This lesson is a follow up to the prior Density of Solids lesson and must be completed in succession in order for it to be effective. This lesson works on a few practical standards within the NGSS. Measurement is a practical skill the students need to be familiar with in order to perform lab activities. The metric system should something they are familiar with in elementary school, however, they are not always exposed. This lesson is great review for those who have and is an excellent entry point for those who have not.
To start off this lesson, the students will begin by making predictions about density of 3 different liquids. On their tables (I have previously filled 3 different plastic cups with the materials) they have 3 plastic cups of 3 different liquids: one has corn syrup (about half full), one has water (half full and colored with red food coloring) and one has vegetable oil (about half full).
They rank the liquids from 3 (most dense) to 1 (least dense). They will use the information from the previous lesson (what density measures, how to measure it and how to calculate it) to help them make their predictions.
They can enter their predictions in their notebooks or on their student notes sheet for more organization of their information. After they have predicted, they will explain why they ranked them in that order.
Most students rank corn syrup as most dense, oil as second water as first. They base the predictions off of the movement of the liquids (by shaking the cups) and the apparent "weight" of the cups (even though they are not weighing them, they claim they can feel how heavy each cup is).
In the previous lesson, students ranked the solid objects from 1-4. In the Engage section of this lesson, they rank the liquids from 1-3.
For this section of the lesson, I have the students combine their rankings of the solid objects to their newly formed rankings for the liquids. They rank them from 1-7 (1 being least dense and 7 being the most). They are predicting the lineup of objects and liquids in a density column they will be creating in a later part of this lesson.
After they rank them, you can have the students draw a graduated cylinder and actually draw the liquids and objects in it in order to show their predictions in a more visual manner. Have them label their drawing by identifying the various layers and objects within.
After about 5 minutes, I ask a few students to share what they think will be at the bottom, middle, top, etc.
Students now calculate the actual densities of the liquids by completing the data table within the ELABORATE section of the student notes sheet. This can be quite a task for students as they have to follow the procedure exactly as directed or it will not turn out. I walk the students through the procedure step-by-step in order to ensure 100% on task behavior.
The density column directions are as follows:
1. Measure the mass of the empty graduated cylinder and record the data in the chart.
2. Pour EXACTLY 20mL of water into the empty graduated cylinder.
3. Measure the mass and record it in the chart. Subtract the mass of the cylinder to find the mass of only the water.
4. Pour the excess water in the cup into the cup labeled "WASTE."
5. Pour the water in the cylinder back into the cup labeled "H2O" (should be empty, make sure it is or they will have to repeat the process again).
6. Repeat steps 1-5 for the oil.
7. Repeat steps 1-4 for the syrup, but LEAVE THE SYRUP IN THE CYLINDER (partly because it's the last one to measure and mostly because it is extremely difficult to get back out of the cylinder.
Once all the data has been collected, have the students calculate the density of the 3 liquids and re-rank them according to their calculated densities. After they have done this, have them pour in the liquids in any order they want and then drop in the solid objects.
After the column has been created, students should reflect on what they see and what they've done and why the column looks the way it does. The questions they answer are:
1. Which object/layer is the MOST dense? How do you know?
2. Which object/layer is the LEAST dense? How do you know?
Students should explain what density is through their answers. After they answer, I ask a few kids to share out their responses.