Water Density and the Ocean


Developed by: Katie Gardner, Kate Florio, Cathy Yehas, and Aly Busse

Download the pdf of this lesson for High School Students
Dowload the pdf of this lesson for Middle School Students
Dowload the pdf of this lesson for Elementary School Students

Topic:

Explore the role of temperature and salinity in changing the density of water samples. Observe and discuss the impact of density differences in the ocean.

Audience:

Ages 8 and up. This lesson can be scaled with ability level. Lesson support for elementary, middle, and high school students are included.

Length:

45 to 60 minutes

NJ State Standards:

5.1.A – Understand Scientific Explanations
5.2.A – Properties of Matter
5.4.G – Biogeochemical Cycles

Objectives:

  • Establish some properties that define an ocean habitat.
  • Observe interactions different water masses have with each other based on density.
  • Learn techniques to determine water properties like salinity and temperature, calculate density of water samples. (for middle and high school students)
  • Use collected measurements and calculations to explain observations and support reasoning. (for middle and high school students)

Introduction:

Ocean water properties such as salinity and temperature create a spectrum of different water masses and habitats in the ocean. In this experiment, we focus on how different water masses interact with each other in the ocean. Different water samples will be prepared and experimented with, controlling variables of temperature and salinity. Students will answer questions such as “what can you tell me about ocean properties?” and “why do scientists care about, and measure properties like salinity and temperature?”

Background:


The focus of this lesson will be the impacts of properties like temperature and salinity on seawater. The term water mass refers to a body of water that has consistent physical properties, such as salinity and temperature. There are multitudes of water masses in the ocean that are divided by density differences; they do not require a physical barrier to conserve their unique physical properties. The differences in density are one mechanism that drives vertical circulation of the ocean. The physical properties of the ocean are important to the biology as well. Many marine animals either remain within a water mass favorable to their needs or must adapt to changing water masses.

Materials:

  • One 250mL volume beaker per student
  • Several stirring rods
  • 2 different colors of food coloring (i.e. red and blue)
  • One density tank* per pair of students
  • Salt – prefer kosher or aquarium salt (dissolves faster)
  • One Small Petri dish or watch glass per 2 students plus a few extras
  • Hot pot
  • One 2L beaker (for ice water)
  • Ice
  • Ready supply of water
  • Paper towels (in case of spills)
  • Scales with maximum mass rating of at least 3000g (for middle and high school students)
  • Probe ware with conductivity and temperature probes. (for high school students)
  • One 400mL beaker per Probe ware setup (for high school students)
  • The elementary school worksheet
    Intended for students up to grade 5
    • Record starting conditions, hypothesis, observations, and explain results.

    The middle school worksheet
    Intended for students of grade 6 and up.
    • Record starting conditions, including measurements of mass and volume, hypothesis, observations, calculations of density, and explain results.

    The high school worksheet
    Intended for students of grade 9 and up.
    • This worksheet includes more calculations, and asks for more detailed support of the explanation of results.

* A density tank is a divided tank with moveable center barrier. Small versions of these can be purchased from science teaching supply catalogs, or instructions to make one can be found online. They are used to teach ocean and meteorological science concepts. We use: http://sciencekit.com/ig0029656/p/IG0029656/

Procedure:

I. Preparation

  1. Set out materials including student beakers and density tanks.
  2. a. Set up stations with scales (for middle and high school students)

    b. Set up stations with Probe ware and rinse water in 400 mL beakers (for high school students)

  3. Pre-measure salt into small Petri dishes or watch glasses.
  4. a. Vary the quantity of salt in the dishes from a little to a lot.

    b. The largest quantity of salt available to students should still dissolve easily into their water sample.

  5. Immediately before the lesson:
  6. a. Fill the 2L beaker with ice and top off with water.

    b. Fill hot pots with water, and turn on.

II. Activity

  1. Instructor will introduce the lesson, and explain that students will work in pairs for this experiment. Explain all the steps of this experiment to students in the order in which they need to be performed, and briefly explain how to use the density tank when they reach that step.
  2. Pass out worksheet.
  3. a. Have students decide with their partner what kind of water sample they will make. Each student will make their own water sample, but they MUST vary at least one variable from their partner’s sample. They may choose hot or cold water, and fresh or salty water. They must also decide who will have red water, and who will have blue water (they must have two different colors between them).

    b. Have students record their water sample decision on their worksheet.

    c. Have students record their hypothesis on their worksheet.

  4. Students will prepare the water samples as planned.
  5. a. Measure the mass of the empty beaker (for middle and high school students)

    b. Add salt, if any, to the beaker, then fill to the 200mL line of beaker

    i. For Hot Water Samples: Instructor should fill to 125 mL mark from Hot Pot then dilute the temperature with tap water to the 200 mL mark. (This is to prevent burns to students, and make beakers comfortable to handle)

    ii. For Cold Water Samples: Fill to the 200mL mark from the ice water. If ice water runs out, refill the 2L beaker with more water, the ice will chill it fast.

    c. Add several drops of food coloring. (Instructor may wish to do this step to prevent stains to clothes and skin)

    d. Use stirring rod to ensure sample is well mixed and salt, if any, dissolves.

    e. Measure the mass and volume of the filled beaker (for middle and high school students)

    f. Use probe ware to measure conductivity and temperature. (for high school students)

  6. Pass out density tanks to each pair of students; ensure the center barrier is in place.
  7. Perform density experiment. (Instructor may need to demonstrate how to use the density tank).
  8. a. Students pour water samples into the tank simultaneously, one on each side of the divider.

    b. Once the water has stilled, pull out the divider in one smooth motion.

  9. After observing the results, students will record their observations on the worksheet, and perform the calculations listed on worksheet. (for middle and high school students)
  10. Students will record their conclusions about the relationship of density to the observed behavior of the water samples in the density tanks.

Evaluation:

Have students record whether their hypothesis was correct. Why or why not? What affect does density have on the behavior of water in the density tanks? What can students think of that water density might impact in the ocean?

Extension:

Show visuals of real-time data of the New Jersey Coast displaying sea surface temperature and salinity. Display data collected by gliders of temperature and salinity at depth (along Rutgers University, Institute of Coastal Marine Studies IMCS endurance line*). See if students can explain any observed differences between the surface data and that collected from depth. What impact do differences in water density have in the ocean? How would you relate the concepts of temperature, salinity, and density to ocean habitat?

Safety Precautions:

Hot water could cause burns; care should be exercised with this portion of the activity.

If salt water gets in eyes, flush immediately with plenty of fresh water. The salt should not be harmful to vision, but can sting.

Resources:

Link to glider data: http://marine.rutgers.edu/cool/auvs/ including endurance line (eline project)

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