The Disintegration of Polystyrene Lab
This inquiry based lab shows students how acetone breaks down the cross-links holding together polystyrene. Students will add acetone to either packing peanuts or pieces of foam cups. The result is a tiny bit of material (polystyrene) in the beaker. Discussions following the lab can cover the conservation of matter (the mass should stay the same), percent yield and cross-link bonding.
This lab is designed for students to make observations about a chemical reaction. They should be able to use those observations to form an idea of what happened to the material. Students will also understand the need to do experiments more than once if they do not understand what happened in the experiment. Key concepts for this lab are: All mass is conserved and never destroyed or created. There are intermolecular forces (cross-links) that form between some molecules. These are not true bonds, but act similarly.
Context for Use
This lab should be done in a high school Chemistry class. It could work with any size class, but students should work with lab partners or in groups. There should not be more than 4 people in a group. This lab can be done in one class period with time for introduction and discussion when finished.
Subject: Chemistry:General Chemistry:Bonding & Molecules
Resource Type: Activities:Lab Activity
Grade Level: High School (9-12)
Description and Teaching Materials
As this is an inquiry lab, I do not give a lot of introduction to this lab. I make sure to discuss safety in the lab and the fact that we are using very flammable and poisonous liquids. I also make sure to remind students that they need to READ the lab in order to know what to do. Tell the students to work together and answer the questions from the lab. Also, make sure to tell students NOT to answer the Final Question. This should be done on their own at their seats. Students should take ~15 mL acetone and place it in a beaker. They then should add packing peanuts to the beaker. If packing peanuts are not available, one Styrofoam cup (ripped into pieces) should work for each group. The acetone dissolves the cross-links between the polystyrene molecules and the material "dissolves." What they are left with, after about 10 peanuts is a very small bit of a gooey material. This is pulled out of the cup and placed on the lab table to dry. I have placed the material on a paper towel, but it tends to stick to the towel and we want to keep all the material together. I have a balance at each lab table but there is nothing that says students need to use it. There is also an unlimited supply of peanuts or cups. If the students know what to do, they will measure the mass of the foam before placing it in the acetone and measure the mass after the "dissolving". The masses should be the same. If the material is measured right after it is out of the acetone, it will probably have a higher mass, because there is still acetone in the polystyrene. If students squeeze the material or press it against the table, this should make some of the acetone come out of the material. Over time, it will evaporate and to masses should even out. After the clean-up, have students take 4-5 minutes to answer the Final Question on their own. Make sure to discuss the lab and talk about what happened and what should have happened. (As the acetone interacts with the cross-links in the polystyrene, it breaks that bond down, allowing the polystyrene material to clump together. There is no actual chemical reaction in this case. Most results will show an increase in mass when weighing the materials after mixing the acetone and the polystyrene. This is caused by some acetone still being in the polystyrene. Given time to dry out, the masses should be the same.)
The source of this lab is from the United States Air Force Academy's Chemistry Dept. I modified a lab from the Mini-Labs website, www.mini-labs.org
, which was originally designed by Matt Morgan and Dan Branan. Student Handout for Disintegration of Polystyrene Lab (Microsoft Word 30kB Aug3 09) Instructor Idea Sheet for Disintegration of Polystyrene Lab (Microsoft Word 25kB Aug3 09)
Teaching Notes and Tips
If the class has not done many inquiry based labs, this may be difficult for them. There are not detailed directions and that may cause confusion. Make sure to tell students to take their time and allow for some groups to repeat their experiments. The main part where students will gain an understanding of the concept is in the post-discussion. Make sure to leave at least 10 minutes of class time to talk about what you found and what happened. Some safety issues to keep in mind: While this lab may not have hugely dangerous chemicals like some other chemistry labs, it does have a chemical which has fumes that are flammable. Make sure to keep the lab free of heat and flames. Also make sure that all lab rules are followed. The main problem I can see happening when I do this with students (I have not done this before) is the fact that some of them may not be used to the fact that this lab is inquiry based. There are many possible answers to the questions. If they are looking for one specific answer, they may not find it.
The lab can be graded in a variety of ways. One of the best ways to determine if the students are achieving learning goals is by having the wrap up conversation. This works if many, if not all the groups participate in the conversation. If only a few students talk about their results, it will not be effective. The students should hand in their lab reports at the end of the discussion.
9C.126.96.36.199 The Law of Conservation of Mass
References and Resources