Density Dynamics

Introduction: Bodies of water form layers based on differences in density, usually affected most by temperature in fresh water and both temperature and salinity in salt water. Water will become more dense as its temperature is lowered until it reaches its maximum density at 4° C. One of the unique properties of water is that it becomes less dense as its temperature decreases from 4° C until it freezes at 0° C. This allows many aquatic life forms to survive through the winter. If ice were more dense than water, it would freeze and sink over and over until the entire lake was frozen. This would eliminate many aquatic organisms and produce a system with far fewer life forms in lakes which freeze periodically.

Changes in water density also cause vertical movement of water and nutrients within a lake, commonly called turnover. In the spring as the ice melts, water at the surface becomes more dense as it warms to 4 degrees C and thus sinks. This sinking water moves to the bottom and forces nutrient-rich water at the bottom to move up. This same process occurs in the fall as the surface water cools and becomes more dense; it will sink and cause the same movement or turnover of the lake’s water.

What to Expect: This activity works best when students have had the opportunity to look at and experiment with the interactions of water masses of different densities (for examples, see Lesson 3). If this is their first experience with that topic, give them at least one extra class session to use liquids of different densities and come up with some ideas about how they interact.

This activity can be done by groups of students or as an interactive demonstration with the class. Five gallons (about 20 liters) is a good capacity for a demonstration in a tank or aquarium; if students are using their own tanks, one gallon (about 4 liters) is sufficient capacity.

As always, the structure of the lesson should be altered to allow students to pursue their own questions about density of liquids. Encourage them to state their question and plan how they will test it; you or their peer can offer advice on their experimental design before they try it. This does not have to be a lengthy process, but simply a way to promote careful testing of questions.

Materials:

• Two water samples, one at room temperature (about 20-22° C and one warm but not boiling, about 35° C)
• Food Coloring
• Clear water tight container at least the size of a goldfish bowl (could be an aquarium, tank, or clear shoe box size container)
• Thermometers
• Ice

Procedure:

Pose the question: what happens to lakes and ponds as they cool down and freeze in winter, and then warm up in the spring? What effect does this have on the organisms living there? Invite ideas and comments from the class, and discuss ways to find out more. Introduce this activity as a way of modeling some of these processes.

1. Prepare two water samples, one at about 20-22° C and one at about 35° C; put a few drops of different food coloring in each batch.

2. Have students measure the temperature of the samples, then carefully pour some of each sample into the aquarium or tank to create two layers of different colored water. It may help to float a sponge and pour the water on to the sponge. You can guide them to put the coldest sample in first or let them work with the materials to solve this problem themselves. This represents a stratified or layered water formation found where the sun warms the upper layers of water.

3. Once the students have distinct water layers in the container, add ice to the surface at one end of the tank. As the ice melts into denser cold water, they should see the cold water move toward the bottom, causing visible circulation patterns in the colored water layers. Next, have students compare this process with what happens when ice is covering the entire surface of the tank. This represents changes occuring during Spring ice melts.

4. Have students describe in written or diagram form the process of mixing and turnover they observe, and relate it to the formation of cold deepwater masses in high ocean latitudes and spring and fall turnover of lakes and ponds.

5. You may wish to have students record temperatures at the surface, mid-depth, and near the bottom throughout the day to track stratification and warming of the water.

6. Allow time for testing of individual questions and reporting of results.

Evaluation: Students should use diagrams and written reports to describe their observations. They should include any related personal experiences: swimming in a stratified pond in summer; testing the thickness of ice on a pond; or fishing through ice.

Extensions:

1. Introduce the variable of salinity. Use salt water and vary the salinity of water samples, or use some fresh water and some salt water.

2. Use a small fan to investigate wind effects on lake circulation (use a battery operated fan).

3. Freeze rocks and sand into ice and observe sedimentation as ice melts.

4. Use a hose to add water to the tank to simulate a stream=s effect on lake water (you could use water with different temperatures and salinities or even sediment content).

5. Ideally, this could be followed up by observations and measurements in a nearby pond or lake.