Title of Lesson
The Effect of Salinity on the Density
Subject Area
Chemical or Physical Sciences, Properties of Matter
Age or Grade Level
Middle school
Brief Overview
In this lesson student are introduced to three water samples
from a river, the ocean, and the dead sea. These water
samples have different salinities and as a result different
densities. Students practice measuring density, learn about the
concept of salinity, and make a hypothesis about how the
three water samples will behave when combined. Students get
hands on measurement experience, and experience a visually
striking demo.
Estimated Time
One 50-minute class period plus possible follow up
discussion in a few days (15 minutes)
NGSS Standards
MS. Structure and Properties of Matter
Disciplinary Core Ideas
PS1.A: Structure and Properties of Matter
• Each pure substance has characteristic physical and
chemical properties (for any bulk quantity under
given conditions) that can be used to identify it.
(MS-PS1-3) (Note: This Disciplinary Core Idea is
also addressed by MS-PS1-2.)
C
Cause and Effect
Standards/Unit Goals
• Cause and effect relationships may be used to predict
phenomena in natural or designed systems. (MSPS1-4)
MA Science and Technology/Engineering Standards
6.MS-PS1-7(MA). Use a particulate model of matter to
explain that density is the amount of matter
(mass) in a given volume. Apply proportional
reasoning to describe, calculate, and compare
relative densities of different materials.
8.MS-PS1-2. Analyze and interpret data on the properties of
substances before and after the substances
interact to determine if a chemical reaction has
occurred.
Clarification Statements:
• Properties of substances include density,
melting point, boiling point, solubility,
flammability, and odor.
Developing and Using Models
• Develop a model to predict and/or describe phenomena.
(MS-PS1-1),(MS-PS1-4)
Essential Questions
Skill Objective(s)
By end of this lesson, students
will be able to
Knowledge Objective(s)
By end of this lesson, students
will know
How does salinity affect density?
-
Measure the density of liquids using a graduated
cylinder and an electronic balance.
Anticipate behavior of liquids based on their density
-
The concept of salinity
How salinity affects density of water
-
A demo tank with a removable center divider such as
the ones available at this website:
http://www.stevespanglerscience.com/store/splitdemo-tank.html
Several gallons* of fresh water (ideally from a local
lake or river, could also be tap water)
Several gallons* of sea water (ideally from an ocean,
but could be made by dissolving approximately 30
grams of table salt per liter of water)
A gallon* of saturated salt water (ideally from the
Dead Sea, or the Great Salt Lake or another natural
water body, but could be made by dissolving
approximately 300 grams of table salt per liter of
water, or as much as you can get to dissolve)
*exact water volumes needed will depend on the
capacity of the demo tank, and the number of
classes. Less “Dead Sea” water is needed because
only a small slug is poured into the tank.
Conductivity probe, refractometer, or another way of
measuring salinity (optional)
Graduated cylinder and balance for each student/
pair/small group.
-
Materials Needed
-
Technology Tools Used
Useful Vocabulary
New Vocabulary Word 1
Meaning
Salinity
Amount of salt dissolved in water
Saturated Solution
When a solution contains as much of a dissolved substance as
it can hold
Prerequisite knowledge/skills
Students have been introduced to density already conceptually
and/or with the D=m/v formula. Students have used graduated
cylinders and balances before to measure volume and mass,
and may or may not have used these tools together to measure
density.
A common point of confusion has to do with perspective on
the personal experience with buoyancy – many students are
Anticipated Student
initially confused about why it’s easier to float on sea water if
Preconceptions/Misconceptions it’s denser. The key is that your density doesn’t change when
you swim in different places, so your buoyancy results from
your density relative to the body of water.
Instructional Tips/Strategies
for the Teacher
Opener.
Tell the students where the three water samples were
collected (or where they could have been collected), and ask
them what they know about those locations. What’s it like to
swim in ocean vs fresh? Has anyone been to the Dead Sea or
the Great Salt Lake, or know what it’s like at those places?
The discussion should lead to salinity and temperature, among
other characteristics of the water.
If anyone has experienced the difference in buoyancy
swimming in these types of water, that leads to discussion of
the characteristic of density. Swimming in a pool or in a lake,
you have to tread water to stay at the surface because you are
a similar density to the water. But, in the ocean, it’s easier to
float at the surface because both salinity and cold
temperatures increase the density, so you are less dense by
comparison.
Lesson Sequence
Development:
1.
2.
3.
4.
5.
Ask students if they can tell the difference between
the density, salinity, and other properties of the water
in the beakers by looking. If they weren’t labeled,
how could you tell?
Use the conductivity probe to measure salinity and
temperature. Have students record these values on
the board. If no conductivity probe is available, the
values can be reported as approximately: 30% in the
dead sea, 3% in the ocean, and 0.03% in the fresh
water.
Explain how to measure density of the liquids with
the cylinder and balance. Model this technique and
the calculation of density from the formula for the
intermediate (ocean) sample.
Have students work in pairs or teams to determine
density of the other two samples. With good lab
technique, they should be able to detect difference
between saturated salt water (~1.2 g/mL) and
freshwater (~1 g/mL), but may not be able to detect
the difference between fresh and seawater (~1.03
g/mL) – hence it’s use in step 3 above to model.
When everyone is done, have the students share their
answers for the different samples, record these on the
board. Discuss how when salinity measurements are
6.
7.
8.
high and density measurements are too, and vice
versa.
Ask the students to predict what will happen if these
three types of water were combined in one beaker.
Have them draw a picture of it. Will denser water go
to the top or the bottom? Will they mix right away or
go into layers first?
While they are sketching their hypothesis, pour river
water and seawater on the 2 sides of the demo box –
divider in place. Put in a few drops of food coloring
on each side and mix (yellow=river and blue=ocean).
Mix red food coloring into ~1L of saturated salt
water in a separate beaker and set aside.
As a demo, pull up the divider and observe the river
water go on top of the seawater. Discuss how
hypotheses are doing so far. Pour the red saturated
salt water carefully down the side, it sinks to the
bottom!
Closure
Reinforce that it was DENSITY that made the three types of
water stack up in the order that they did. The least dense
water was the freshest and it floated on top.
Source of demo tank:
http://www.stevespanglerscience.com/store/split-demotank.html
Resources
Weblinks
Reproducible Materials
References
Assessment
We found several excellent images on google image search of
people floating in the Dead Sea, and of salt crystallizing on
the edge of the Dead Sea and the Great Salt Lake. These were
very helpful for the students, who had personal experience
swimming in freshwater and seawater, but not saturated
water. The images of salt crystallizing on the edges of the
water was a good illustration of the saturation of the solution.
Possibilities include having the students turn in their density
calculations or their hypothesis sketch for credit, or another
formative assessment to gauge understanding of the density
concept.
One could explore the effect of temperature on density with a
similar demo tank, or even at the same time.
Extensions/Explorations
Fill petri dishes with water from each of the sites (use the
same volume for each) and let the water evaporate. Compare
the amount of salt crystals left behind. This reinforces the
concept of salinity as the amount of salt dissolved in the
water. The salt wasn’t visible, but it was there.
To follow up the next day, build a "density tower” with the
class. Bring in 5 substances with a range of densities – for
example rubbing alcohol, cooking oil, dish soap, corn syrup,
and molasses. These substances will form layers when poured
into a tall, narrow beaker or glass. Ask the students to predict
the order that the layers will be in based on their knowledge
of these everyday substances. (Alternatively, look up the
densities of the substances you select, and have the students
base their predictions on these values.) See if their hypotheses
are confirmed.
Discussion of relative density in bodies of water that you can
swim in leads naturally to a discussion of buoyancy.
Reflections
Contact Information
Hollie Emery [email protected]
Wendy Hanlon [email protected]