The purpose of this Warm-up is to get students to think more about what they mean by “light” and “heavy,” to prepare for later activities that explore density.

The purpose of this discussion is to introduce the term "density." Select students to share reasons that each might have more mass. It may be helpful to discuss how mass is measured, to conclude that each, by definition, has the same mass. Then ask students to discuss what it means, specifically, when we say that feathers are lighter than steel, and how much volume a thousand kilograms of each substance might occupy.

Add the following definition to the class reference chart, and ask students to add it to their reference charts:

The density of a substance is the mass of the substance per unit volume. That is, . (Definition)

For example, a metal object whose mass is 150 kilograms, with a volume of 1000 cubic centimeters, has a density of , or 0.15, kilograms per cubic centimeter. Each cubic centimeter of the metal contains 0.15 kilograms of mass.

The goal of this activity is for students to use concepts of volume and unit conversion to enhance their understanding of density. During the Activity Synthesis, students must attend to precision (MP6) as they discuss the relationship between mass, volume, and density

This task presents a different way to think about density. Instead of considering mass per unit volume, students analyze animal population density. They use unit conversion and volume calculations to solve a problem. As students choose and track common units of measurement, they are attending to precision (MP6).

While students work, monitor for a variety of strategies such as:

• Converting the density of 16 fish per 100 gallons of water to 0.16 fish per 1 gallon.
• Multiplying the tank’s volume in gallons by 16, then dividing by 100.
• Calculating that if 275 fish were used, the density would be about 14 fish per 100 gallons.