SLIME

Materials Preparation

Polyvinyl Alcohol (PVA)
Solid Borax
Hot Plate
250 mL Beaker

Presentation

PVA Solution
NaB(OH)₄, 4% Solution
Food Coloring
250 mL Beaker
Large Glass Stirring Rod

Preparation

Heat ~100 mL of water in a 250 mL beaker to 95°C on a hot plate.
Weigh out 4.0 g of PVA (polyvinyl alcohol) into a paper cup.
Remove the water from the hot plate, and slowly add the PVA while stirring.
Return the beaker to the hot plate (~90°C) and stir the solution until the PVA has completely dissolved.
Using solid borax and water, prepare a 4% by weight solution of NaB(OH)₄.

Presentation

Pour 100 mL of the PVA (polyvinyl alcohol) solution into the 250 mL beaker.

Add a few drops of food coloring.

Add 10 mL of the sodium borate solution NaB(OH)₄ to the beaker.

Stir constantly until the correct consistency. You will probably need to add additional NaB(OH)₄. Add a small amount (~5 mL) each time while stirring until there is only one large glob of slime.

*This procedure was adapted from:

E. Z. Casassa, et al., J. Chem. Educ., 63 (1986) 57-60.
G. G. Stroebel, et al., J. Chem. Educ., 70 (1993) 893.

Background

Polymers, both natural and synthetic, play an integral role in our daily lives. Naturally-occurring polymers include cellulose (mentioned in gun cotton demo), rubber, skin, hair, DNA, etc. There are also many synthetic polymers that have been incorporated into every aspect of our lives, such as nylon, rayon, polyester, plastics, PVC, chewing gum, etc. In this demonstration, we are showing some of the properties of a common synthetic polymer, polyvinyl alcohol (PVA). PVA (structure seen below) is a straight-chain polymer that is often used as a thickener, stabilizer, and binder in cosmetics, paper cloth, films, cements, and mortars.

PVA:

The straight-chain PVA will cross-link with the tetrahedral borate anion, which is generated from borax as follows:

B₄O₇^2-(aq) + 7H₂O --> 4H₃BO₃(aq) + 2OH^-(aq)
H₃BO₃(aq) + 2H₂O --> B(OH)₄^-(aq) + H₃O⁺(aq)

Borax, or sodium borate, occurs naturally in the deposits of evaporated seasonal lakes. However, we are most often accustomed to thinking of borax as a cleaning agent. Sodium borate is commonly used in detergents, water softeners, soaps, disinfectants, and pesticides. The reaction between borate and PVA is:

The solution of PVA in water is viscous, but free-flowing. Once the borate and PVA have been combined, the cross-linked polymer is produced. When the concentration of cross-linked chains is high, water molecules become trapped inside the 3D network, thus producing a semisolid gel. Eventually the water will evaporate leaving behind a crunchy ball of slime.

You can invite a few students to handle the slime, but it is probably a good idea to have them wear safety gloves. Avoid getting the slime on your clothes, and make sure the students leave the slime with you at the end of the demonstration. Be sure to wash all glassware immediately following the show to prevent the slime from drying.

References

http://en.wikipedia.org/wiki/Borax

Safety Concerns

MSDS sheets for all chemicals involved can be found in the Appendix. There are no known toxic effects of borax, PVA, or the slime. However, we should make sure the students who touch the slime thoroughly wash their hands afterward.