Density Curriculum

Section 2—Lesson 5: Why Is Density Considered a Property of a Particular Kind of Matter?

Background Information

Confusions About Whether Density is Independent of Size and Shape

Students are often confused about how the size or shape of an object affects its density. For instance, they typically think that cutting an object in half must affect its density. This suggests that they are still focusing on either the mass or the volume and not acknowledging the relationship of mass to volume. They may also get confused when they are dealing with the same material in a very different shape. For instance, in talking about a piece of steel wool, they'll say it is not very dense because of all the air inside of it. In this sense, they don't think about the steel wool as very thin pieces of pure steel. In fact, the density of a pure piece of steel is the same as the thin fibers of pure steel. Of course it feels lighter to the students because the overall space of the steel wool pad is a mixed density that includes air and steel.

Introducing the Idea of Assigning Numbers for Density Under Standard Conditions

This lesson introduces the concept of assigning numbers for density under standard conditions and presents the idea that density can be used to help identify different substances. However, it also addresses confusions that students often have after learning about assigned numbers for density. Some students may wonder how density can be a relationship between mass and volume and yet be a set property of particular substances. This lesson reinforces students' understanding of the difference between properties of materials and properties of objects.

Introducing the idea of specific numbers for density can invite confusions about the nature of density. Students often take away the mistaken idea that density cannot change under any condition. Instead, we want them to realize that these numbers refer to the density of a substance under standard conditions of temperature and pressure. Even textbooks often miss this point. Missing the caveat about temperature and pressure makes it difficult to understand instances in the real world where density does change, such as in heating of gases, freezing of certain liquids, etc. For this reason, it is important to teach Lesson 12, Can the Density of Solids, Liquids, and Gases Change? and to discuss with students what it does and does not mean to say that density is a property.