Activity:  Poke,  But  Don’t  Soak!    Question  that  students  will  be  investigating:    Can  you  poke  a  pencil  through  a  plastic  bag  without  it  leaking?    Related  NGSS  Content  Standards:    MS-­‐PS1-­‐1:    Develop  models  to  describe  the  atomic  composition  of  simple  molecules  and  extended  structures.  MS-­‐PS1-­‐3:  Gather  and  make  sense  of  information  to  describe  that  synthetic  materials  come  from  natural  resources  and  impact  society    Related  NGSS  Science/Engineering  Process:  Developing  and  Using  Models:  Modeling  in  6-­‐8  builds  on  K-­‐5  and  progresses  to  developing,  using  and  revising  models  to  describe,  test,  and  predict  more  abstract  phenomena  and  design  systems.  Develop  a  model  to  predict  and/or  describe  phenomena.  (MS-­‐PS-­‐1),  (MS-­‐PS1-­‐4)    Materials:  

-­‐ 2  pencil  sharpers  to  be  used  periodically  -­‐ 1  small/snack-­‐sized  Ziploc  bag  per  student  -­‐ 10  standard-­‐sized  wooden  pencils  (2  per  pair)  -­‐ 1  large  pitcher  of  water  -­‐ tray  to  catch  dripping  water  -­‐ paper  towels  

Disposables:  -­‐ Ziploc  baggies  (small/snack-­‐sized)  

 Procedure:  

1. Show students a Ziploc bag filled about half way with water and ask them the following questions:

a. Do you think you can poke a pencil through this plastic bag without it leaking?

b. Why or Why not? 2. Provide students with their own Ziploc baggies in groups of 2, and have them fill

their bag about ½ to ¾ of the way full with water and zip it up tightly. 3. After ensuring their pencil is sharp, they will poke it through the middle of one

side of the bag. Note that no water spills out! 4. Take a short moment to discuss these results using the following discussion

questions. a. How does the bag fit around the pencil (tightly or loosely)? b. Do you think you can keep pushing the pencil out the other side of the bag

without water spilling? (Have students try this after they answer) c. Do you think you can poke more than one pencil through the bag without

any water spilling? (Have students try this after they answer) 5. Discuss the results. Explain how polymers make this experiment possible.

a. Is this what you expected to happen? Why or why not?

b. What did you expect to happen? c. Why is it helpful to push the pencil through quickly rather than slowly?

i. Potential answers should be about stretching out the bag while pushing the pencil through so the pencil won’t fit as tightly and water will leak out. Help guide students to this conclusion.

d. What do you think will happen when you pull the pencil out of the bag? 6. Follow up: Ask students if they think this same experiment is possible with other materials as well. Suggest that they try it at home with a balloon.

 Anticipated  Results:  Students should be able to poke their pencil through the entire water-filled plastic bag in a quick, stabbing motion without the water leaking or spilling out of the baggie through the hole they formed.

Developmentally Appropriate Explanation of Science Content: A  polymer  is  made  up  of  many  smaller  molecules,  called  monomers,  which  are  bonded  together.  For  example,  if  I  have  one  paper  clip,  do  you  think  it  will  represent  a  monomer  or  a  polymer?  (Students  should  guess  ‘monomer’)  So  then  what  do  you  think  a  chain  of  paper  clips  all  linked  together  would  represent?  (Students  should  guess  ‘polymer’)    The  two  main  types  are  synthetic  and  natural  polymers.  Natural  polymers  include  things  like  wood,  wool,  and  silk.  Do  you  think  that  Ziploc  bags  are  made  up  of  synthetic  or  natural  polymers?  (Students  should  guess  ‘synthetic,’  but  if  they’re  struggling,  give  them  some  examples  of  synthetic  polymers;  i.e.  Tupperware)      In  order  to  make  Ziploc  baggies,  monomers  must  undergo  chemical  reactions  to  become  polymers.  Once  the  polymers  are  produced,  they  create  flexible  fibers  that  are  all  intertwined  together  through  physical  interactions  (demonstrate  this  with  your  fingers),  which  are  the  fibers  inside  of  the  Ziploc  bag.  (You  may  also  mention  the  difference  between  chemical  and  physical  reactions/interactions  here.)    By  pushing  the  pencil  through  the  plastic  bag,  you  are  not  breaking  the  bonds  in  the  fibers,  but  rather  are  just  pushing  through  them,  like  how  you  would  squeeze  your  way  in  between  two  people  that  are  standing  close  together.  You’re  not  breaking  the  

Figure  1:  Buggy  and  Buddy  

people  or  causing  them  any  harm,  you’re  simply  forcing  them  to  stand  just  far  enough  apart  for  you  to  fit  through.  In  this  experiment,  you  are  the  pencil,  and  the  two  people  that  you’re  trying  to  get  through  are  the  fibers  in  the  Ziploc  bag.    

     College  Level  Explanation:  A polymer consists of many monomers chemically bonded together. There are two main types of polymers: synthetic and natural. Natural polymers can be found everywhere, so that there are many familiar examples readily available to point out to young students. The proteins that we use in our bodies are polymers made up of amino acid monomers, and glucose monomers make up the cellulose polymers in plants. However, based on the huge variety of plants all over the world, we can infer that different types of glucose, such as amylose and cellulose, interact differently to create distinct properties of the polymers. Figure 3 provides an example of glucose monomers composing the polymer, amylose, through a condensation reaction.

Natural polymers make up things like wool and wood that people have been using for hundreds of years and continue to use today. Synthetic polymers on the other hand, were not put into practical use until the 1900’s, but these are what make up our Ziploc bags and all other types of plastics. The molecules most commonly used to form plastics include ethylene, propylene, styrene, phenol, formaldehyde, ethylene glycol, vinyl chloride, and acetonitrile. These

Figure  3:  Ohio  State  University  

Figure  2:  Chemistry  Need  

monomers form polymers through the process of either condensation reactions or addition reactions. Condensation reactions occur when two molecules form a covalent bond. They do this by getting rid of smaller molecules attached to them which leave and form either water or alcohol, leaving an open bonding position for the remainder of the two molecules to create a bond with each other. Figure 3 shows the water molecules forming in between each monomer of amylose above. Addition reactions involve rearranging double bonds to form single bonds, lengthening and forming a polymer chain. Ziploc bags are a thermoplastic invented by John Rex Whinfield, made up of polyethylene and terephthalate, created through a condensation reaction between glycol and terephthalic acid. Once the reaction is completed and cooled, the polymers are formed, leaving relatively flexible fibers that can be used to make Mylar, film, Dacron, and of course, Ziploc Bags.

     Figure  4:  Polymers  and  Plastics  

Additional  Resources  for  this  Topic:   Chelsey. "Science For Kids: Baggie and Pencil Magic." Weblog post. Buggy and Buddy. Buggy and Buddy, 20 Apr. 2013. Web. 28 Oct. 2013. <http://buggyandbuddy.com/author/cmarashian/>.  References:   Abedon, Stephen T. "Carbohydrates." Carbohydrates. Ohio State University, n.d. Web. 28 Oct. 2013. <http://www.mansfield.ohio-state.edu/~sabedon/biol1025.htm>. Fruedenrich, Craig, Ph.D. "How Plastics Work." HowStuffWorks. A Discovery Company, n.d. Web. 24 Oct. 2013. <http://science.howstuffworks.com/plastic4.htm>. Lower, Stephen. "Polymers and Plastics: A Chemical Introduction." Polymers and Plastics: A Chemical Introduction. Creative Commons Attribution, 2009. Web. 24 Oct. 2013. <http://www.chem1.com/acad/webtext/states/polymers.html>. "Polymers | Chemistry.need.org." Polymers | Chemistry.need.org. Need, 2013. Web. 24 Oct. 2013. <http://chemistry.need.org/curriculum/polymers>.