|Author(s): Marissa Mullen, Marc Stephenson||
Demonstration Equipment - Teacher's Guide
Density is the amount of mass per unit volume.
Standards addressed: 8th Grade Physical Science
8. All objects experience a buoyant force when immersed in a fluid. As a basis for understanding this concept:
a. Students know density is mass per unit volume.
b. Students know how to calculate the density of substances (regular and irregular solids and liquids) from measurements of mass and volume.
d. Students know how to predict whether an object will float or sink.
Explanation of principles involved
Density cube set
Density is equal to the mass per unit volume of the object.
It can be calculated in two different ways: using length x height x width to find volume or using the displacement method to find volume. The displacement method is performed by dropping the object into a graduated cylinder of water and measuring how much the water raises (or how much water is displaced by the object). This amount is equal to the volume of the object.
The density of an object determines whether that object will sink or float in water. Density is also described as the amount of matter per amount of space the object takes up. In this lab, all of the blocks are the same size, so they all take up the same amount of space (all have the same volume). Students are investigating how differing masses of same-sized objects give those objects different densities.
Also, students are investigating which objects will float and which will sink. This has to do with the amount of water displaced by each block. If the object is the same weight or lighter than the displaced water, it will float; if it is heavier than the displaced water, it will sink.
Different combinations of blocks can change the densities, causing something that would normally float to sink.
Why is it that some things float in water while other things sink? The answer has to do with density.
1. Today we will be calculating the density of ten cubes. The cubes are all the same size, but they are made of different materials: copper, brass, steel, aluminum, acrylic, oak, nylon, pine, poplar, pvc.
Which of these materials do you think is the least dense? ____________________________
2. Find the volume of your group's density cube.
Type of substance: ___________________________
Volume = length x width x height
Measure the sides of the cube (in centimeters) and use your measurements to calculate the volume. Show your calculations below.
3. Place the density cube on the electronic balance, and find its mass in grams.
Mass of cube: __________________
4. Now find the cube's density.
Density = mass/volume
Density of cube: ________________
5. Now drop your cube into the beaker of water. Does the cube sink or float? __________________
6. Using the data from other groups in the class, complete the table below.
7. What value was the same for each cube? Why?
8. Which cube was the most dense? ______________________
9. Which cube was the least dense? ______________________
Do They Sink or Float?
Now use the rubberband to connect two blocks together.
1. Which combinations sink? Why?
2. Which combinations float? Why?
Use odd-shaped objects, such as crumpled up aluminum foil, and have students use the displacement method to calculate volume, the electronic balance to find the mass, and then use those numbers to calculate density.
Have students test two soda cans - one diet soda and one regular soda - to see which can will float and which will sink. The diet soda floats, while the regular soda sinks because of the difference in sugar content.
Applications to Real Life:
Building things that need to float - like boats. What materials are best to use?
References & Links:
To purchase density cubes: