1.5 class periods

2-3 Volume and Density

The purpose of this section is to introduce students to the concepts of volume and density. Students will learn that volume measures the amount of space taken up by matter and that matter can be defined as anything that has mass and volume. They will also learn the units in which volume is measured. Students will then be introduced to the fact that mass and volume can be used to measure density, another important general property of matter. Density is then defined as mass per unit volume of an object. Finally, the concept of density will be used to explain why some objects float in water while other objects sink. Students will learn that objects with a density greater than 1 g/mL sink in water; whereas objects with a density less than 1 g/mL float.

The themes that can be focused on in this section are patterns of change, scale and structure, systems and interactions, and stability.

Patterns of change: Make students aware that the volume of an object can change if matter is added to or taken away from the object. The density of an object, however, will not change if the same kind of matter is added to or taken away from the object. Demonstrate this fact using an unequal size to show that their density is the same.

Scale and structure: Tie the concept of scale and structure to patterns of change in that while the size (scale) of an object may change, its density will remain the same if the structure of the object (the kind of atoms in that object) does not change.

Systems and interactions: After using the concepts of density to explain why an object floats or sinks in water, make sure students understand that the same concept can be used to determine if an object will float or sink in fluids (air or liquids) other than water. Stress that while the system (the fluid) and the interaction (object placed in the fluid) may change, the basic concepts that govern the interactions of matter based on density do not change.

Stability: You may want to use the theme of stability, rather than patterns of change, to demonstrate that the density of a particular type of matter does not vary despite any change in the volume of that matter. Again, using wooden blocks of unequal size would be a good way to demonstrate this important concept.

Performance Objectives 2-3

1. Define volume and give the metric units used to measure volume.
2. Describe matter in terms of mass and volume.
3. Define density and compare the densities of various objects.
4. Describe why an object sinks or floats in water, using the concept of density.

Science Terms 2-3

Volume, p.49

Density, p.49

If 96.5 grams of gold has a volume of 5 cubic centimeters, what is the density of gold?

Step 1: Write the formula

Density = mass/volume

Step 2: Substitute given numbers and units.

Density = 96.5 grams/5 cubic centimeters

Step 3: Solve for unknown variable

Density = 19.3 grams/cubic centimeter

• If 96.5 g of aluminum has a volume of 35 cm³, what is the density of aluminum? How does its density compare with the density of gold?
• If the density of a diamond is 3.5 g/cm³, what would be the mass of a diamond whose volume is 0.5 cm³?

• What is density?
• What determines whether an object floats or sinks in water?