Introduction:

A floating balloon, a sinking submarine, a swimming fish and a floating iceberg... All of these things appear to defy the laws of the universe as we know it, but how? In this series of lab activities students will explore the concepts of density and buoyancy. In the first section of the lab students will explore liquids that appear to float and sink, looking at relative buoyancy. Later in the lab, students will calculate actual densities of unknown materials. By mixing and calculating their own formula's densities the students will have a chance to explore the concept of salinity.

Purpose:

The purpose of this lab is for students to reacquaint themselves with lab techniques such as measurement and data keeping. Students will also further their graphing skills and understanding of the fundamental concept of density.

Pre- lab Questions :

these questions should be answered by students BEFORE beginning the lab, usually done as homework the night before the lab.

1. What is the density of water? How can you find this number using experimental techniques?
2. What is the density of ocean / salt water? Will it be more or less than fresh water?
3. What is your density compared to ocean water ? how do you know?
4. How does a concrete or metal boat float?

Part 1. Layering the Liquids

Procedure:

A. Layering the Liquids - Beginner's level

In this part of the lab students will receive a set of 4 different liquids that have been colored. They will be challenged to layer the liquids without mixing them in a clear plastic straw or small test tube. Students will record their trials on the accompanying data sheets or on lined paper.

Once students have found the correct layering order they should repeat their procedure a second time to confirm the validity of their results. After they are sure they have found the correct relative densities of their liquids, the students should return the materials to the front table and check out a second set of liquids for Part B of this activity.

B. Layering the Liquids - Intermediate's level

In the first session the liquids were of 4 different types, making them insoluble in one another and easy to layer. In this part of the lab the liquids are similar so the techniques for layering will need to be much more carefully executed. (HINT: work slowly and tip your tubes slightly)

Once again, students will receive a set of 4 different liquids that have been colored. They will be challenged to layer the liquids without mixing them in a clear plastic straw or small test tube. Students will record their trials on the accompanying data sheets.

Once students have found the correct layering order they should repeat their procedure once to confirm the validity of their results. After they are sure they have found the correct relative densities of their liquids, the students should return the materials to the front table.

Data:

Layering the Liquids

data should be recorded on lined paper.

Discussion Questions :

Layering the Liquids

1. (part A) Can you guess the names of the materials in your sample cups? What types of clues did you use? Be specific.
2. Why was the second challenge more difficult to solve? What were some of the difficulties that your team had?
3. Which is heavier a pound of puffed rice cereal or a pound of marbles? Samples are on the front table. How can you restate this question correctly ?
4. How can you make a cup of marbles heavier? Suggest two methods after looking at the cups on the front table. Draw a sketch of your solution.
5. Describe density in your own words. Include examples of materials that are more and less dense. Add sketches to help with your explanations.

Procedure: Part 2. Relating density and salinity

In this part of the lab students will mix their own secret formulas of salt and water to exchange with another lab team.

Students will calculate the densities of their secret formulas using the equation density = mass / volume. This calculation should allow students to predict the layering order of the the secret formula they have mixed / received.

Notes:

• You will want to develop a standardized procedure.
• measure carefully.
• use only one or two drops of food coloring
• record your predictions before trading with another group so they will have a key to follow

Procedure :

1. Use the measuring cups and graduated cylinders to measure the volume of liquids and salts that you are mixing in your formulas.
2. record all amounts on a separate sheet of paper / data table of your own design
3. weigh a standard volume of your formulas once mixed.
4. calculate the density of each cups - record
5. exchange with another team.
6. predict the layering order of the cups you have received
7. try out your predictions
8. weigh and calculate the density of the liquids in the cups
9. compare with the team that created the secret formulas

Materials:

Straws or test tubes
Eye droppers
4 cups of liquids
potato slice to hold the straw upright or test tube rack
sink or other means of rinsing the materials
balances
graduated cylinders
measuring spoons

Hypothesis:

Students should begin the lab by hypothesizing the densities of the liquids using other senses and observation skills.

Data: Relating density and salinity

Record your results here. Then, using apple works , create a side by side bar graph that compares this information. Post this graph in your discussion and state why you think there were differences in the results.

	red	blue	yellow	green
your formula's densities	gms /ml	gms /ml	gms /ml	gms /ml
actual results found by your team	gms /ml	gms /ml	gms /ml	gms /ml
predicted results from team that made formulas	gms /ml	gms /ml	gms /ml	gms /ml

Discussion questions: Relating density and salinity

1. Discuss your results from the lab.
2. Name three ways that density affects your everyday life.
3. Describe how water of different densities might be found in the ocean and what the effects of this might be on the ocean and ocean life.

Extension Activities: choose one to complete

1. Calculate the densities of the unknown liquids on the front table. Use the balance and cylinder at your lab station table.
2. Calculate the densities of the unknown solids in your lab basket. Use the balance and cylinder at your lab station table.
3. Compare the densities of hot water and ice water. Which is more dense how do you know?

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