecules
being captured by the ice (being frozen) than there are
ice molecules turning to water, the net result
is that the
amount of water decreases, and the amount of
ice increases.
Adding
salt disrupts the equilibrium. The individual
particles that
make up salt (known as ions) arrange themselves around the
water molecules. In doing so, they shield the
water molecules
from interactions among themselves, making it less likely
that they will find each other and form
ice. This means
that the number of water molecules captured by
the ice (frozen)
is reduced, so the rate of freezing is also reduced.
The rate of melting of the ice is unchanged by
the presence
of the salt.
In order to return the system to equilibrium,
where the number
of molecules of water that are freezing is equal
to the number
of ice molecules that are melting (remember this
is the freezing/melting
point), we must lower the temperature sufficiently to slow
down the water molecules so that they can become
part of the
ice formation.
The
higher the concentration of salt, the lower the
freezing point
drops. Any foreign substance, such as sugar,
alcohol, or any
chemical salt, added to the water, forms a
solute,
which will lower the freezing point and melt
ice. This
is why salt is used to melt snow and ice on
roadways and sidewalks.
Salt is also inexpensive and readily available.
The melting
action of salt forms brine, or a
strong saline
solution, that can penetrate below the surface of ice and
packed snow. Brine prevents water from freezing
and bonding
to a surface. When ice melts due to the presence of salt,
heat is absorbed. This is called an
endothermic reaction.
Does the
size or quality of the salt placed into solution affect
the freezing/melting point, e.g. iodized table salt
vs. kosher
salt vs. rock salt? Actually it doesn't matter how they are
packaged or processed, once dissolved the salt molecules are
so tiny compared to the distance they move through
the liquid
that size is not a factor. (Adapted from
Worsley School
Online http://www.worsleyschool.net)
Activity
1 - Salt content changes the freezing point of water - one
45 minute class period
Materials:
Per Student Group - 2 plastic beakers or
similar containers, crushed ice, 2 unfrozen "freeze
pops," measuring
cup, thermometer, graph paper, rock salt, balance - **Lab Safety Reminders** The freeze
pops are not to be consumed if they are being used in a laboratory experiment.
Also use a plastic beaker or scoop to distribute crushed ice, do not
use a glass
beaker to scoop the ice.
Preparation: It should be noted that this
activity requires a significant amount of crushed ice. It is suggested
that bags of crushed ice be purchased (if not available on
site) and stored
in a cooler. Distribute the materials to each student
group.
Activity
-
Obtain two large beakers (1000 or 500 ml.)
and fill them
with equal weights of ice, about 2/3 full.
Label the beakers
A and B.
- Place
an unfrozen "freeze pop" in each of the beakers
so that it is surrounded by ice. (Freeze pops
are popsicles
with a plastic wrap, and can be purchased in
most grocery
stores.)
- Place
a thermometer in each beaker and record the
temperature
of the ice. Students should take their temperature measurement once the ice has
stabilized or after they've added it to the beaker (not during).
- Mix
230 grams of rock salt with the ice in Beaker
A. When adding the salt do so carefully to ensure that the salt is distributed
throughout the beaker and to prevent piling up on top of the ice.
- Record
the temperatures in each of the beakers
every 5 minutes
for a total of 30 minutes. Graph temperature
versus time
for Beaker A and for Beaker B.
- 15
minutes into the experiment, pour another 115 grams of
salt into Beaker A.
-
25 minutes into the experiment, pour another 115 grams
of salt into Beaker A. Continue taking
temperature readings
until you have 30 minutes of data
collected.
-
Examine the freeze pops at the end of the experiment,
and determine which one has frozen and its
degree of hardness.
Record the results.
Assessment / Questions
Discuss why the salt-ice mixture produced a
lower temperature
than the ice with no salt added.
(Explanation:
Dissolving salt in water lowers the temperature at which
the water freezes, or at which the ice
melts. Melting
ice takes heat energy from the surroundings; in
this case,
heat from the unfrozen "freeze pop" is transferred
to the colder brine mixture, helping to freeze
the popsicle.
The rate at which the ice melts depends on the
rate at which
heat energy flows in - the greater the temperature
difference, the faster the heat transfer will
be. When ice
melts, the amount of heat transfer is 80
calories per gram
of water, regardless of the salinity. If the
ice has salt
on it, it will melt at a lower temperature, and will be
colder than ice melting without salt on it.)
How might salt be used for food transportation?
(For example,
freight and refrigerator cars use ice and salt
to cool foods.)
Ask students to think of other applications of salt-ice
interactions for industrial uses or uses around
their home
or neighborhood. Students may have used salt to
deice sidewalks
and driveways. Have students describe any
observations they
have seen of salt and ice interacting, or how
they may use
these principles in their lives.
Discuss why oceans tend to freeze at a
lower temperature
than freshwater bodies of water.
Activity 2 Background
Sea
ice is simply frozen ocean water. It
forms, grows,
and melts in the ocean. Sea ice forms in the Polar
Oceans, known in the North as the Arctic
and in the
South as the Antarctic region. In the Arctic,
there is a permanent
mass of ice covering the region. Nearly all of
the Southern
Ocean or Antarctic sea ice is "seasonal ice," or
it melts away and reforms annually. While Arctic
and Antarctic
ice is of vital importance to the marine mammals and birds
for which it is a habitat, sea ice in the Arctic
appears to
play a more crucial role in regulating climate.
Sea
ice forms a boundary between the relatively warm ocean and
the cooler atmosphere. Sea ice floats for
the same reason
the ice in your drinking glass floats. It is less dense in
the solid phase than it is in the liquid phase.
Although sea
ice is made from salty seawater, the salt
molecules are rejected
back into the liquid as ice forms, resulting in a
briny solution.
There isn't much room for salt molecules to be trapped in
the close-knit structure of ice. Sea ice,
like regular
ice, is made of oxygen atoms and hydrogen atoms
that are joined
together to form a rigid net or lattice. Hydrogen bonding
provides ice with its unique properties. In all
known forms
of ice, the water molecules retain their
identities as distinct
molecules and bond to other molecules by hydrogen bonding.
Unlike sea ice, icebergs,
glaciers, ice sheets,
and ice shelves all originate on land. Icebergs are broken
pieces of glaciers, which originate from masses
of snowfall
on land.
So
why are sea ice and icebergs important to
scientists and to
you? Even though sea ice occurs primarily in the
polar regions,
it influences our global climate.Sea ice is
important because
it regulates exchanges of heat, moisture and
salinity in the
polar oceans. It insulates the relatively warm ocean water
from the cold polar atmosphere except where
cracks, or leads,
in the ice allow exchange of heat and water vapor
from ocean
to atmosphere in winter. Sea ice affects the
movement of ocean
waters. It can also be an obstacle to normal
shipping routes
through the Northern Sea route and Northwest
Passage. Sea ice also affects climate because of its high albedo or
reflectivity.
Finally, wildlife and people rely on sea ice to hunt and
travel.Glaciers
can provide clues about the effects of global warming due
to their sensitivity to temperature
fluctuations. Increased
iceberg calving is an important indicator of
climate change.
Satellite imagery has allowed scientists to
observe how icebergs
in a remote corner of Antarctica can disrupt an
entire marine
ecosystem. (Adapted from National Snow and
Ice Data Center
website http://nsidc.org/sotc/sea_ice.html)
For
the article "Satellite imagery shows how
icebergs affect
Antarctica's food chain" visit http://news-service.stanford.edu/news/2002/april24/icebergs-424.html
For
more information on how scientists study polar
ice formation
visit http://nsidc.org/sotc/sea_ice.html
For
a visual tutorial on water molecules and ice formation visit
http://www.edinformatics.com/math_science/water_ice.htm
Activity 2a - Effects of Salinity on Freezing Rate
- two 45 minute class periods to collect data
Materials:
Per Student Group - 230 grams of ordinary table
salt, balances,
containers/boats for measuring salt, graduated
cylinders,
2 plastic ice cube trays with divided
watertight sections
(it's important that they are standard cube
trays and will
not create unusual shapes), tap water, thermometer, 12,
beakers (at least 300 ml) or cut 2L plastic
soda bottles,
metric ruler, graph paper, food coloring
(optional) to color
ice cubes to be used in buoyancy activity,
Earth globe will
be helpful during discussion.
Preparation:
Students will need access to a
freezer.
Activity
- Begin
with a background discussion with the class
of the locations
of the North Pole (Arctic) and South
Pole (Antarctic).
Use a globe to illustrate the geography of
these locations
during this discussion. Ask the
students which Pole
is NOT covered by sea ice (Antarctic)
and which is
covered by sea ice (Arctic). Ask
the students
to explain their answer. Ask students which
Pole is more
likely to have glaciers nearby (South Pole - the glaciers of the
Queen Maud
Mountains begin to the south of 85 degrees south latitude; the nearest glacier
supporting landmass to the north pole is Greenland which does not extend north
of 84 degrees north latitude; therefore, the south pole has glaciers in closer
proximity than the north pole. ).
-
Ask the class to hypothesize about the affect of water
salinity - or amount of salt - on the
formation and buoyancy of sea ice. Do they
think ice formed
from salt water will freeze more quickly or
more slowly
than fresh water? Why? Do they think that sea
ice (i.e.,
formed from salty water) will be more buoyant or less
buoyant than ice formed from fresh water
(e.g., icebergs)?
Why? Do they think that the size of sea ice
or icebergs
affects their buoyancy (e.g., percentage
above and below
the water line)? Explain.
- Label
12 jars as follows: 2 marked "A"; 2 marked
"B"; 2 marked
"C"; and 2 marked "D." In each jar mix salt and water
solutions as follows:
- In
each jar marked "A": mix 128
grams salt with
250 mL water
- In
each jar marked "B": mix 85 g
salt with 250
mL cup water
- In
each jar marked "C": mix 43 g
salt with 250
mL cup water
- In
each jar marked "D": 250 mL pure
tap water
NOTE:
One set of jars A-D will be used to fill an ice cube
tray, it's not necessary to be exact. The other set
of jars should be covered with lids,
regardless of where
they are stored, to preserve volume.
Put the second
set in a refrigerator if available,
otherwise set them
aside in a cool, shaded area of the
room.
-
Label each quarter section of both ice cube
trays as sections
A, B, C, and D.
-
Pour one set of solutions A - D into their
labeled sections
of the ice cube tray. Ask the students to
make a variety
of ice cube sizes using each solution. Place the tray
in the freezer.
-
Using a data chart (click
to access "Freezer Experiment"
table), students
observe and record water temperature and
conditions after
1 hour, 24 hours, and 48 hours. **STOP
Review Safety Procedures** Water
temperatures are
taken in the liquid water, under any ice
formations, if
present. If not present, draw an "X"
through that
data entry. Students should describe the quality
of the ice formations including degree of
solidity, color,
texture, volume, layering, etc.
-
At the end of 48 hours and using graph paper,
plot a line
graph of temperatures.
Activity 2b - Buoyancy - one 45 minute
class period
- Using
the second set of solutions (A - D) that
were set aside
from Activity 2, conduct a series of
buoyancy tests. On
data chart (click
to access "Buoyancy Experiment" table),
test and record
how well each kind of ice* (A, B, C, D) floats in each
solution (A, B, C, D). Students should use
their own judgment
on how to qualify the relative buoyancy of "icebergs"
in each solution. (The teacher could suggest
a quantitative
evaluation, using a metric ruler to measure
in millimeters
the visible height of ice above and below the
water line.
Convert these measurements to percentages to
account for
discrepancies in uniformity of size and shape
of the ice.)
*It may be helpful to initially add food coloring
to these ice cubes to aid visibility, however
as the ice
melts, the color will obscure the view.
Assessment
/ Questions for Activity 2
Did any solution not freeze? If so,
which one?
Compile a list of explanations on the board
as students
elaborate on why this one didn't freeze. (Although it
depends upon the experimental conditions, the highest
salt concentration, "A", will
likely not completely
freeze. Highly saline solutions such as this
are called
"brine.")
Which solution was first to freeze? Why
(Usually "D"
is the first to freeze.) Compile a list of
explanations
on the board as students elaborate.
- Explain
what happens to the ability of water to
freeze as you
add more and more salt. The graphed
results should
assist the students in their explanation.
(The higher
the salinity, the longer it takes to freeze. This is
because the higher the salinity, the lower
the freezing
point.)
OPTIONAL:
To access an interactive animation depicting
the interaction
of ice, water, and a solute (e.g., salt),
visit the "General
Chemistry Online!" webpage, "Why
does salt melt
ice?" (http://antoine.frostburg.edu/chem/senese/101/solutions/faq/why-salt-melts-ice.shtml).
Raise and lower the temperature and "add
solute"
by clicking on the banner below the
"dancing"
water molecules. Ask students how this
animation relates
to their experiment results.
Have groups discuss whether the findings confirm their
initial hypotheses on salinity. (Students
may have been
surprised to find that salt water freezes more slowly
than fresh water and, when salt content is
high enough,
may not freeze at all.)
Have groups discuss whether the findings confirm their
initial hypotheses on buoyancy. They be
surprised to learn
their ice cubes are most buoyant in the
saltiest solution,
"A" in this case. Many will not have guessed
that all the ice cubes would be free of salt and thus
"ice type" did not affect buoyancy. In
addition, they may not have guessed that the
size of the
ice cube does not affect the percentage
above and below
the water line.
Ask students to design an experiment that
would help them
determine the salinity of ice cubes made from
saltwater.
**Tasting should
not be accepted
as a method or strategy**
Is ice formed from salt water salty or fresh? (Fresh.)
How does this relate to sea ice and icebergs
in the Arctic
and Antarctica? (Both sea ice and icebergs
are essentially
salt-free.) Click
here to see: "Sea ice photos from the
Arctic"
and "Iceberg photos from the Antarctic".
How can you explain differences in their
physical appearance?
(The sea ice shown is thinner and flatter
having formed
from seawater; the icebergs are large and layered from
years of snow accumulation.) These photos could
also be used as a discrepant event at the
beginning, which
would provide a lead-in to the activities.
How does this relate to sea ice and icebergs
in the Arctic
and Antarctica? (Both sea ice and icebergs
are essentially
salt-free.) Click
here to see: "Sea ice photos from the
Arctic"
and "Iceberg photos from the Antarctic".
How can you explain differences in their
physical appearance?
(The sea ice shown is thinner and flatter
having formed
from seawater; the icebergs are large and layered from
years of snow accumulation.) If earth's
climate is warming,
how will this affect the formation of sea
ice? What effects
might this have on people and animals that
live in polar
regions?
**Lab
Safety Reminders**
General Thermometer
Use
- No food items generally deemed edible are to be consumed
if they are being used in a laboratory experiment.
- Glass
thermometers that are alcohol filled
are recommended.
If mercury thermometers must be used
for purposes
of higher accuracy, Teflon® coated are recommended.
- Resistance
thermometers, or temperature probes, should be
considered as alternatives to glass
- Never
use a thermometer as a stirring
device or swing
or shake down a
thermometer.
- Never
use an open flame on a thermometer
bulb.
- Use
extreme care when inserting or
removing a thermometer
from a rubber stopper.
- Choose
a thermometer with an appropriate temperature
range.
- Use a plastic beaker or
scoop to distribute ice.
|
Original
sources: Activity
1 is Adapted with permission from the Salt
Institute's activity
"Salt: The Essence of Life." Activity 2 is adapted
from UCLA
Marine Science Center's OceanGLOBE
"Density & Salinity
Lesson Plan."
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