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Procedure:
1. Prepare 25 mL of a saturated solution of sodium borate by mixing borax with water until no more dissolves.
(solubility ~1 g/16 mL H20)
2. Add 100 mL of distilled water into the empty glass juice container.
3. Add 3 drops of red food coloring.
4. Add 0.6 g of guar gum and stir until dissolved.
5. While swirling the liquid in the bottle, slowly add 5 mL of the saturated borax solution prepared in step 1.
6. Cap the juice bottle.
Expected Results:
The addition of sodium borate to an aqueous suspension of guar gum produces a cross-linked gel. The resulting cross-linked
polymer forms a red gel that looks like juice.
Boric acid is such a weak acid that it doesn't give up H+ but instead accepts an OH- from the water. The borate ion can make
weak bonds with the OH groups in the polymer chains to link them together. In the presence of borate ions, the guar gum forms a
weak gel. The chains of the polymer react with borax forming a three-dimensional gel network.
Follow up:
• Polymers, cross-linking.
• Borax solution and Elmer's glue is an example of cross-linking where varying the amount of borax solution produces the effect
of slime or Silly Putty.
• The Nylon Demonstration Kit (available from Ward's Science) is another great example of polymer synthesis.
Students in higher grades can consider these questions:
• How might biodegradable polymers help reduce the environmental pollution caused by plastic waste?
• How does the cross-linking process affect the volume of the polymer?
• How does the chain molecule's length affect how it attracts water? How does that impact the viscosity of the resulting gel?
Teaching notes:
The objective of the activity is to prepare a cross-linked polymer and examine its physical properties. The juice bottle can
be opened and quickly shaken back and forth, causing the gel to partially leave the bottle walls and quickly snap back.
You can also use the activity to help demonstrate the difference between joining monomers by the processes of addition polym-
erization or condensation polymerization.
Disposal/Clean-Up:
The natural polymer can be disposed of in the garbage.
Joke Juice (continued)
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