• Students will make predictions and test their hypotheses.
• Students will follow a procedure to create a density tower.
• Students will use a density tower as a tool to determine the density of solids.
• Students will be able to describe how mixtures like plastics and blood can be separated by density.

Density is a property of matter that describes how closely packed atoms are within the substance. Plastics are a good example of a material that can be separated in water based on density. In this lesson, students will create a density tower to observe the relative density of different liquids. They will also learn how scientists can use the concept of density to separate different components of a mixture, such as blood.

Teacher Preparation

Prepare 100 mL of each liquid:

• To prepare the salt water, dissolve 1 g table salt in 100 ml of water. Then, add a drop of green food coloring and stir.
• To prepare the water, add 1 drop of red food coloring into 100 ml of water and stir.
• To prepare the rubbing alcohol, add one drop of blue food coloring to 100 ml of rubbing alcohol and stir.
• The corn syrup, dish soap and vegetable oil do not need any preparation.

Every student should receive a small sample (2 mL) of each liquid, three objects, and a graduated cylinder or cup in which to layer their liquids. Each liquid and item will be dropped into their density tower, starting with the densest liquid. Suggested objects are beans, Orbeez beads or cork.

Tip: It is strongly suggested that an adult add the first layer of corn syrup to the student cup, as it is sticky and hard to pour for students.

Lesson

Start by showing students the Plastic Density Video and the Density Slide Show. Using the Density Student Lab Notebook, ask students to list and discuss everything they noticed in the video.

Ask students what they know about plastic:

• Have they ever noticed the recycle symbol on the bottom of a plastic bottle?
• Why do they think some plastics floated while others sank?

This phenomenon will help drive the rest of the lesson.

Review matter with students and discuss the difference between matter and energy. For more on this, see the lesson Properties of Matter: Mass vs. Weight. Discuss density as you move through the Density Slide Show.

Density Tower Activity
Prompt students to pour a small amount of each liquid into their cup. It layers nicely if they allow the liquid to run down the side of the cup. If possible, use pipettes to add in liquid. Ask students to draw their tower in their Density Student Lab Notebook.

Tip: Lay down puppy pads or newspaper for easy clean up.

Density tower key: If students pour slowly, the tower should look like this.

Sink or Float Activity
Gather up your objects (cork, beads and beans) and a cup of water. Ask students to predict whether the object will sink or float in the water. Drop the object in the water and ask students to record their observations in the Density Student Lab Notebook. Continue this process with the rest of the objects.

Class Discussion
Discuss the questions that are listed in the slideshow as a class. Allow students to compare with their classmates and make observations about their density tower. This is a great time to answer any questions students may have.

Each of the liquids has a different density. The liquid with the lowest density is rubbing alcohol. The densest liquid is corn syrup, which causes it to remain on the bottom. The density tower shows the density of each liquid in relation to another. The density of a solid can be estimated by dropping the solid into the liquids. The object will land in a particular layer of liquid. This means the object and liquid have a similar density.

Allow students to add each object to their density tower to see where it lands. Students should add this information to their drawing. As a class, discuss that this means those objects have a density lower than the liquid below it but higher than the liquid above it. Have students record the relative density of the objects in their lab notebook.

Discussion
What happened in the phenomenon video? Plastics are not all created equal. They are each made of different chemicals. The number on the plastic tells recycling plants how to treat them. Plastics can be sorted initially by density. Some plastics like #1 and #3 plastics are denser than water and can be sorted out.

The Sanford Connection
Think about how the scientific concept of density can be applied to health care. When doctors take a blood sample from a patient, they are interested in different parts of the blood. Blood is made of different liquids and solids. The first step is to spin the blood at a high speed to allow the three parts of the blood to separate. The densest layer is the red blood cells (erythrocytes). The next layer is the buffy coat, which is made of white blood cells and platelets. The rest of the blood is plasma, which is water with dissolved particles. This layer is very similar to the salt-water layer from the density tower. Sometimes, doctors at Sanford Health use blood plasma as a treatment for disease.

Learn more here.

Clean up
All liquids are safe to pour down the drain. Pour the top layers into the drain and dispose of the bottom layer and solids in the trash. Be careful not to pour solid materials, such as Orbeez, down the drain.

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