Introduction
Why does water in a lake turn over with the seasons? Why does warm air rise? Why does wood float but metal sink? It all has to do with density. This demonstration makes density differences visible and distinct.
Concepts
- Density
- Convection currents
- Differential heating
Materials
Food coloring, blue
Food coloring, red
Water, tap, cold, 500 mL
Water, tap, hot, 500 mL
Beakers, 600-mL, 2
Density Box Demonstration Device
Grease (petroleum jelly)
Safety Precautions
The materials used in this demonstration are considered nonhazardous. Care should always be taken with hot water. Food coloring can stain clothing and other materials. Always follow safe lab procedures in any demonstration. Wear chemical splash goggles, a chemical-resistant apron and chemical-resistant gloves.
Disposal
Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The water may be flushed down the drain according to Flinn Suggested Disposal Method #26b.
Procedure
- Locate the density box demonstration apparatus. Slide the central divider in and out of the apparatus to be sure it is functioning properly. Use a thin layer of grease (such as petroleum jelly) to lubricate and help to form a seal between the two chambers. Note: The seal does not have to be 100% water tight for the demonstration to work.
- Place the center dividing wall securely in place.
- Obtain about 500 mL of cold tap water in a beaker. Add enough blue food coloring to make the cold water a dark blue color.
- Obtain about 500 mL of hot tap water in a different beaker. Add enough red food coloring to make the hot water a dark red color.
- Simultaneously pour the hot water and the cold water into the separated halves of the chamber—red on one side and blue on the other (see Figure 1). If water is not poured simultaneously, there may be seepage from one side to the other.
{12933_Procedure_Figure_1_Density chamber with center wall in place}
- Wait about 20–30 seconds to allow any turbulence to subside.
- Carefully lift the dividing wall from the chamber. Observe the results (see Figure 2).
{12933_Procedure_Figure_2_Density chamber after removing center wall}
Teacher Tips
- Usually the two extremes of tap water (hottest and coldest) work fine without needing to heat or cool the water by other measures. You might experiment with greater extremes.
- Practice your technique of slowly removing the center wall prior to doing the demonstration with students.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Asking questions and defining problems
Developing and using models
Disciplinary Core Ideas
MS-PS3.A: Definitions of Energy
MS-ESS2.C: The Roles of Water in Earth’s Surface Processes
HS-PS3.A: Definitions of Energy
HS-PS3.B: Conservation of Energy and Energy Transfer
HS-ESS2.A: Earth’s Materials and Systems
HS-ESS2.C: The Roles of Water in Earth’s Surface Processes
Crosscutting Concepts
Patterns
Cause and effect
Scale, proportion, and quantity
Systems and system models
Stability and change
Energy and matter
Performance Expectations
MS-PS1-4: Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.
MS-PS3-4: Plan an investigation to determine the relationships among the energy transferred, the type of matter, the mass, and the change in the average kinetic energy of the particles as measured by the temperature of the sample.
MS-PS3-5: Construct, use, and present arguments to support the claim that when the kinetic energy of an object changes, energy is transferred to or from the object.
MS-ESS2-4: Develop a model to describe the cycling of water through Earth’s systems driven by energy from the sun and the force of gravity.
HS-PS3-4: Plan and conduct an investigation to provide evidence that the transfer of thermal energy when two components of different temperature are combined within a closed system results in a more uniform energy distribution among the components in the system (second law of thermodynamics).
HS-ESS2-3: Develop a model based on evidence of Earth’s interior to describe the cycling of matter by thermal convection.
Discussion
This demonstration clearly illustrates differences in density in two different temperature liquids. Each cubic centimeter of warm water is less dense than each cubic centimeter of cold water and thus the hot water rises to the top of the chamber and the cold water sinks. Since the food coloring is dispersed in the two different density fluids, the food coloring moves with the water and thus creates the distinct color separation. If you let the chamber sit long enough for the temperature to become homogeneous, the color differentiation will disappear. Concepts of molecular motion and temperature effects can be discussed and related to this demonstration. This demonstration also shows the high heat capacity of water in that it retains heat and resists temperature change.
This demonstration illustrates the principle of convection currents in the real world. Convection patterns occur continuously but they often go unnoticed. This demonstration provides a visual image for students as they discuss real world convection patterns. Air, for example, is warmed by the sun and rises from the earth as surrounding colder air masses sink. This continually changing temperature differential causes the constant mixing of the atmosphere. Water temperature differences will vary with the seasons. Warm water will be on the top during the summer but at the bottom while ice-covered in the winter. Warmer water at the bottom of an ice-covered lake will rise to the surface in the spring causing a complete turnover of the water in the lake. Actually, all bodies of water are in continual density turnover as night and day warming/cooling cycles occur. Many other examples of differential heating resulting in different densities in fluids can be discussed and related to this demonstration.
References
Special thanks to David A. Katz, retired, Wilmington, DE, for bringing this demonstration to our attention.
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