Introductory Activity : Duration: 1 hour
Description: Students will learn that density is a property of liquids and solids. After conducting an experiment with colored water, sugar cubes, and straws, students will learn that if a liquid has a smaller density it will float above liquids with greater densities.
Goals: National Science Education Standards (NSES) - Science as Inquiry: Properties and changes of properties in matter; a substance has characteristic properties, such as density.
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Body : Focus Phase:
Fill a fish tank with water; it needs to be large enough to fit two cans of soda (widthwise and lengthwise). Ask students to predict whether both (diet and regular) cans of soda will sink or float and why. Students write their predictions and explanations on a data sheet (see Materials ). After predictions have been made, place both cans in the water. Have students record the results on their data sheet. Ask students for their ideas about what happened. Write students' ideas on the board.
Challenge Phase:
Each student will receive four cups. Three of the cups will contain one cup of water with either red, yellow, or blue colored water. One cup will just be an empty waste cup. Students will put the following amount of sugar cubes in each cup:
* the red colored water will contain 5 sugar cubes
* the yellow colored water will contain 3 sugar cubes
* the blue colored water will not have sugar cubes
Students will stir their mixtures until the sugar dissolves completely. Give each student a straw. Inform students, "The goal of this activity is to layer your colored water in the straw." (Demonstrate what is meant by layering using other colors of water -- different colors than what the students have.)
"What order will the colors layer in your straw? Why?" Students will record their predictions and explanations individually. Students begin the activity by placing their clear straws in the cups of colored water. They will use their thumb to get the colors to layer. The goal is to get the water to layer in the straw. (It should layer blue, yellow, and red from top to bottom.) Students will write down the order in which the colors layered and why they think they layered in this way.
[ Author's Note: Students layer the colors by placing their straw in the cup so that the straw is halfway submerged in the liquid. Then they place their thumb (or a finger) over the top of the straw so the liquid stays in the straw when they pull it out of the liquid. Keeping their thumb on the straw and the straw upright, they move over to the next cup and place their straw in the liquid (about two thirds submerged into the cup). Then they lift their thumb from the top of the straw so the next color fills that part of the straw. Students then put their thumb over the top of the straw before lifting it out of the liquid. This is continued until each liquid has been placed in the straw.]
Concept Introduction Phase:
Ask students to share their results from the focus phase and challenge phase. Write the results on the board.
* Cans: "Why did one sink and one float? We will weigh the cans at this time. What makes the regular soda weigh more than the diet?"
* Straws: "Did your solutions layer? In what order? Why did they layer?"
"Are there any similarities between the two experiments?" Students should make the connection that the amount of mass in the heavier of the two liquids affects the density, and it sinks below the substance containing less mass. The density of the regular soda is heavier than the diet soda because it contains more sugar. The density of the red liquid is heavier because it has the most sugar. Discuss the concept of density and the relationship between mass and volume. The amount of mass in an object makes it denser.
Concept Application (Reinforcer):
In order to reinforce students' understanding, ask the following questions: "In this activity did you notice that the colors mixed to create a brownish liquid? If so, why did they mix in this way instead of forming distinct layers? What did you do differently to get the colors to layer?"
[ Author's Note: If students do not layer the liquids correctly (from least dense to most dense from top to bottom), then the denser liquids will fall to the bottom of the straw, thus mixing the colors to form a brownish liquid. The goal of this activity is to get the students to realize the correct order needed to layer the liquids to achieve layers and not a brown liquid. This is a trial and error science experiment.]
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