Summary

In this lesson, students will explain phenomena using their knowledge of how changes in mass and volume can affect the density of an object.

Grade Level

Middle School

NGSS Alignment

This activity will help prepare your students to meet the performance expectations in the following standards:

• MS-PS1-1: Develop models to describe atomic composition of simple molecules and extended structures.
• Scientific and Engineering Practices:
  • Developing and Using Models
  • Analyzing and Interpreting Data
  • Engaging in Argument from Evidence

Objectives

By the end of this lesson, students should be able to

• Explain that objects with a greater density are more likely to sink and objects with a lower density are more likely to float.
• Explain that increasing the mass in the same volume results in a greater density (and vice versa).
• Explain that decreasing the volume of the same mass results in a greater density (and vice versa).

Chemistry Topics

This lesson supports students’ understanding of

• Density
• Mass
• Volume

Time

Teacher Preparation: 60 minutes

Lesson:

• Warm-Up with Discussion: 15 minutes
• Demonstrations with Discussion: 45 minutes
• Homework: 10-15 minutes
• Lab Activity: 120 minutes
• Homework: 15-30 minutes

Materials

Warm-up and Demonstrations:

• Aquarium filled with water
• One large, heavy object that floats in water (10-pound or lighter bowling ball, campfire log, etc.)
• One small but relatively dense object (marble, ball bearing, etc.)
• Piece of foam
• Sheet of paper
• Sheet of aluminum foil
• 20-25 Pennies
• Empty film canister or other small plastic container
• Balloon

Lab activity:

• 3 hardboiled eggs
• 3, 500 mL beakers
• Salt
• Large bowl (alternatives: dish pan or bucket)
• 2 oranges
• 10 mL graduated cylinder
• 2, 100 mL graduated cylinders
• Larger container such as bucket
• Can of regular Coke
• Can of Diet Coke
• Container of vegetable oil (needed for two of the lab stations)
• 2-3 Ice cubes
• 50 mL or similar sized beaker
• Box of raisins
• 4 Small cans of 7-Up
• 2 empty 2-Liter bottles
• Eyedropper
• Small ketchup packet
• Bottle of corn syrup
• Bottle of rubbing alcohol

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• Do not consume lab solutions, even if they’re otherwise edible products.
• Food in the lab should be considered a chemical not for consumption.

Teacher Notes

Warm-Up:

• Show the students a 10-pound bowling ball (a campfire log would work well, too) and a marble or ball bearing.
• Ask the students to think about which object is more dense. Pass the objects around the class so that the students can feel their relative mass and size.
• After a few minutes, ask students to share their thoughts with a partner.
• After the share time, have a class discussion regarding their ideas. Do NOT give them the answer at this point.

Demonstrations:

• Pass out the “Density Concept Notes” handout. Answer Key has been provided for expected student responses.
  • Students should complete the first bullet point (adding dots to the two squares).
  • Discuss student responses.
• Read the second bullet point with the students.
  • For the demonstration, hold up a piece of foam and ask the students what will happen if the foam is placed in the water. Do the same with a penny.
  • Discuss how the foam is less dense than the penny (evidence: the foam floated while the penny sank).
  • Students should record what happened on their handout.
• Read the third bullet point with the students.
  • For the demonstration, hold up a piece of paper and ask the students if it would sink or float.
  • After discussing the students’ ideas, show that it would float.
  • Then remove the paper and tear it in two.
  • Ask the students if the density of the paper changed.
  • After discussing the students’ ideas, show that the two pieces of paper still float.
  • Students should record what happened on their handout.
• Now read the first of the density principles with the students.
  • To demonstrate it, place a film canister or other small container with two or three pennies into the water.
  • Discuss with the students about how its density would compare to that of water.
  • Now start adding pennies one at a time until the container sinks.
  • As you are adding pennies, ask the students to explain what is happening to the density of the container as you are adding pennies. Make sure that they provide evidence and reasoning for their responses.
  • Explain that the container kept the same volume but increased in mass which increased the density of the container.
  • Students should record what happened on their handout.
• Now read the second of the density principles with the students.
  • To demonstrate it, make a small raft out of aluminum foil.
  • Place two or three pennies on the raft and place it in the aquarium. The raft should float.
  • Ask the students how we could make the raft sink.
  • Crumple the foil with the pennies into a small ball. It should now sink when placed in the aquarium.
  • Discuss how the volume was reduced while the mass stayed the same.
  • Students should record what happened on their handout.
• For a second demonstration of the same principle, place two or three pennies in a balloon and drop it into the aquarium. The students should note that the density of the balloon with pennies is greater than that of water.
  • Now remove the balloon from the water. Ask the students how we could get the balloon to be less dense than water.
  • Inflate and tie the balloon.
  • Place the balloon in the water and discuss how the volume of the balloon increased while the mass stayed about the same. This decreased the density of the balloon allowing it to float.
  • Students should record what happened on their handout.
• Now read the third of the density principles with the students.
  • To demonstrate it, return to the warm-up objects (10-pound bowling ball/log and marble/ball bearing). Place both objects in the aquarium (the students will be surprised that a 10-pound bowling ball will float).
  • Explain that, even though a bowling ball/log is much heavier than a marble, it was less dense due to its large volume. Therefore, you cannot always predict density solely by an object’s mass.
  • Students should record what happened on their handout.
• For homework, have the students think of one real world example for each principle.

Lab Activity:

• Use the following directions to set up each of the lab stations:
• Station #1:

1. Boil two eggs.
2. Fill two beakers with the same amount of water.
3. Put an egg into one of the beakers. It should sink.
4. Label this beaker “Fresh Water.”
5. Put the other egg into the second beaker.
6. Dissolve enough salt into the water so that the second egg floats.
7. Label this beaker “Salt Water.”

• Station #2:

1. Fill a container with water.
2. Place one unpeeled orange into the water. It should sink.
3. Remove the peel from the second orange.
4. Place it into the container of water. It should float.

• Station #3:

1. Fill a 10 mL graduated cylinder with water.
2. Fill a 100 mL graduated cylinder with water.

• Station #4:

1. Fill a container with water.
2. Gently place a can of regular Coke into the water. It should sink. Do NOT shake the can or it will float.
3. Gently place a can of Diet Coke into the water. It should float.

• Station #5:

1. Fill 1/3 of a 500 mL beaker with vegetable oil.
2. Carefully place an ice cube onto the surface of the vegetable oil. The ice should float.
3. As the ice melts, the liquid water sinks to the bottom of the beaker.

• Station #6:

1. Gather a small beaker (50 mL), small cans of 7-Up, and a box of fresh raisins.
2. Avoid using 2-L bottles, plastic bottles, or larger cans of 7-Up since it rapidly loses its fizz.

• Station #7:

1. Fill a 2-L bottle all the way to the top with water.
2. Fill an eyedropper about ¼ full with water.
3. Place the eyedropper into the 2-L bottle. It should float. If it sinks, put less water into the eyedropper.
4. Cap the 2-L bottle.
5. If the eyedropper does not sink when the bottle is squeezed, add more water to the eyedropper.

• Station #8:

1. Obtain one 100 mL graduated cylinder
2. Gather corn syrup, water, vegetable, and oil.
3. One density column could be set up and observed by the students instead of each group making one. See the directions on the attached student lab handout for directions.

• Station #9:

1. Fill a 2-L bottle all the way to the top with water.
2. Gently place one ketchup packet into the water. Be careful not to compress it too much. The packet should float. When the bottle is squeezed, the packet should sink.

• Divide the class into nine groups.
• Pass out the student lab handout to each student.
• Assign each group a station number where they will begin.
• Have the students follow the directions for the station as written on the attached student handout. They will be making a labeled sketch of what they observe, indicating if the mass or volume is changing more as well as how the density is affected, and indicating which density principle is being demonstrated (see the attached key for expected student results).
• After about 10 minutes, have the students switch stations.
• Repeat until the students have completed all nine stations.

Homework:

• This is listed on the lab handout as “Analysis”. Alternatively this can be done in class rather than for a homework assignment. Teachers can decide if this is done before or after a class discussion of what the students observed and how they answered the questions at each station. See the attached answer key for the expected student results.
• For the assignment, each student will choose the station that they understood the best and write an analysis of what was observed in claim, evidence, and reasoning format. See the attached student lab handout for more detailed student instructions.
• Each student will also revisit their drawings for each station and add dots to the appropriate objects to show the differences in density.