Water Density and Vertical Mixing
Cascades to Coast GK-12 Curriculum
Fellow: Whitney McClees (PSU Environmental Science)
Teacher: Kirk Ordway (Mt. Tabor Middle School)
Advisor: Catherine de Rivera (PSU Environmental Science)
Learning Goals:
Students will learn about water density and how both temperature and salinity can alter
density. They will aim to create a small version of a thermocline and a halocline.
Target Grade: 6th grade
Oregon State Standards:
6.1P.1 – Describe physical and chemical properties of matter and how they can be measured.
6.3S.1 – Based on observation and science principles propose questions or hypotheses that can be
examined through scientific investigation. Design and conduct an investigation that uses
appropriate tools and techniques to collect relevant data.
6.4D.2 – Design, construct, and test a possible solution to a defined problem using appropriate
tools and materials. Evaluate proposed engineering design solutions to the defined problem.
Activity Summary: A hands-on activity using the scientific method to investigate water density
in order to determine whether it is possible to have two distinct layers of water that allow three
separate objects to float on the surface and in the middle and sink to the bottom.
Activity Plan:
Groups of 2-3 students, but 4-5 can also work.
Set up (5 minutes):
1) Label the plastic containers. Each group should have two, one labeled Top and one
labeled Bottom.
2) Assemble the following for each group:
a. Two plastic beakers, one labeled Top and one labeled Bottom
b. Sediment tube
c. Ring stand
d. Thermometer
e. Hydrometer
f. 1 100mL plastic beaker with salt
g. 1 100mL plastic beaker with aquarium rocks
h. Spoon
i. 3 vials with caps
j. Data sheets and pencils
3) Each group should also have access to ice and hot and cold water.
Introduction (5-10 minutes):
1) Read through/teach beginning paragraphs with students
2) Introduce Davey Jones’ Locker, density, and the density formula.
3) Introduce temperature and salinity and how they change water density.
4) Introduce thermocline and halocline.
5) Have students fill out questions in the introductory paragraphs and come up with
hypotheses.
Work time (45 minutes):
1) Go over rules
a. You need 2 layers of water in the sediment tube Top layer and Bottom Layer with
different densities.
b. You can change the density of the water by using the provided salt, hot and cold
water at the sinks, and ice in the bucket
c. Object: get 3 vials floating in the sediment tube, 1 at the bottom, one at the top,
and one in the middle between the 2 layers.
d. You can change the density of the vials using the aquarium rocks. ROCKS
ONLY GO IN THE VIALS. NOTHING BUT ROCKS GO IN THE VIALS!
e. Must fill out data table showing what Temp, Salinity, and density each layer is in
order to complete the experiment
f. Thermometers are glass so use extra care with them.
g. Once they fail have them dump the sediment tube out in the sink and start all over
again.
2) Let students attempt it in groups. They will attempt multiple times.
3) Give clues every 10-15 minutes or at your discretion. Not everyone will be successful.
Suggested clues:
a. Test Temperature and Salinity in the beakers before pouring them into the
sediment tube.
b. As temperature goes up and salinity goes down density goes down: As
temperature goes down and salinity goes up density goes up.
c. Cold, salty water on bottom. Hot, fresh water on top.
d. You can test the whole experiment in the beakers before putting it in the tube. Fill
beakers with appropriate water. One vial floats in both the top and the bottom
water. One vial sinks in the top and bottom water. One vial sinks in the top water
and floats in the bottom water.
4) Have students clean up and start working on questions.
Related Concepts: Climate change and how it impacts temperature and salinity of water. How
seasons can affect thermocline and halocline.
Materials: Plastic beakers (one labeled Top and one labeled Bottom), sediment tube, ring stand,
thermometer, hydrometer, 100mL plastic beaker with salt, 100mL plastic beaker with aquarium
rocks, spoon, 3 vials with caps, ice, hot water, cold water, data sheets and pencils
Extensions: Water properties, water testing. Students will start to see how salinity and
temperature affect water density and how that might apply to the larger ocean ecosystem.
Assessment Questions: See worksheet below.
Handouts and worksheets: Next page.
Davy Jones’ Locker
An Experiment on Density Layering and Vertical Mixing
For many centuries, the belief has existed that bodies lost or buried at sea end up in Davy Jones’
Locker, a specific depth between the surface and the bottom of the ocean. What we are going to
explore is whether or not Davy Jones’ Locker is a myth. Is it actually possible for an object to
stop sinking before it reaches the bottom?
Density is defined as the mass of a substance divided by its volume (D = mass/volume).
The mass is measured in grams and the volume is measure in cm3 or mL. Therefore, density is
measured in units of g/cm3 or g/mL (mass/volume). Any factor that changes or effects the mas or
the volume of a substance will cause a change to its density. Can you think of factors that might
change the mass or volume of water?
______________________________________________________________________
______________________________________________________________________
In many areas of the ocean, there are noticeable layers of different densities. For example,
surface waters can be less dense and bottom waters can be denser. These relatively stable density
layers occur due to rapid changes in temperature, called a thermocline, or rapid changes in
salinity, called a halocline. Due to these changes in density, the water tends not to intermix from
one layer to another. How do you think these layers might affect animals that live in the water?
______________________________________________________________________
______________________________________________________________________
Your task is to try to recreate Davy Jones’ Locker by building a layered ocean with objects
floating in each layer.
Objective:
Get three vials floating in the sediment tube: one at the bottom, one at the top, and one in the
middle between the two layers.
Hints:
A. The formula for density is ___________________.
B. How can you change the density of the water? Of the vials?
C. Think ahead – how will you know if your vial will float in each layer?
Hypothesis:
______________________________________________________________________
Rules:
1) You need two layers of water in the sediment tube (top layer and bottom layer) with
different densities.
2) You can change the density of the water by using the provided salt, hot and cold water at
the sinks, and ice.
3) You can change the density of the vials using the aquarium rocks. However, ONLY
ROCKS can go in the vials. NOTHING else but the rocks.
4) You must have your data table filled out to complete the experiment.
Use the following data tables to record your tests. A diagram would also be useful in answering
the questions.
Trial 1:
Layer
Temperature
Salinity
Density
1 (Top)
2 (Bottom)
Trial 2:
Layer
Temperature
Salinity
Density
Temperature
Salinity
Density
1 (Top)
2 (Bottom)
Trial 3:
Layer
1 (Top)
2 (Bottom)
From your observations, come up with different hypotheses for the conditions occurring in the
water column by answering the following questions.
1) What conditions are necessary for the test tubes to be in different locations in the water
column?
2) What two major properties of water can change the density of the oceans?
3) Will the different layers within the water column eventually mix if left alone? Explain
your answer.
4) What major weather events can cause the ocean surface layers to mix with deeper waters
below?
5) Do you believe that Davy Jones’ Locker can exist in nature?
6) Define the following:
a. Thermocline
b. Halocline