On the whiteboard, this picture is displayed.
By answering the questions below, the students spend the first 3-4 minutes analyzing and making inferences about the image. Students answer the following questions on their own one their student notes sheet or in their notebooks.
1. Which column, A or B, is larger?
2. Which column, A or B, has MORE particles in it?
3. Which column, A or B, do you think has more MASS?
4. Which column, A or B, looks more crowded?
After 4 minutes of working, I ask the kids to share their responses with their table mates for (about a minute). The students share their observations and responses with one another in quiet voices.
Note: This image and activity helps students to see that mass is determined by the amount of matter (represented by the red dots) in an object. Because both objects are the same size, they can assume they are the same volume. Students make simple observations about the cylinders and when we share our responses, we connect their observations to the bigger picture of density. They can see the dots (matter) are more crowded in B. I tell them that crowded means dense in science and we can think of thinks that are dense being more crowded than things that are not dense. This seems to drive home the point.
I pass out a cup full of 4 different materials. After having a preliminary conversation about density (see ENGAGE section for conversation details), I ask the students to make the following predictions before we actually calculate the densities according to the formula density=mass/volume. They can answer on their student worksheets or create a table in their notebooks.
Before starting the lab, rank the following items based on their predicted densities (1=least dense, 4=most dense). (These items are distributed to the students in a plastic cup so they may see and touch them to get a better idea for making their predictions.)
Wooden Stick =
After the students make their rankings, I have them explain why they chose the rankings that they did. I ask them to answer the following question in writing and supporting their response with evidence.
Why did you rank the objects in this order?
After 8-9 minutes, I make a chart on the whiteboard and have the kids give me their rankings which I write on the board for everyone to see. After a student shares their responses, I have them provide me with their evidence.
Based on their predictions and the image from the Engage activity, students will work together to determine what density actually is. When they find out, they will write down their definition in their notebooks.
What is density?
I ask them questions like "If cylinder B is more dense than cylinder A (referring to this image) what do you think density means? What does it have to do with? (amount of stuff inside of something) Even though both objects have the same volume, they have different densities. What else about them must be different? (mass) "
At the end, I would like them to have something that resembles: Density is a measurement of how much matter is packed into a certain amount of volume.
With the materials in the cups, the students now work together to calculate the actual densities of the objects (marble, popsicle stick, a piece of macaroni, half of a crayon) using the triple beam balance to calculate mass and a graduated cylinder and the water displacement method for volume. Students complete the chart in their notes sheet and answer the questions that follow.
I float from table to table at this point ensuring that students are working together efficiently and effectively. If any student is unsure of how to use the tools, I refresh their minds at this time, as well.
Now that the students have calculated the actual densities, they make a new ranking of the objects based on their actual densities. They then compare the actual densities with their original rankings and reflect on what they had correct or incorrect.
Rank the following items using your data to help you make informed choices. (1= least dense, 4= most dense)
Wooden Stick =
Were your predictions correct? Explain which object is the MOST DENSE and explain what that means in terms of volume and mass.
These answers can be shared allowed at the end of class, or I will ask for 1 or 2 kids to share.