Density Curriculum

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

Background Information

Density as a Property of Material Kind

Earlier, we said that density was a property of material kind. A property is a characteristic used to describe a substance or material. A property of a material can't be changed by physical changes such as cutting it. This is true for density. It doesn't matter what size the piece of the material is, it still has the same density. However, this does not mean that density always stays the same. Density can change when conditions of temperature and pressure change.

What Causes Changes in Density?

One way that density can change has to do with kinetic energy—the vibration or movement of the molecules. In a solid, the movement of the atoms and/or molecules is described as 'vibration' because they remain in the same position relative to the other atoms and molecules. The vibration of structures results in expansion. In a liquid atoms and/or molecules move around each other, changing their relative positions but can be described as 'touching' each other at all times. The cause of the expansion has to do with vibrations in the bonds. In a gas, atoms and/or molecules move independently of one another. Their only contact is when they happen to collide as they move. They spread out so that other gases are in between them (Cause #3). Changes in density are caused by: 1) changes in temperature, and 2) changes in pressure.

This lesson includes a demonstration that shows a brass ball that is able to pass through a brass ring. The ball is heated on a hot plate for a few minutes, after which the hot ball cannot pass through the ring. The mass of the heated ball is compared to the mass of the unheated ball and they are shown to be the same.

Density as a Numeric Value

Students are often taught numeric values for density. They may be taught these in lieu of developing a deep understanding of density and the inherent relationship between mass and volume. Some students may miss the subtle point that the numeric values are assigned under standard conditions of temperature and pressure. Not understanding the causes of density and relying on simplified models of "material kind" can create an apparent contradiction—density is a property of material kind and therefore characterized by a steady state model—while at the same time density is potentially a dynamic feature of that same material when certain conditions such as temperature and/or pressure change. Not viewing density as dynamic supports a static model that contributes to a range of difficulties for students later. It makes it difficult to understand weather patterns, ocean currents, the make-up of our atmosphere, and other complex scientific phenomena.