Explorers Club

In January 2010 the Explorers investigated the difference between “physical changes” and “chemical changes,” demonstrating a chemical change with a combination of plain old glue, water, and a powder known as borax.

The Explorers started by discussing the difference between the two types of changes that a substance can undergo—an especially pertinent topic since the fourth graders have been learning about that very thing in science class.

The simplest category of change is physical—in a physical change, the shape or form, or even the “phase,” is changed, but the chemical make-up of the substance remains the same. One good example is ice melting: both in solid form (ice) or liquid form, the substance itself (water) is still the same, even if the form is changed. Another example is tearing a piece of paper—you have changed its shape, but you have not changed its chemical composition.

In a chemical change, on the other hand, different substances combine in a chemical reaction that leaves you with something entirely different than what you started with. Burning a log in the fireplace is a good example: the heat causes the wood to chemically combine with the oxygen in the air, and what you end up with is some ash, some energy, and lots of gases—but you no longer have any wood.

A good general rule of thumb is that if you can do something to change a substance, but then you can do something else to get back what you had originally, then it was probably a physical change. You can melt the ice into liquid water, but you can then freeze the water to get the ice back. If the change is irreversible, though, it was a chemical change—once you burn it, you cannot bring the wood back. The Explorers mulled over the tricky question of pouring salt into water and stirring it up: is this a physical or chemical change? It might seem that you have a new substance—salt water—but in fact the salt is still present in its original form, only now it is dissolved into the water. The proof that this is actually a physical change is that if you put your pitcher of salt water in the sun for a few hours, the water will evaporate into a gas (another physical change) and all you will see is the salt left at the bottom.

With all of those scientific concepts floating through our brains, it was time to experiment!

We started with two bowls, into one of which we poured a half cup of water and the other a quarter cup of water. We added a quarter cup of plain white glue to the bowl with less water. A fourth of a teaspoon of borax powder (actually a crushed mineral that is often used to enhance the cleaning powers of detergent) was then added to the bowl with more water. Both bowls were stirred thoroughly until we had a mix of water and dissolved borax in one bowl and an even mix of glue and water in the other. Finally, we added a couple drops of food coloring to the water/glue mix just for decoration’s sake.

When everything was mixed, it was time to initiate our chemical reaction. Mr. Ramsey instructed the Explorers to pour the water and borax mix into the bowl with the water and glue, and then immediately start digging in with their bare hands to mix it all together. The reaction takes very little time to begin: the club members could instantly (and almost magically) feel the watery glue (or gluey water, if you prefer) begin to transform into a more solid form—a slippery, gooey, slimy sort of solid, but a solid nonetheless. We had succeeded, in fact, in making our own blobs of slime.

What had just happened? Glue is actually a fairly complicated substance, made of molecules that form long chains. (The technical name for this material is polyvinyl acetate, for those keeping score.) A similar material is gelatin, which is the stuff that Jello is made from. In Jello’s case, being heated while dissolved in water causes the long molecular chains to “cross-link” so that they form a solid, and within this solid the molecules of water are trapped so that they cannot evaporate like they would if they were just sitting in a bowl. This explains why Jello stays wiggly and sort of wet inside for days and days.

What we did with our glue is very similar, except that this time it is the borax powder, rather than heat, that serves as the cross-linking agent. Once in contact with the long chains of molecules in the glue, the borax links them together into a solid. Just as with the Jello, though, this new solid has the water molecules trapped within the new substance, and it is this that makes our slime feel, well, slimy.

The food coloring does nothing chemically to the slime, but just gave us some nice shades that were more interesting than boring old white. While the club members cleaned up both themselves and their workspace, Mr. Ramsey went around with baggies for them to drop their slime into for the trip home (which undoubtedly thrilled all of our parents, but Mr. Ramsey’s philosophy is that once the slime walks out the classroom door it is no longer his problem) and gave them instructions for keeping the slime in good shape. The main issues here are to keep the slime sealed tightly in the baggie and refrigerated when it is not being played with—otherwise, the slime can start getting moldy and otherwise furry.

And then, with a final warning that the food coloring can stain material and that the slime can be tricky to get out of such things as clothing, carpets, a dog’s fur, and so on (again, not really Mr. Ramsey’s problem!), everyone was on their way.

To see some photos from our slimefest, click on the gallery link below.

Gallery

To find more details on the specific chemical reactions involved in this activity and all the molecules that are playing a part, click on this link.

The Science Behind the Slime