SNC 3MO LESSONS

Students continue the explorations of chemicals that began in Grade 10. Further investigations into the properties and reactions of everyday chemicals serve as the basis for discussions leading toward the detection, classification of these materials as well as an explanation of their properties and reactions.  Technologies associated with the production, detection and use of chemicals will be highlighted along with environmental concerns related to production and disposal.  The unit culminates in an activity where students compile information on the properties, preparation, uses and hazards associated with a chosen material.

- Define & give an example of each:  (using Science Spectrum textbook)
o element
o compound
o molecule
o atom
o chemical formula
o physical property
o chemical property
o physical change
o chemical change

Polymer Gels
The following information was part of our display for the SET96 and SET98 Scientific Power to the People Exhibition, The Galleries, Bristol, 20th and 21st March 1998.

A POLYMER is a very long molecule rather like a very long piece of string.
The borax joins two molecules together to make a large network - rather like a flexible scaffolding. This is cross-linking.  This turns the fluid dissolved polymer into a GEL.
In oilwells water is pushed down to push oil up. If this breaks through to the production well bore, the well will just produce water. A solution is to block the rock pores with polymer gel to make the well produce oil again. These gels can be made from naturally occurring sugar-based polymers such as Guar and they can be cross-linked with Cobalt or Chromium.
Other everyday examples of polymer gels are to be found in non-drip emulsion paints, foods, car tyres and agrochemicals.
The polymer that we are using is POLYVINYL ALCOHOL and we are joining the strands together with BORAX. This gel, when coloured, is sold in toyshops as SLIME.

HOW TO MAKE SLIME

Prepare:
3% PVA (Poly Vinyl Alcohol) in Water
(use [-CH2CH(OH)-]n fully hydrolysed PVA,molecular weight approx. 125,000 , dissolve at 90 degrees C)
2% Borax in Water
(Na2B4O7 ·10H2O molecular weight = 381)
Mixing Ratio:
Take 100 ml of the 3% PVA Solution
Squirt in 3 ml of the 2% Borax Solution
Stir vigorously for 2 minutes  AND PLAY
(Adjust texture by adding water or 2% Borax solution)
Experiment 2:  Slime Away
Cross-Linking Poly (vinyl alcohol) with Sodium Borate
Objective: The objective of this experiment is to explore the change in physical properties of a polymer as a result of cross-linking. The result of adding more cross-linking agents to a polymer is considered and another model of cross-linking is viewed.
Applications:
There are a number of uses of the PVA polymer we are studying:
1. They may be used in sheets to make bags to act as containers for pre-measured soap you simply throw into a washing machine.
2. The PVA sheets may be made into larger bags to be used by hospitals as containers for the cotton cloth used in the operating rooms or to hold the bed linen or clothing of infected patients.
Time: This experiment will require approximately 15-20 minutes to run and clean up.
Materials and Supplies:
-100 ml/group of poly (vinyl alcohol) 4%
- 10 ml of sodium borate 4%
- Styrofoam cups and wooden stir sticks (tongue depressors)
- Zip lock bags or latex gloves (surgical)

General Safety Guidelines:
- Laboratory aprons and goggles should be worn in this experiment as in all procedures.
- Both the borax and the PVA will burn the eyes. Hands should be washed at the end of the experiment.

Procedure:
The polyvinyl alcohol and sodium borate are mixed together in approximately a 10 to 1 ratio.
1. 100 ml of the 4% poly (vinyl alcohol) is added to a Styrofoam cup.
2. Food coloring can be added to the PVA in the cups to make different colors. Simple food coloring is recommended. This coloring should be added before any of the borax solution has been added, or it can be added directly to the borax solution.
3. Add 10 ml of the 4% cross-linker (sodium borate) to each cup. Begin stirring the mixture immediately with your wooden tongue depressor.
4. Make observations as to what is occurring as the reaction proceeds.
5. Within a couple of minutes the slime will be formed. Lift some of it out with the tongue depressor and make your observations. Record your observations on your data sheet.
6. Take some in your hand and stretch the slime slowly. Record your observations on your data sheet.
7. Repeat the stretching exercise only this time do it rapidly. Record your observations on your data sheet. Compare the results of the two tests. The slime is non toxic and is safe to handle, so you can put it in a Zip-lock bag (or latex glove) and seal it to take home.
8. Follow good laboratory procedure and wash your hands with soap and water. It is recommended that this procedure be followed whenever handling this material. Keep it in the glove or bag until it is discarded. The sodium borate or PVA could burn your eyes.
9. Place a small amount of the PVA on a paper towel and set it off to the side to dry until tomorrow. Upon returning to class the next day, record in the data section your observation of the slime.

Video Clip

Data and Analysis:
Observation of the PVA before the sodium borate is added:
Observation of the PVA after the sodium borate is added:
Observation of stretching the cross-linked PVA slowly:
Observation of stretching the cross-linked PVA rapidly:
Observation of the cross-linked PVA left out in the air overnight:
Questions:
1. What are the physical properties that change as a result of the addition of sodium borate to the poly (vinyl alcohol).
2. What would be the effect of adding more sodium borate to your cup (your thoughts only)?
3. After making the observations on the dried PVA, how does the water affect the elasticity of the polymer? What is elasticity?
4. Find and circle the repeat unit in the polymer molecule below?

5. What is the formula of the poly (vinyl alcohol) monomer circled above? (Your teacher may want to show you how to alter this slightly after you have drawn the structure.)
6. In the picture below, circle the borax cross-linking agent.
 

Teacher Notes:
Objective: The objective of this experiment is to explore the change in physical properties as a result of cross-linking polymers. The results of the addition of more cross-linking agents are considered and another model of cross-linking is viewed. Students also have an opportunity for monomer identification.
Experimental:
1. The Polyvinyl Alcohol as a solid is mixed in water to make a 4% solution. That is 40.0 grams of PVA per 960 grams (milliliters) of water. The best results are obtained by heating the water to about 80oC on a hot plate with magnetic stirrer. Sprinkle the PVA powder in very gently and slowly on the top of the solution while stirring so as not to cause the mixture to clump together. Temperatures above 90oC may result in decomposition of the PVA and perhaps the creation of an odor to the solution. Continue to sprinkle the PVA into the hot solution while it is stirring. After all of the PVA has been added to the water, place a top on the vessel. If the water evaporates off, a skin of PVA will form. This PVA sheet might also be a nice item to lift off and show the students. Continue stirring until the mixture is uniform (note also that it will be somewhat viscous). Allow the solution to cool, and the resulting solution will be ready for the students to use.
2. If students are adding a dye to their PVA, make sure they do this before the addition of borax.
3. The borax (sodium borate) can be obtained from your grocery store as "Twenty Mule Team Borax," a laundry bleaching agent. The borax is mixed at a 4% concentration in water. To do this measure out 4 grams of borax and dissolve in 96 grams (milliliters) of water (note: Water has a density of 1 g/mL).
4. The material becomes more viscous as we mix the PVA and the borax. It will reach a maximum level of viscosity and will not thicken further without more cross-linking agent. The addition of a higher ratio of Borax will result in a very viscous polymer (like Jell-O).
Theoretical:
The polymer used is "poly (vinyl alcohol)". The monomer has a formula of:
- Borax is sodium borate, Na3BO3. The borax actually dissolves to form boric acid, H3BO3. This boric acid-borate solution is a buffer with a pH of about 9 (basic). Boric acid will accept a hydroxide OH- from water as indicated on the next page.

The hydrolyzed molecule will then act in a condensation reaction with PVA as indicated in the last question on the student laboratory.

- In the above illustration, two PVA molecules are shown being cross-linked by a hydrated borax molecule. Four molecules of water are also produced.
- The resulting material is about 95% water. It is the water that gives the polymer flexibility. Note that as the polymer dries it returns to its solid phase now as a sheet that is rigid and almost transparent.
- The PVA does not dissolve easily in water. Prepare the PVA solution at least one day in advance.
- Guar Gum dissolves in water much more easily than PVA, but seems to "jell" at a much more unpredictable rate than the PVA mixture does. For t
this reason, PVA is preferred.
Additional reading for more in depth information can be found in:
Journal of Chemical Education, Jan. 1986, #63, pp. 57-60.
Sample Data and Analysis:
Observation of the PVA before the sodium borate is added:
The solution is fluid.
Observation of the PVA after the sodium borate is added:
The mixture becomes more viscous (thicker).
Observation of stretching the cross-linked PVA slowly:
The slime flows and stretches.
Observation of stretching the cross-linked PVA rapidly:
The slime breaks.
Observation of the cross-linked PVA left out in the air overnight:
It became a dry film.
Answers to Questions:
1. The mixture becomes more viscous (thicker).
2. The mixture would jell.
3. The ability of the cross-linked polymer to stretch decreases. The polymer becomes more brittle and will break.
4.
5. C2H3OH
6. The hydrated borax, minus the four hydrogens are shown on the previous page bonding two chains of the PVA polymer together.

Experiment 3: A Silly Polymer
Cross-Linking a Polymer to Create Everyone's Favorite Childhood Toy, Silly Putty
Objective: The objective of this experiment is to cross-link a polymer and observe the changes in the physical properties as a result of this cross-linking. The changes in physical properties of a cross-linked polymer are also studied as the temperature is varied.
Review of Scientific Principles:
If a substance springs back to its original shape after being twisted, pulled, or compressed, it is most likely a type of polymer called an elastomer. The elastomer has elastic properties (i.e., it will recover its original size and shape after being deformed). An example of an elastomer is a rubber band or a car tire.
The liquid latex (Elmer's glue) which you use contains small globules of hydrocarbons suspended in water. The silly putty is formed by joining the globules using sodium borate (a cross-linker). The silly putty is held together by very weak intermolecular bonds that provide flexibility around the bond and rotation about the chain of the cross-linked polymer. If the cross-linked bonds in a polymer are permanent, it is a thermosetting plastic, even if above the glass-transition temperature (Tg). If the bonds are non-permanent, it can be considered either thermoplastic or an elastomer.
Time: A 20-25 minute period is required to perform the mixing/making of the silly putty.
Materials and Supplies:
- 55 % Elmer's glue solution in water
- 4 % borax solution (sodium borate)
- Styrofoam cups
- zip lock bags
- food colors

General Safety Guidelines:
- Since borax solid (a bleaching agent) and solution will burn the eyes, goggles and aprons should be worn.
- Hands should always be washed after kneading the silly putty and finishing the experiment.

Procedure:
1. Wear goggles and lab aprons.
2. Pour 20 ml of the Elmer's glue solution into a Styrofoam cup.
3. Add 10 ml of the cross-linker (borax solution) to each cup.
4. Immediately begin stirring the solutions together using the wooden stick.
5. After a couple of minutes of mixing, the silly putty should be taken out of the cup and kneaded in the hands. Don't worry about the material sticking to your gloves as these pieces will soon mix with the larger quantity with which you are working. Continue to knead until the desired consistency is reached.
6. Using a ruler to measure, drop the ball from a height of 30 centimeters. To what height does it rebound?
7. Stretch the silly putty slowly from each side.
8. Compress the silly putty back into a ball.
9. Pull the silly putty quickly from each side and compare the results.
10. Place the silly putty on some regular news print and press down firmly.
11. Remove the silly putty from the news print and make observations.
12. Repeat the same procedure on a comic section of the newspaper. The silly putty is non-toxic and safe to handle so you can put it in a zip-lock bag and take it home.
13. Follow good laboratory procedure and wash your hands with soap and water when you have finished the experiment.

Video Clip

Data and Analysis:
Height of the rebound _________ cm.
Observations of pulling the silly putty slowly:
Observations of pulling the silly putty quickly:
Observations of the silly putty on newsprint:
Observations of the silly putty on the comic's section of the newspaper:
Questions:
1. How do the physical properties of the glue, water mixture change as a result of adding the sodium borate?
2. What would be the effect (your thoughts) of adding more sodium borate solution?
3. What is the ratio of the height of the drop to that of the rebound distance?
4. Who in the class had the ball with the most elasticity?
5. How did you come to the conclusion of whose ball was most elastic?
At Home:
-Place your ball in the refrigerator for 10 minutes. Recheck the bouncing portion of this experiment.
6. What are your observations?
7. Why do you think this was observed?
-Now place your ball about 6 inches from a light bulb for about 5 minutes and again recheck the bouncing portion of this experiment.
8. What are your observations?
9. Why do you think this happened?
Explain the Following:
1. Why does a car tire appear to be flat in the summer even though the gas inside is hotter than in the winter.
2. Why does a basketball bounce differently inside a gym than it does outside on a cold wintry day.
3. Why will a tire sometimes bump during the winter as a car is moving, only to smooth out its ride after the car has been traveling for a distance.
Teacher Notes:
Objective: The objective of this experiment is to investigate cross-linking using a similar technique as was used in the making of slime. The same parameters are worked again with a formal and a quantitative measurement used to describe elasticity. The added home investigation of the effect of temperature on the elasticity also includes concepts of molecular motion and intermolecular bond strength.
Review of Scientific Principles:
If a substance springs back to its original shape after being twisted, pulled, or compressed it is a type of polymer called an elastomer. The elastomer has elastic properties. It will recover its original size and shape after being deformed.
The liquid latex used contains small globules of hydrocarbons suspended in water. Joining these globules forms the mass with which the students will be working. The covalent bonds along the chain are strong, but the bonds between chains are normally weak. However, additives such as borax allow the formation of strong "cross-links" between chains, such as C-B-C. As the number of cross-links increases, the material becomes more rigid and strong.
 

If the rigidity of a polymer is noticed to decrease when a critical temperature is reached, the polymer is called a thermoplastic. If the bonds between polymer molecules are very strong, the material decomposes before any softening occurs. Such a material is called a thermoset plastic.
Natural sources of this liquid latex are milkweed, rubber trees, pine trees, aloe plants, and many desert plants. This latex is used to quickly mend and repair any damage to the outer covering of the plant.

General Safety Guidelines:
- The materials used in this experiment are all non toxic. It is a good idea always to exhibit good laboratory technique when working with the students. Make sure the laboratory.

Experimental:
There are many variations of this experiment.
1. The original silly putty was prepared using sodium silicate and mixing this with borax.
2. A variation also exists using laundry starch and mixing it with borax.
3. Similar variations also exist by sprinkling the borax evenly and gently over the solution of latex then working it with the hands. This does not require as much kneading to dehydrate the sample.
Time: - About 15 minutes are required to ready solutions, cups and tongue depressors.
10-15 minutes will be required in lab for testing and clean up.
The students will require 10-15 minutes of work at home in order to finish all of the experimental work on this laboratory and the write up.
Answers to Questions:
1. The liquid type of starting material should jell and become more viscous as cross-linking occurs.
2. The material will become more solid or rigid.
3. Student answer. This is only a method of measuring elasticity of the polymer. Stretching gives a similar means of comparison.
4. Student answer.
5. Greatest rebound to drop height ratio.
6. Here the student will be studying the effect of temperature variation on elasticity. Students are sometimes surprised if they place their sample into a freezer rather than a refrigerator. The results are that the ball will shatter rather than bounce.
7. The ball should be more elastic.
8. Contrary to what some students will predict, should the ball become too warm, the resulting ball will deform rather than continue to increase in elasticity.
9. The ball deformed rather than rebounding.
-All of the answers to the questions in the EXPLAIN THE FOLLOWING section involve the use of principles previously presented in this laboratory.

Day #11:  Types of Reactions Lab (OE2; SE3; SE7; SE9)
? FINISH lab from last day
? Hand in the completed lab (in-formal) by next day