Activity:  Adventures in Density

Materials Needed

Eye droppers

Food coloring

Salt

Beakers or jars for sample solutions

Beakers, graduated cylinders or narrow jars for each small group

Access to microwave oven and/or refrigerator, or a faucet that produces very cold and very warm water; (ice cubes may also be used to make cold water)

Student Worksheet

• Demonstration

Fill a jar with clear, room temperature tap water. Place the tip of an eye dropper filled with one of the colored solutions halfway into the tap water. Ask students if they think the solution will float or sink, and why.  Discuss what happens.

• Procedure

Divide the class into groups of four or five students, and ask each group to:

1. Make two separate beakers of salt solution by mixing about one tablespoon salt into one cup water. 

2. Fill two more beakers with fresh water. 

3. Review the formula for measuring density:  Density = mass (in grams)/ volume (in cm³)

Find the density of both the fresh and saltwater in the beakers. (Remember to subtract the mass of the beaker when determining the mass of the water.)

4. Color the water in each beaker a different color. 

5. Make one fresh- and one salt-water beaker warmer or colder than room temperature.  You will end up with something like the following: 

• Warm, fresh water (green)
• Cold, fresh water (yellow)
• Warm, salt water ( red)
• Cold, salt water (blue)

6. Ask each group of students to design at least two of their own experiments in density using the materials provided.  They can use the Student Worksheet to guide them through the scientific method.  They might layer warm water on cold water, salt water on fresh water, time how long it takes layers to mix if sitting undisturbed, layer all four solutions, add more salt to the salty water, etc.

7. Ask students to summarize their experiments and share their findings with the rest of the class. Ask them to explain how water density plays a role in coastal environments.  In what ways would density affect how pollution is dispersed in the water? How would seasonal changes affect water density? What kind of adaptations would animals living in an estuary need in order to live there? What variables were used in the experiments?  Is it possible that warm, salty water could be denser than cold fresh water? Why or why not?

(This activity was adapted from the Connecticut Sea Grant College Program's publication "Long Island Sound in a Jar" by Heather M. Crawford.)

Student Worksheet

Adventures in Density

1. Observation/Question:
2. Hypothesis:
3. Predictions:
4. Materials:
5. Methods:
6. Observations:
7. Conclusions:

Student Activity

 1.  By using the internet, newspaper articles, and journals, students can work in groups to research what kinds of problems exist in the nearest estuary or coastal environment.  They can also use the Additional Resources listed below. Assign each group a separate issue (issues may include overdevelopment, contamination from toxic substances and pathogens, habitat loss, poor land use, dredging, and the presence of non-native species). Ask them 1) to determine if or how the problem is being managed, and 2) how they might convince a local/state politician to allocate funding for it. Have each group present their findings to the class.

Assessment

1. Explain the criteria that define an estuary, and why estuaries are important environmental areas. What role does water density play in an estuary?

2. Explain the relationship between temperature and density.

3. How can density be used to explain why, on lakes and ponds, only the surface water freezes, but life persists in the deeper waters?