Chemistry Experiment

General Chemistry I CHEM XXI Project Report Worksheet (4 pages max.)

Experiment title: How Do We Identify an Unknown Substance?
Guiding questions: State the question or questions that that you were trying to answer in this lab.
How do you determine the two unknown types of plastic? What are the two unknown plastics?

Safety considerations: Briefly describe the safety measures you had to take to perform the experiments.

Before entering the lab, I put on my lab coat and practiced using my goggles in case of any spills or splashes. I also used proper methods for dispensing my mix liquids. /2p /2p

Procedures: Briefly describe the experimental procedures you followed to answer the guiding questions. Outline the specific problems you were trying to solve and how you did it.

1. /2 p /2 p
Find the volume of the four known substances using a vernier caliper to make measurements. (For rectangular prism measure length, width, and height; For cylinder measure diameter, divide by half to find radius, and height) 2. Find the mass of each known substance using the electric balance. Make sure the balance is clean and calibrated to zeros. 3. Graph the volume and mass on an excel sheet to help determine the density of each plastic. The y-axis is the dependent and the x-axis is the independent, which in this case the y-axis is the mass and the x-axis is the volume. In the graph, add a linear trendline. Determine the slope to find the average density. 4. Once you have found the density for each known plastic, you will find the density of the two unknown plastics. First, use 50mL of water and place each plastic in the water and determine if they float or sink. Make sure to take note if they float or sink. 5. If the plastic floated, you will place it in a mixture of ethanol and water using the following steps(this is creating a mixture with a density between 0.920 g/mL and 0.953 g/mL): a. Place 40mL of ethanol into a beaker, and then place 60mL of water into the same beaker. Mix these two liquids together. b. Place the plastic, which floated in the water, into this mixture and note if it floats or sinks. c. If it floats the plastic is less dense than the ethanol-water mixture. If it sinks the plastic is more dense than the mixture. 6. If the plastic sank, you will place it in a mixture of salt and water using the following steps(this is creating a mixture with a density between 1.04 g/mL and 1.42 g/mL): d. Place 20mL of NaCl into a beaker and then add 20mL of water and stir. e. Place the plastic, which sank in water, into this mixture and note if it floats or sinks. f. If it floats the plastic is less dense than the salt water. If it sinks the plastic is more dense than the mixture. 7. With the information you got in step 5 and step 6, compare the densities to the information you gathered in step 3 to analyze the unknown plastics with the known plastics.
Lab section: F

Data, observations, calculations, graphs, and tables: Present the most relevant data for the different experiments that you completed. Include at least one data table and the corresponding plot that allowed you to determine the density of one known plastic sample. Include all of the data that allowed you to identify the two unknowns.
Known plastic PP Volume (mL) | Mass (grams) | 12.1 | 11.250 | 7.27 | 6.712 | 9.70 | 8.896 | 4.85 | 4.477 |
Volume=length x width x height
1 cubic centimeter = 1 mL

/6p /6p

Density of Known Plastics-
PETE: 1.42 g/mL
HDPE: 0.953 g/mL
PP: 0.920 g/mL
PS: 1.04 g/mL
Density of water: 1 g/mL
Unknown plastics-
Sample 1 (disposable biomedical syringe): Floated in water and in ethanol-water mixture. Therefore, the sample was less dense than water and the ethanol-water mixture.
Sample 2 (drinking water bottle): Sank in water and in saltwater mixture. Therefore, the sample was more dense than water and the salt water mixture.

Claims: Answer your beginning questions as concisely as possible and in complete sentences.
State what you think is the identity of the two unknown plastics analyzed in this experiment.

To determine unknown plastics, you can find the density of known plastics and compare their density with the unknown plastics densities. Sample 1 is PP, or polypropylene. Sample 2 is PETE, or polyethylene terephthalate. /4p /4p

Lab section: F

Evidence and analysis: Clearly describe how the data that you collected supports all of your claims. You are expected to build convincing arguments to justify your claims.

Knowing that sample one floats in both the water and the ethanol-water mixture means that the plastic is less dense than the liquids. Using the density of the known plastics and water, you can compare the plastic of sample. Because the sample floated in water, we know the sample’s density is less than 1 g/mL. Because it floats in the ethanol-water mixture, we know that the sample’s density is a smaller value. When we compare this to the smallest density of the known plastics we get that the sample is polypropylene.
For sample two, because it sinks in water, we know the density is greater than 1 g/mL. When it sinks in the salt water mixture, we know that it has the greatest density. When we compare this to the greatest density of the known plastics we get that the sample is PETE because that has a greater density than water and is a higher density than the other plastic that has a greater density than water.

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Final reflections: Using a narrative style, summarize what you learned by completing this experiment. Explain how your knowledge, ideas, and abilities have changed in the following areas: a) how to design experiments to find the identity of unknown substances; b) how to make accurate and reliable measurements; c) how to record data and make calculations using the proper number of significant figures; d) how to use a spreadsheet program to analyze data.

Some sources of error could be when we placed the plastic into the water, and there was surface tension because we did not fully submerse the plastic in the water. This would cause the plastic to rise and float, when it should have sunk. Another error could have been in the calculations of volumes. Another error could have been when calculating the density through a graph and getting the answer as 1/density.
Just like our experiment, where we tested plastics and placed them into their categories, people in recycling companies do this as well. Another place we see separating substances, could be chemical plants. There are many places where we see people try to identify a substance. Like in our lecture, there are many different ways to identify a substance. In our experiment we identified unknown plastics using density. There are also other factors we could have used such as boiling point, solubility, and viscosity.

/8p /8p