Lesson Plan #:AELP-PHY0052.html

## Density

Lesson Focus: Density

Science Understandings:

1. The same volume of different liquids may have different masses, therefore different densities.
2. When two liquids have different densities, the heavier liquid will sink below the lighter liquid.
3. Adding the liquids together in different orders doesn't change the final positions of the liquids.
4. The layers of liquid can be used to roughly determine the density of an object.

Objectives:

The students will demonstrate these understandings by:

1) Explaining why different liquids settle in distinct layers;
2) Explaining why the different liquids do not mix together; and,
3) Predicting where an object will settle when it has a different density than the liquids it will be placed in.

Materials:

Light corn syrup (add red food coloring) Small paper cups
Blue lamp oil 1 glass of fresh water
Milk 1 glass of salt water
3 clear drinking glasses 1 piece of cork, candle, and metal

Background Information:

"Density" is the term used to compare the mass of the same volume of different substances. The same volume of two different substances can have a different mass. The heavier the liquid, the greater the density. A liquid that is less dense will float on a liquid that is more dense. For example, a glassful of water has a greater mass than a glassful of oil. Since water is denser than oil, two layers will form with oil being the top layer (Bosak, 1991). If you drop an object into the density column, where it rests can tell you about its density. For example, if a piece of cork floats on the oil then you can roughly determine its' density to be less than that of the oil's (Liem, 1987).

When the same volume of milk, lamp oil, and corn syrup are combined in any order, three separate layers form. The reasoning for this is the three liquids have different masses, causing different densities. Oil forms the top layer, milk the middle, and corn syrup the bottom. When you drop a piece of cork into this density column, it will float on the oil. A piece of candle will sink below the oil but will float on the milk. A piece of metal will sink to the bottom of the column, therefore having the greatest density. When you slowly combine a glass of salt water with a glass of fresh water, two separate layers will form. The fresh water is less dense and will float on the salt water.

What do you notice about these liquids? What do you think would happen if you combined these different liquids together?

Concept Exploration:

Have the students try it out. Provide the students with a variety of liquids, such as corn syrup, lamp oil, and milk. Ask them to predict what will happen when the three liquids are combined. Does it matter what order the liquids are added to each other? Have the students discuss this in small groups. How might you find out if order matters? Students record their data on the attached data sheet. Then, have them develop additional questions and choose one to investigate.

Concept Introduction:

Have the students share their results from the investigative phase. Ask:
What happened to the liquids when they were added together?
Did it make any difference when you changed the order of combining them?
Why do you think different layers formed instead of mixing?
Can you explain why this is happening?

Have the students weigh 10 mL of each liquid. Provide a data sheet that they can record their findings on. Introduce the concept of density and explain that by using the formula Density = Mass/Volume, one can determine where certain liquids and objects will settle.

Sample data sheet
 Mass (grams) Density (g/mL) Volume (mL) Milk Corn Syrup Oil

Concept Application:

Show students a glass of fresh water and a glass of salt water.
Can they predict what will happen when the two liquids are poured together?
Can they explain their response?
Have them test their predictions.
Make sure they combine these liquids very slowly.

Show students the original density column of oil, milk, and corn syrup. Ask them to predict what will happen if you drop a piece of cork into the layers of liquid? How about a piece of candle? A piece of metal? Students will draw their predictions on the attached student data sheet. Have them test their predictions and observe where the objects end up settling. Then have them draw these findings on the data sheet. Ask students what this shows about the densities of the different objects?

Resources: