Density & Bouyancy of Soda Cans (Debbie Brenner, Pam Collins)

Questioning Script

Author

Pam Collins, modified by Debbie Brenner

Principle(s) Illustrated

1. Density

2. Bouyancy

Standards

1. Science & engineering standards

• Asking questions and defining problems

• Develop and use of models

• Engaging in arguments from evidence

2. Cross-cutting concept standards

• Cause and effect: Mechanism and explanation. Events have causes, sometimes simple, sometimes multifaceted. A major activity of science is investigating and explaining causal relationships and the mechanisms by which they are mediated. Such mechanisms can then be tested across given contexts and used to predict and explain events in new contexts.

• Systems and system models. Defining the system under study—specifying its boundaries and making explicit a model of that system—provides tools for understanding and testing ideas that are applicable throughout science and engineering.

Procedure

The basic procedure is the same for each of the following five combinations of liquids:

1. Water

2. Isopropanol

3. 50:50 mixture of water and isopropanol

4. Salt water

5. 50:50 mixture of salt water and isopropanol

• Pour about 600 mL of liquid to be tested into a clean dry 1-L beaker.

• Carefully place the regular can of Coke® into the liquid. Be sure that an air bubble is not trapped underneath the can.

• Note whether the can floats or sinks.

• Transfer the liquid to a second clean dry 1-L beaker.

• Carefully place the can of Diet-Coke® into the liquid. Be sure that an air bubble is not trapped underneath the can.

• Note whether the can floats or sinks.

• For the 50:50 mixture of salt water and isopropanol, stir the mixture vigorously before adding the can of soda. Then allow time for the mixture to separate.

• For the final step (after discussion), pour the mixture into the empty beaker, stir vigorously, and then allow it to sit undisturbed. The interface between the salt water and the isopropanol becomes visible.

A Density compared to water

B Density compared to isopropanol

C Density compared to salt water

D Salt water and isopropanol mixed thoroughly

E Isopropanol and salt water separate creating an interface where the cans come to rest

F When the mixture of isopropanol and salt water is left standing, the interface is clearly visible

Disposal

Empty out the cans of soda used in the demonstration, rinse the outside and place in a designated recycling bin. The isopropanol should not be poured down the drain. Consult local regulations for appropriate means of disposal.

Questioning Script

Prior knowledge & experience:

Students understand that some things float and some things sink.

Root question:

Why does cold water sink and warm water rise? Why does salty water sink and fresh water rise?

Target response:

Because of differences in density.

Common Misconceptions:

Students think things may sink or float because one is a solid and the other is not.

Photographs and Movies

References

Middle School Science

Educational Innovations: Poly-density bottle