If I give you glue, water and Borax, can you come up with a recipe for perfect polymer slime based solely on what you know about the science?
Last week, I did hands-on science with 150 third graders at Success Beyond the Classroom’s Creativity Festival at the University of Minnesota. As they came into the room, I asked them to draw a picture on chalkboard of anything related to science. They drew test tubes, trees, volcanoes, and even scientists!
Then we dug into the science. After doing my favorite large group hands-on experiment with purple cabbage juice to warm them up, I announced that we’d be making polymer slime, but that they would have to invent the recipe for the perfect goo.
To begin with, I talked about the science. We learned that one ingredient in the slime is glue (polyvinyl acetate), which is a polymer, or long chain of molecules. Then, we talked about the fact that adding water to the glue makes it less viscous, or thick. As usual, I had them repeat the vocabulary after me. Finally, I explained that the sodium tetraborate in Borax laundry detergent is a cross-linking substance that makes glue molecules stick together, and that we’d mixed up some Borax and water for them to use as a crosslinker for the slime.
Their challenge was to come up with one “recipe” to make a gooey, soft slime and a second recipe for a harder, rubber-like slime that could be rolled into bouncy balls. They each had a note card and pencil to keep track of their work, plastic teaspoons as measuring tools and paper cups for mixing slime. First, they’d stir up different proportions of glue and water, and then they’d add the Borax solution as a cross linker and mix it all together with a popsicle stick. To make it a little more colorful, they could add a drop of food coloring or some cabbage juice.
Needless to say, there were failures and successes and the kids had a blast. We talked about the fact that experiments often don’t work on the first try, and each kid explained to the group how they’d made their perfect slime as they demonstrated how it bounced and stretched.
You’ll be amazed when you fill a plastic zip-lock bag with “blood” and poke sharp skewers through, only to find that the bag doesn’t leak! All you need is a ziplock bag, water, food coloring and wooden skewers. Heavy-duty ziplocks work best! Fill a quart-sized ziplock bag with water, add a few drops of red food-coloring, and seal it. Slowly poke several wooden skewers completely through the bag, from one side to the other, avoiding the part with air in it. See how many you can push through! (Remember to be careful with the sharp points and I’d recommend putting a bowl underneath to collect drops.)
Why doesn’t the bag leak? Plastic is a polymer, made up of long, elastic molecules that form a seal around the spot where the skewer is poking through. In addition, the bag is sealed and contains very little air, so there isn’t much air pressure pushing on the water. If you make a hole in the part of the bag with air in it, surrounding air pressure (atmospheric pressure) can push on the liquid and make the bag leakier!
What could be more fun than creating your own green slime to play with? It’s easy to synthesize your own green goo using only Elmer’s glue (the non-washable kind), Borax (found in the laundry detergent section of most stores), green food coloring and water.
In a bowl, have your child mix together about 1/3 cup glue and 1/3 cup water with a spoon or Popsicle stick. These measurements don’t have to be exact. Add a few drops of green food coloring and mix well.
To make the Borax solution, add around a cup of water to a jar. To the water, add about a Tablespoon of Borax. Have your child shake the jar to dissolve as much of the Borax as possible. You are making what is called a saturated solution, so it may not all dissolve! Don’t worry, it will work just fine.
Have your child add about a teaspoon at a time of the Borax solution to the glue/water mix. After each addition, have them stir the mixture together. You should see long strings begin to form and stick together. Keep adding Borax until the mixture doesn’t feel gluey any more. It will form sort of a shiny playdough-like substance. If you add too much Borax solution, it will feel wet. You should be able to just knead it a little to absorb the extra water! The slime is not toxic, but Borax is soap, so don’t let your kids eat it!
I am a biologist and not a chemist, but here is the science, as I understand it.
Mixing Elmer’s glue with water forms a substance called a polymer, which is a long chain of molecules. (A molecule is the smallest amount of a specific chemical substance that can exist alone, like H2O, a single water molecule). The polymer formed by water and glue is called polyvinyl acetate.
The Borax solution (sodium tetraborate) is a cross-linking substance that makes the polymer chains stick together. As more and more chains stick together, they can’t move around and the goo gets thicker and thicker. Eventually, all the chains are bound together and no more Borax solution can be incorporated.
You can store the slime in plastic bags. If you want to make a larger batch, just remember to mix equal amounts of glue and water and add as much Borax solution as needed.
To make your child feel like a “real” scientist, find an old, button up shirt for your child to use as his or her “lab coat”. It’s fun and will protect their clothes. You could even try to find some old safety goggles in your garage for your child to wear, although the ingredients for this project are relatively safe. (Very young children should always be supervised while doing science projects.)