Vertical Density Gradients

Density

Put a lid on it! In small groups students will explore density in a water system.

Objective
Students will understand the physical processes associated with hypoxic zones, as well as the concepts of density and stratification. Students will also be challenged to think critically about a simple demonstration, and hypothesize about various outcomes of the demonstration.

Materials

• Water
• Salt
• Scales
• Food coloring
• 10 ml graduated cylinders
• Medicine droppers or pipettes
• Paper cups

Procedure

1. Add 1 gram of salt to 5 ml of cold water. Mix thoroughly in a cup to dissolve salt. Add 1 drop of food coloring.
2. Pour into a 10 ml graduated cylinder.
3. Add 5 ml warm water to another cup. Add 1 drop of a different colored food coloring.
4. Using a medicine dropper or pipette add fresh warm water to the graduated cylinder containing the cold salty water (the two waters should not mix).
5. Check that the graduated cylinder with separated water masses looks similar to picture.
6. Students think/pair/share to brainstorm different ways they can mix the water (earthquake, tornado, hurricane, winter change top water cold).
7. Ask the students to try to mix the water with one of the ideas from their think/share/pair.

Tips
You can try separate density layering scenarios using only warm/cold or salty/fresh water and compare how these are the same/different from the combined scenario.

You can experiment with tools to aid mixing, like ice cubes or straws.

Discussion Questions

1. On a hot summer day in a deep body of water, where is the warm water and where is the cold water?
2. Which is heavier: hot water or cold water? Salty or fresh water?
3. What is the property that looks at the relationship of occupied space and mass?
4. Why didn’t the two layers of water mix?
5. How does oxygen get into water?
6. How might stratification contribute to hypoxia?
7. Why do hypoxic regions typically occur in deep water?

National Science Education Standards

• K-12: Unifying Concepts and Processes
• Evidence, models, and explanation
• 9-12 A: Science as Inquiry
• Abilities necessary to do scientific inquiry
• Understandings about scientific inquiry
• 9-12 B: Physical Science
• Structure and properties of matter