Density Curriculum

Section 4—Lesson 13: Dropping an Object into a Liquid: How Does Density Affect Sinking or Floating?

Background Information

Entity-based Versus Relational Conceptions of Sinking and Floating

It is common for students to focus on the object that is doing the sinking or floating rather than on the relationship between the densities of the object to the liquid. The activities in this lesson attempt to help students develop a Relational Causal model of sinking and floating by offering evidence that is discrepant with a focus merely on the object and the Linear Causal Model that follows from it. By holding the object constant and varying the liquid, students' attention is directed to the role of the liquid, thereby revealing the underlying causal structure as relational. In Lesson 14, students are asked to layer liquids so they can see the densities of the liquids as relational. These understandings are important for comprehending a variety of phenomena such as why oil floats on water, why ships float differently in salt water than in fresh water, and so on.

Relational Causality

Relational Causality involves recognizing that an effect is caused by a relationship, often one of balance or imbalance, between elements of a system. Neither element is the cause by itself. Thinking about Relational Causality requires a departure from linear, unidirectional forms of causality where one object or entity acts as a causal agent on another affecting an outcome in one direction only, in a domino-like pattern.1

Research shows that students typically assume simple linear, unidirectional cause and effect models when analyzing scientific phenomena. These assumptions are evident from infancy.2 Causes are often perceived of as embedded in objects or entities, in this case, the object or substance that is sinking or floating. Attempting to reason about sinking and floating with an entity-based, linear causal model leads to viewing the surrounding fluid as playing a passive role. This reinforces a linear conception. Only in dramatic contexts, such as dropping an object into a very dense liquid, does the liquid's role in the relationship as part of the causal agent become obvious enough to challenge the notion that equates the entity with the cause.

Helping Students Develop a Relational Conception

Students' problematic tendencies are compounded by certain teaching practices. For instance, referring to certain objects as "sinkers" and others as "floaters" without reference to the liquid encourages students to view the object itself as the only thing that determines sinking or floating. A common activity in the primary grades is to make a list of objects that sink and objects that float. This encourages a linear, entity-based, static model that can contribute to a range of difficulties for students later.

As you talk with your students, it will be helpful to have different examples of relational causality to draw upon. The example used in Section 2 is about what makes two people sisters. In our experience, social examples have helped students to grasp the notion of relational causality because they are able to understand that you can't be a younger or older sister without having another person to relate to. Similarly, in density, you can't determine whether or not an object will sink or float unless you compare the object's density with the liquid's density. Here are some other examples to help your students think about Relational Causality that you may want to refer to:

  • The outcome of certain sports series. For instance, when determining how well a baseball team is doing in a particular season, you can't just look at their win-loss record. You must also think about how the other teams in the league are doing and variables that impact how they might progress and grow over the season.
  • Food production and population growth. Determining how much food to produce depends on how big the population is. If food production increases, so will the population. Likewise, if the population increases, food production will also need to increase in order to sustain the population.