|
Grade
Level: Elementary
| Time:
45
min. in class (after prep.) | Content Standard NSES Physical Science, properties of objects and
materials | 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
When
solutions of two different densities meet, the
lower density
(less dense) solution will move on top of the
higher density
(more dense) solution, resulting in a layering or
stratification
of the solutions. Density is an important feature
of seawater
since many physical and biological processes are
affected by
it, such as moving heat around the globe
influencing climate
and feeding and reproduction by marine
organisms.
Key
Concepts
-
Different densities of water, or solutions
will stratify
to form layers.
- Density
of ocean water is influenced by
temperature and salinity.
- Cold
water with dissolved salts (higher
salinity) is denser
than warm water without dissolved salts
(low or no
salinity).
- In
the oceans, the deep, bottom layer is colder and
saltier than the surface layer.
Essential
Questions
- Why
do some objects float or sink relative to
other objects?
- How
can knowing specific properties of
solutions be useful
to our everyday life?
- What
properties of water make our daily life
easier?
- How
is salt water different from fresh
water?
- Would
the Earth be different if the oceans
contained fresh
water?
Knowledge
and Skills
- Hypothesize
what will happen if 4 liquids, each a
different color
and density, are poured together.
- Plan
and conduct a simple investigation of
density.
- Demonstrate
changes in density through
experimentation.
- Compare
and contrast results of experimentation
with hypotheses.
- Describe
how salt affects the density of water.
Prior
Knowledge
- Identify
floating and sinking objects.
- Describe
attributes of matter, which are qualitative, such
as color, smell, size, and texture.
- Use
simple measurement devices to make
measurements in
scientific investigations.
Common
Preconceptions
- Students may
think that objects
sink in water because they are
"heavier"
than water.
- An object
floats because it has
air in it.
- Students
often confuse density
and thickness, assuming that
thick liquid is denser
than thinner, less viscous
liquids.
- An object
floats because it is
"lighter" than water.
- All liquids
mix.
Concept
Map
This
map is best used for the teacher's
benefit to understand
how salinity affects the properties of water. A
concept map could be constructed with
the students
and centered around the essential
question - How
can knowing specific properties of solutions be
useful in life? - as a precursor to the
activity. From the comprehensive
Aquarius Concept
Map: Water and its patterns on Earth's
Surface.
Background
Liquids
have density too. Unlike the densities of solids,
which remain relatively constant, the
densities of
many fluids can be easily changed. Do
objects float
the same way in fresh water as they do in
salt water?
If you have the same amount of each,
saltwater weighs
more than fresh water. Salt water
is described as being more dense than
fresh water.
In the case of ocean water, heating, cooling, and
salinity all influence density.
Circulation in the
ocean depends in part on differences in
density of
the water. Water with more salt is
denser (heavier)
and sinks while fresh water is less dense
and "floats"
on the surface. These buoyancy
differences can result
in the separation of water into layers
(stratification)
within an estuary or ocean. Stratification can
be disrupted by tidal mixing, heating and cooling of
surface waters,
and / or by wind generated water
movement, such as
waves and currents. This action results
in vertical
mixing. Density driven currents are an important
feature in coastal waters, affecting
the physical,
chemical, and biological dynamics in the
ocean. Many
marine organisms use density currents for
migration,
reproduction, and feeding.
Materials:
4
large containers (e.g., pitchers or milk jugs),
food coloring
(4 colors), transparent drinking straws,
pickling salt (preferred),
5 vials or test tubes per student group (4 for solutions
and 1 for waste)
Preparation:
Before
the students arrive, the teacher will prepare 4
solutions,
each with a different density, as
follows:
- Container
#1: 1 gallon water + 0 cups of salt +
bottle of yellow
food coloring
- Container
#2: 1 gallon water + 1/2 cup of salt +
bottle of green
food coloring
-
Container #3: 1 gallon water + 1 cup of salt + no food
coloring (i.e., clear)
-
Container #4: 1 gallon water + 2 cups of salt + bottle
of blue food coloring
Mix the
solutions thoroughly, until all salt is dissolved.
The solutions
must be distinctly colored. Use the entire
contents of one of
the small bottles usually sold in sets of four at
the grocery
store. Pickling salt is preferred for this activity because
it does not have any additives and will not make
cloudy solutions.
Clear or translucent drinking straws must be used
so that the
colors of the different solutions can be observed
when in the
straw. Activity
- For
best results, do not reveal how much salt is
in the solutions
while the students are crafting hypotheses. Also, do
not place the solutions in their containers
in any manner that may divulge the relative
density of
the solutions.
- Distribute
a sub-sample of each of the four solutions
to students.
Demonstrate and then instruct them to
practice drawing
solution into their straw by placing a
finger over the
end of the straw. They will use their index
finger like
a valve. Lab Safety Reminder - Mouth
pipetting is a
hazard and should not be
allowed.
- Direct
students to select two of the solutions at
random. While
holding the solution in the straw, lower the
end of the
straw into the second liquid. Draw a sample
of the second
solution into the straw. If the first solution floats
on the second, the first is less dense. If the first
mixes or falls through the second; the first is more
dense.
- By
making systematic comparisons of all four
liquids and
recording each trial (i.e., noting which
colors float
above others in the straw), students will establish
an order of density for the four liquids.
- As
an extension, challenge students to get all
four solutions
layered in the straw.
Assessment
/ Questions
- Ask
students if they can define density. If not, ask them
to name a pair of things that are
approximately the same
size (i.e., volume) but one floats in water while the
other sinks (e.g., ping pong ball and a small rock; a
paper boat and a brick; a cork and large
pebble; a Cheerio
and a dime). Ask the students how these paired items
differ from one another. (Although they have
the approximately
the same size or volume, their masses are different;
thus they have different densities.) Explain
that "density" is
"mass per volume."
- Ask
students if the solutions they
"stacked" in
their straws had different volumes. (No, the straw
itself constrained the volume of the
solutions.) Which
color of solution was the least dense?
(Yellow.) Which
color of solution was the most dense?
(Blue.) Ask students
to hypothesize about what made the
density (or mass)
of each solution different –
was it the color? (No.) Ask the students to hypothesize
about what might have been added to the
solutions to increase
their densities. Ask "What experiments might you
conduct to discover the 'secret ingredient'
used to increase
the density of your solutions?"
Lab Safety Reminder
- Tasting the blue solution would be effective
but should
not be encouraged.
- After
the students have discovered that salinity
– or salt
content – affected the density of their
solutions,
ask which color had the highest salinity
(Blue.) and which
had the lowest salinity (Yellow.). Explain that
as the salinity
of water increases, so does its density, making
it heavier
and more likely to sink. Ask the students to
name examples
of high salinity water (Oceans.) and low-to-no salinity
water (Lakes, rainwater, tap water). Ask
"What happens
when rain falls on the ocean?" (It would
float on the
ocean water until it was mixed by wind or
waves.)
Lab Safety
Reminder:
1. Students should never taste any chemical.
2. Presenting the liquids as different colors
eliminates the
potential for students to pour the chemicals back into the
dispensing container thereby contaminating a
larger quantity.
Tell them that all of the liquids used in this activity are
colorless and that you have added color for safety and that
the color has no effect on the results of the lab.
3. Remind students to never mouth pipet any substance in a
lab.
Tip: This activity is indirectly
adapted from the
activity "Layering Liquids" featured in
the Great Explorations in Math and Science (GEMS)
curriculum
"Discovering Density." Find data sheets
to supplement and guide this activity in this publication.
Original
source: Adapted
from Mid-continent
for Research
and Education (McREL)
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