Density Curriculum

Section 4—Lesson 15: What Happens in Sinking or Floating When the Relationship Between Densities Changes?

Background Information

Manipulating Relationships Between Densities

The activities in this lesson reinforce many of the concepts in the module and those in the previous lesson. This lesson also introduces the question of what happens when one part of the system changes—in the demonstration case, the density of the liquid. Students are given an opportunity to use materials they see daily (soft drinks) and relate them to the concept of density. First, they will observe that one soda can (diet) floats while the other soda can sinks (regular) in water. This pushes them to see that the densities of the cans of soda must not be the same. Upon examination of the cans, students will find that they are of equal volume so the only way they can have different densities is through different masses. The students may realize this by reading the ingredients on the side of the can. To push the students' attention to the role of the liquid in the equation, corn syrup is added to the water to increase its overall density. The students will observe the regular soda now rises to the top of the water due to the change in the relationship between the density of the water/corn syrup mixture and the density of the regular soda.

Connections to the Real World

There are many instances in the real world where humans or animals manipulate overall density to shift the relationship between two densities to suit their needs. For example, some fish have a swim bladder that they can inflate to a greater or lesser extent to rise or sink in the water column. Submarines mimic this design. Hot air balloons also manipulate the density of the air in the balloon to cause sinking and rising. Encouraging students to make connections such as these will help them to see how the concept of density as dynamic is a powerful tool for understanding scientific problems in the real world.

A Threshold for Changing the Relationship

One of the important lessons about relational causality and density is that it involves aspects of what is referred to as the "tipping point" phenomenon. If we manipulate the density of a substance, it will not affect its relationship to another substance until it reaches the point where the differential between the densities changes direction. This threshold or tipping point can create sudden effects even if the causes have been accumulating over time. This has implications for how and when we notice changes in density. It also has important implications for the constitution of our atmosphere, the release of chemicals into water bodies, and the temperatures of our oceans, among other applications.