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LAB in Observations, Density, Inferences, Identifying an Unknown, Measurements, Scientific Method, Error Analysis. Last updated January 17, 2020.

### Summary

In this lab, students will practice density calculations and put their calculations to test by creating their own density column.

Middle School, High School

### NGSS Alignment

This activity will help prepare your students to meet the following scientific and engineering practices:

• Scientific and Engineering Practices:
• Using Mathematics and Computational Thinking
• Analyzing and Interpreting Data
• Obtaining, Evaluating, and Communicating Information

### Objectives

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

• Calculate the density of unknown solutions given the mass and volume.
• Correctly create a density column based on their calculations.

### Chemistry Topics

This lesson supports students’ understanding of:

• Density
• Observations
• Measurement

### Time

Teacher Preparation: 30 minutes

Lesson: 60 minutes (can split Part 1 and Part 2 over two days)

### Materials

For each lab group:

• Goggles
• 250-mL beaker
• Balance
• 4 empty plastic cups
• 4 unknown solutions in plastic cups
• Density list (separate document in sidebar)
• Student handout (per student or per group)
• Test tube brushes (for cleaning all graduated cylinders)

For class density column:

• Eight unknowns

### Safety

• Safety goggles should always be worn when carrying out an experiment in a 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.

### Teacher Notes

• In Part 1, students will carry out the experiment on a small scale, recording data on the student activity sheet provided. In Part 2 (could be on another day), the class will pool their data to determine how to create a density column with all eight unknown liquids.
• Eight unknown solutions are prepared as follows:
• A – (1.00 g/mL) Colored green water (add two drops of green food coloring)
B – (1.37 g/mL) 100% Maple syrup
C – (1.06 g/mL) Blue dish soap
D – (0.83 g/mL) Baby oil (add one drop of yellow, one drop of red food coloring)
E – (0.92 g/mL) Vegetable oil
F – (1.42 g/mL) Honey
G – (1.33 g/mL) Light corn syrup (add one drop of red food coloring)
H – (0.79 g/mL) Rubbing alcohol (add two drops of blue food coloring)
• The "Density List" document in the sidebar includes the substances listed above, as well as a few extra substances that could be removed to make it easier for students to identify the unknowns.
• For nine lab groups, one way of distributing four unknowns to each group is as follows:
 Group 1: A C E G Group 4: B D G H Group 7: A E G H Group 2: A B D F Group 5: C E F G Group 8: C D F G Group 3: B C F H Group 6: A B F E Group 9: A B E H
• In the “Discussion Questions” document, found in the sidebar, there are some questions you could use as a pre-lab assignment or just for discussion to get students thinking on a more abstract, conceptual level about density.
• In Part 2 of the lab, students will discuss the results of their laboratory experiments and work together to order ALL eight of the unknowns into a giant density column. They will have to work together and rely on their peers’ data to come to a class consensus. They will check the final answer at the end of the period by making a density column with all eight liquids.
• Since each group is working with 4 unknowns and there are 8 total, multiple groups should be measuring the same unknown. If students who worked on the same unknown got very different answers for density, this would be a good opportunity to talk about the importance of error analysis, experimental design, and attention to detail in a laboratory setting.
• You could scale down the experiment and use 10-, 25-, or 50-mL graduated cylinders and smaller sample sizes using a large enough second graduated cylinder (50- or 100-mL) for the density tube. You could also use a smaller graduated cylinder (100- or 250-mL) and less of each solution for the class density column in Part 2 with all 8 unknowns.
• Adapted from a laboratory activity written by Lindsey Hubert, provided to me by Latasha Ford.

### Background

Someone with poor lab habits left a bunch of unlabeled solutions in the lab! I need your help to figure out what they are based on their densities. For Part 1, I have included a list of common solutions and their densities so you can determine the identity of the four solutions assigned to you and your partner. You will then make a density column from them, with the densest material at the bottom and the least dense on top.

For Part 2, you will create a density column with all eight of the unknowns, using your classmates’ solutions as well as your own, so you will have to work together as a class to determine the proper order for the unknowns.

### Materials

For Part 1:

• Goggles
• Four unknown solutions (labeled)
• Balance
• Test tube brushes (for cleaning graduated cylinders)

For Part 2:

• Eight unknown solutions (labeled)

### Procedure

1. Using a balance, find the mass of an empty 100-mL graduated cylinder. Record this mass.
2. Using the graduated cylinder, measure a certain amount (between 30 and 60 mL) of one unknown solution. This will be the volume measured for every unknown solution. Record this volume in the data section.
3. Re-mass the graduated cylinder. Record this mass in the data section.
4. Calculate the mass of the unknown solution. Show your work in the calculations space provided below the data table, and record this mass in the table.
5. Pour the solution back into its plastic cup and wash and dry your graduated cylinder.
6. Calculate the density of this unknown solution. Show your work in the calculations space provided below the data table, and record this in the table.
7. Compare your density to the list of densities provided by your teacher to determine the most likely identity of your substance, and record this in the table.
8. Repeat steps 2–7 for the rest of your unknown solutions.

### Results

 Unknown solution Volume (mL) Mass of empty graduated cylinder (g) Mass of graduated cylinder with solution (g) Mass of solution (g) Density of solution (g/mL) Identity of solution

### Calculations

Show calculations from steps 4 and 6 in the space provided below.

### Analysis

A. Rank your unknowns in order from most dense to least dense.

1. ___________ 2. ___________ 3. ___________ 4. ___________

B. Take your most dense solution and slowly pour a small amount of it into the 250-mL graduated cylinder.

C. Continue adding each solution from most dense to least.

### Conclusion

Answer the following questions in a paragraph or two in the space provided below.

• Were you able to accurately identify your 4 unknowns? Were any of them particularly challenging to identify? Why or why not?
• Did you put them in the correct order/did they stay in the density column in the order in which you added them? If not, how might you evaluate what went wrong?
• Did you see all 4 unknown solutions in the density column you created with your group’s assigned solutions? If not, which ones mixed? Why do you think they mixed?

### Part 2

Using your own and your classmates’ data from Part 1, determine the correct order for a density column that includes all eight of the unknowns tested in Part 1. Create that density column by pouring a small amount of each substance, again starting with the most dense and ending with the least dense, into the large 1000-mL graduated cylinder.

Unknowns, in order of most to least dense:

1. __________
2. __________
3. __________
4. __________
5. __________
6. __________
7. __________
8. __________

### Conclusion

Answer the following questions in a paragraph or two in the space provided below.

• Were your calculations confirmed by other classmates who had the same unknown? Did anyone get different densities from others with the same unknown? How did the class come to a decision on which number was correct? Did your density column confirm this conclusion? How might you evaluate what went wrong for the groups with different densities?
• Did you see all 8 unknown solutions in the density column you created with the full set of solutions? If not, which ones mixed? Why do you think they mixed?
• Why is it important for scientists to keep detailed observations as they complete their experiments? What challenges could detailed record-keeping help scientists overcome?
• Why is it important for scientists to publish their procedures, data, analysis/calculations, and conclusions when they conduct an experiment, not just their conclusions?