Feelin' the Change
In January 2013 the Explorers investigated the differences between “physical changes” and “chemical changes,” culminating with the club members digging their hands into the details of a particular chemical change—literally.
The difference between physical and chemical changes is a fairly simple one. A physical change is one in which the chemical make-up of a material is unchanged—only its physical form or appearance is altered. If you still have the same material at the end of what you’ve done, or if the process you’ve done is reversible so that you can get back what you started with, it’s a physical change. With a chemical change, however, you end up with materials with new chemical properties, and you cannot reverse what you did to get back the materials you started with.
A good example is paper. If you crumple the paper, or tear it into pieces, you have not changed its properties—it is still paper, just in a different form. If you BURN the paper, however, the heat of a flame acts as a catalyst for the molecules of the paper to chemically combine with the oxygen in the air. The result is a pile of ash and assorted gases that escape into the air. In no way can you reverse the burning to get back the paper.
The only problem is that there are lots of tricky situations where it seems that one kind of change has occurred when it actually was the other. Freezing water is one example—ice may seem to be an entirely different substance than water, but in reality it has only undergone a physical change, and melting the ice will bring back your original liquid water. Dissolving salt into water is another example of something that resembles a chemical change but isn’t one. “Salt water” is still made of the individual components salt and water, and if you allow the water to evaporate over time you will again have your pile of salt.
With these changes in mind, the Explorers now considered the case of glue and water—mixing them together gives you, well, watery glue. Or gluey water? Whichever. Either way, if you allow this soupy mixture to sit long enough, the water molecules escape from the glue, evaporate, and you end up with a blob of dried-out glue.
The club members started by making exactly this glue-water mixture, along with a touch of food coloring to give it a more colorful appearance. Next, though, they added one more solution: a mixture of water with a small amount of borax powder (found in many cleaning detergents) stirred in. As soon as they put these two mixtures (glue-water and water-borax) together and started stirring them with their hands, they could feel a startling change happening. Strings and sheets of slippery, slimy goo were literally forming in their hands.
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What had happened? The borax molecules combined with the glue molecules in a process known as “cross-linking.” In doing so, they essentially formed a molecular cage of sorts, and many of the water molecules became permanently trapped in that cage. No matter how long the goo sits out in the open, the water molecules cannot escape and evaporate. Nor is the process reversible—there was nothing we could do in the classroom to get back our original glue, water, or borax. Instead, we had ourselves an entirely new substance—a fairly gross substance, perhaps, but a new one nonetheless.
To conclude this activity, everyone dumped/scooped/poured (some slime mixtures turn out more solid, some more watery) their goo into plastic baggies to take home. Mr. Ramsey warned the students that because of the food coloring the slime can stain materials (carpets, tables, your sister), and that the slime should be kept sealed in the baggie and stored in the refrigerator when not being played with (otherwise mold can grow on it, for yet another level of grossness!).
Check out the Gallery to see some photos of our slimy exploration (thanks to club assistant Lissa for taking most of the pictures). To see earlier slime activities from previous years, or for more details on the chemical changes that occur in this activity (and a recipe to make your own slime at home), click on the links below.