|
Grade
Level: Middle
| Time:
45
min. in class (after prep.) | 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
Seawater contains many dissolved substances and these add
mass to the water producing a greater mass per unit volume, or density,
than that of pure water. The relationship between the density of a fluid, weight of an object, and
buoyancy is critical in understanding the ocean, because density has a
direct influence on the way seawater and objects in seawater behave.
Key
Concepts
-
An
object can both sink and float depending on
its relative
density to the surrounding fluid.
- There
are two main factors that make ocean
water more or
less dense: the temperature and
salinity.
- Less
dense water floats on top of more dense
water.
- Generate
hypotheses and make predictions.
Essential
Questions
- Why
do some objects float in water and others
don't?
- If
we can't detect density with any of our
five senses,
how do we know it exists?
- What
is density a measure of?
- Why
does a ship float in the water and a
paper clip sinks?
- Are
fish "floating" when they are
in the water, even
though they are submerged?
Knowledge
and Skills
- Observe
density by floating an object in variable density
solutions and then in a solution that has
layers of varying
density.
- Examine
the effect salinity has on the density of
water .
- Predict
what will happen to the potato in
varying densities
of water.
- Explain
the role density has in buoyancy.
Prior
Knowledge
- Identify floating and sinking objects.
- Distinguish between mass, weight, volume, heaviness, and
size.
- Describe attributes of matter, which are qualitative,
such as color,
smell, size, and texture.
- Salt has mass and when it is mixed with water it does not
change the
volume of the water, however it does change the mass
of the water.
- The extent to which a substance -- such as salt -- mixes into and
throughout another substance -- such as water -- is
called its solubility. (In discussions
of solubility,
the substance being added is called a solute and the
substance the solute is placed in is called
a solvent.
If the solute mixes and spreads completely
through the
solvent so that the particles are so
intermingled with
those of the solvent that they will not settle out,
then the substance has dissolved in the
solvent.)
Common
Preconceptions
- Objects float
in water because
they are lighter than water.
- Objects
sink in water because
they are heavier than water.
- Mass/volume/weight/heaviness/size/density
may be perceived as
equivalent.
- Wood floats
and metal sinks.
- All objects
containing air float.
- Liquids
of high viscosity are
also liquids with high density.
- An object
that appears to be on
top of the solution is not
floating but is held up
by the solutions "skin."
- Objects
that are completely
submerged but freely suspended,
such as fish or
submarines, are not buoyant.
Concept
Map
Create a preliminary concept map with
the class,
to determine level of student
knowledge. See Assessment
below to revisit the concept map as a
form of evaluation.
This less and activity relates to the
branch "Properties
of Water" from the comprehensive
Aquarius Concept
Map - Water and its patterns
on Earth's surface. From the comprehensive
Aquarius Concept
Map: Water and its patterns on Earth's
Surface.
Background
Seawater
has a higher density than fresh water.
Seawater contains
many dissolved substances and these add mass to the
water in which they are dissolved. This produces a
greater mass per unit volume, or a density
higher than
that of pure water. The amount of salts
dissolved in
water is known as salinity. In waters where
large amounts
of suspended sediments are present, the
density also
increases. This most notable in major
rivers such as
the Nile, Mississippi, and Amazon. In the
oceans, at
the junction between the continental
shelves and the
continental slopes, large, dense, sediment
water mixtures
form currents. When bodies of water with different
densities contact each other, some mixing
may occur,
but it often occurs slowly or not at all. This can
be observed where fresh water flows out
into the ocean
and remains on top of the denser
seawater. Along ice
caps and glacial margins, rivers of dense,
cold water
flow out and sink into the oceans relatively warmer
(and therefore less dense) seawater. Many swimmers,
including your students, have experienced hot/cold
spots in lakes and rivers where water of different
density has formed.
The
relationship between density of
fluid, weight of
an object, and buoyancy is a basic
concept in understanding
the behavior of seawater. Students
often have difficulty
identifying the factors that influence whether
an object sinks or floats in a liquid.
When deciding
whether an object sinks and floats,
they consider
the object's mass or volume instead of density.
Different objects also have different
densities,
and depending on their relative densities
to solutions,
they will either sink or float.
Materials:
3 tall beakers or glass tumblers labeled A, B,
C; water; salt (pickling salt works best to
prevent cloudiness);
spoon; knife; potato (cubed into 3 pieces
that are equal
sizes, experimentation with size of cube may
be prudent
to ensure flotation for the solution you've
mixed); balance;
variety of smaller high-density objects (e.g., made of
metal or glass) and larger low-density objects
(e.g., made
of wood or plastic); ruler; graduated cylinders
if beakers
or tumblers do not have marked volumes, data
sheet
Preparation:
This activity described here is a demonstration.
In
Beaker A, mix salt and water until saturation,
try not to
let the solution become cloudy or have salt crystals on
the bottom. Fill beaker B, with fresh water.
In Beaker C,
fill it half full with a saturated salt
solution, then carefully
pour in fresh water so that the fresh water is
layered on
top of the salt solution. Tip - to minimize mixing, pour
the fresh water down the side of the glass or
use a spoon
(or glass rod) to direct the stream against the side of
the glass. The less dense fresh water will float on the
denser saltwater solution. In Beaker C, the
saltwater solution
should be clear enought to appear to be pure water. All
three beakers are filled to the same level /
volume. Prepare
these solutions before students arrive in the
class so they
are not aware that there is a difference among
solutions.
Activity
- Ask
students to describe their understanding of
density. Record
their thoughts on a chalk or white board.
(It is mass
per volume.) The demonstration they are
about to see
should help them better understand density.
Students should
use the data sheet to record their
observations.
- Carefully place a cube of potato in Beaker A. You should expect the
potato will float at the top of the liquid. Ask
the students
which has higher density –-- the potato
or the liquid?
(The liquid.)
- Carefully place a cube of potato in Beaker B. The potato will sink
to the bottom. Ask the students which has higher density
–-- the potato or the liquid? (The
potato.)
- Ask students to hypothesize about what made the density
(or mass) of each solution
different. Ask them "What might one of the liquids be?" (They
will likely guess water.) Based on their experiences with
placing potatoes in water
before boiling, can they guess which of the two liquids is water? (The
liquid in Beaker B.). Ask the students "What might have
been added to the liquid
in the first beaker to increase its density? What experiments might you conduct
to discover this 'secret ingredient'?" Lab Safety
Reminder -
Tasting the solution in the first glass would be effective
but is not advised.If the students cannot guess correctly,
then tell them they will learn the "secret
ingredient" after the next step of the demonstration.
- Carefully place a cube of potato in Beaker C. The potato will sink
to the middle of the glass. If, during the
previous step,
they didn't guess the composition of the
solutions, identify
the solutions in the first two beakers. (The bottom
layer is saltwater, the top layer is
pure water.)
Then ask again why they think the potato sank
only halfway
in Beaker C. Lead them with questions to arrive at
the conclusion that the solutions are layered in Beaker
C. Following is a suggested line of
questioning. Ask
for a prediction about the potato's behavior in
Beaker C.
Student answers should vary --"sink,"
""float,"
I don't know." Since this last demonstration
is a bit of a trick, don't spend too much time on their
guesses. When the potato sinks halfway in
the beaker,
ask, "What might be the cause?"
I will ask
if they think it is possible for one liquid to float on
top of another, like the potato floats on top
of the water.
Is it possible or likely that there is a smooth gradient
of increasing density from the top to the bottom of the
liquid? Again, a variety of answers, but lead them
to realize that there are two different liquids
of different
densities layered in the beaker, and the density of the
potato must be an intermediate between them.
Assessment
/ Questions
-
Discuss the idea of density (i.e., mass per volume) with the students. Did the
demonstration help them better understand the concept? Density is not the same
as weight because it depends on an object's volume.
- Ask
the students "Which beaker of solution would you
predict weighs more?" (The
solution in the Beaker A.) To illustrate this point, show the amount of
salt that went into the saltwater solution,
explaining
that the saltwater solution has the weight
of the water
plus that of the salt.
- Mass/volume/weight/heaviness/size/density can mistakenly
be perceived
as equivalent–-- unlike weight, mass is
independent
of the pull of gravity. Discuss with students
this misconception,
it may be helpful to start fundamentally with a
definition
of matter. Matter is anything that takes up space
(has volume) and has mass. Recording on a
whiteboard, ask
students to generate a list of examples of matter.
You may want to have a box of items containing
some examples
of matter: rocks (solid), water (liquid), air
(gas), your
desk, your arm, your shoe to display after
they've had sometime
to craft list. Then ask students to list
examples of some
things that are NOT matter (objects that do not take up
space and do not have mass) - examples of
"non-matter"
things include heat, light, an idea.
Because of its definition, matter has four
general properties,
that is, four measurable quantities that can
always be assigned
to it: mass, weight, volume, and density. Mass and
weight are different, but related
quantities. An object's
mass is a measure of its inertia -- the more
mass an object
has, the more resistant it is to changes in
motion. Weight
is a measurement of the gravitational force acting on an
object. So a big rock has more mass than a
small rock.
The big rock also weighs more than the little
rock in the
earth's gravitational field. If you took
both rocks
into the universe, outside of any gravitational
field, both
rocks would not have any weight -- but the big
rock would
still be more resistant to a change in motion
than the little
rock -- that is, the big rock still has more
mass than the
little rock, even though both rocks are
"weightless".
An object has the same mass on earth as it has on other
planets, but its weight is very different. Visiting the
website "Your Weight on
Other Worlds"
–-- is an additional resource to
help illustrate
this point.
- In cooperative teams of 3-5 have the students
use the equation
for density (mass divided by volume) to
calculate the density
of the two solutions –-- water and saltwater, and
the density of the potato. They should record their
results on the data sheet. They will need
balances, rulers,
and graduated cylinders or marked beakers to
determine the
mass and volume. The potato cubes should
be measured
(length x width x height) to determine volume. In their
groups, a second task is to choose among
smaller high-density
objects (e.g., made of metal or glass) and
larger low-density
objects (e.g., made of wood or plastic) to find
two objects
with the same weight but different volumes. Ask
them which
of these objects have relatively higher
densities.
- If the students
are familiar with and the teacher is
comfortable with
concept maps, instruct the students
to construct a concept
map using the following concepts:
matter, mass, weight,
volume, density, water, ocean,
salinity, buoyancy, float.
Each concept should be identified in
the map by having
a circle around it. Lines with
arrowheads (directional)
are drawn between concepts that are
related. One or
a few words are written on the line
to describe how the
concepts are related. There will no
"right" answer per
se. Each of the students will have a
map that looks different.
Encourage students to acknowledge all
the the relationships
that seem important. The teacher can
use the maps to
identify persistent misconceptions or
misunderstood relationships
between concepts.
Wrap
Up :
In their cooperative groups, have students discuss
and record
thoughts on the following questions:
- How
can knowing the density of solutions be
useful in daily
life?
- Why
is density important to life in the ocean?
- How
does temperature affect the density of water?
Have
each group report out to the entire class,
their thoughts
/ answers to these questions.
Lab
Safety Reminder:
Students
should never taste any chemical.
Original
source: Adapted
from Mid-continent
for Research
and Education (McREL)
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