Density Curriculum

Section 3—Lesson 10: How Does Mixed Density Contribute to Overall Density?

Background Information

Mixed Density

The third cause is most easily talked about as mixed density. One example of mixed density is when a gas, such as water molecules in the form of steam, spreads out in a room and there are lots of "air molecules" in between the "water molecules." Other examples include a sponge with holes in it. The state of the molecules affects how spread out they are. For instance, Styrofoam is formed by blowing air into a dense liquid, Styrene. The air greatly increases the liquid's volume without adding much mass, and the density of the resulting Styrofoam is due to the combination of the density of the Styrene and the density of the air. The in-between space in this case is air (sometimes it is another material instead).

While mixed density appears to be a comprehensible way to talk about the third cause with students, it lacks some precision. Here is the compromise that this simpler presentation makes. First, there are cases where molecules or atoms are spread out, but the density of the object or substance is not due to mixed density. Instead of air in between the molecules or atoms, there is simply space. Second, the molecules in many plastics (polymers) are long and curly, so when they fit next to each other there can be spaces (with vacuum, gases or liquids in them).

This introduces a puzzle about where to include molecular bonds. Gases fit in between some molecular bonds, such as those in certain plastics. Therefore, these substances could be categorized as mixed density. However, gases do not fit in the spaces between most molecular bonds, therefore most of them are best described by Cause #2.