Density Curriculum

Section 3—Lesson 12: Can the Density of Solids, Liquids, and Gases Change?

Understanding Goals

Subject Matter

• Density is not static; it can change.
• We assign densities (numeric values) to different elements. This can make it harder to think of density as changeable. The numbers refer to density under a certain set of conditions.
• Changing the temperature (and pressure) can change the density of a substance.
• When the temperature is increased, molecules in solids, liquids, and gases move faster.
• Solids and liquids expand a little when heated. Gases expand a lot when heated.

Causality

• We forget to think of "steady states" as caused. Realizing that density is dynamic can help students get beyond the idea that density "just is," and to help them to think about why it is what it is.
• Recognizing that density is dynamic helps students grasp the role that it plays in many complex systems, such as weather, ocean patterns, and movement of pollutants in the environment.