Explorers

In February 2010, the Explorers did some slippery investigations of physical changes.

The club members had previously done an activity dealing with chemical changes of materials, creating their own slime out of glue, water, and borax powder (see Getting Goo-fy). In a chemical change the substances you start with are changed into completely new ones, while in a physical change the shape or form or even the phase (gas, liquid, solid) might change, but the substance still has the same chemical identity.

A good rule of thumb is that in the case of a physical change, you can usually do something to reverse the process and get back the same material you started with. For example, stirring sugar into water makes sugar water—but then leaving the sugar water in the sun will eventually give you back the sugar, once the water evaporates away.

The Explorers looked at three different things, all of which are made from the same sort of material—the same basic material, in fact, that can be found in disposable diapers. This material’s usefulness comes from the fact that it can absorb great amounts of water—or, ahem, other liquids. When it does this it expands from small crystals into squishy, gooey blobs. The liquid is still there, but now it is more or less confined within the blobs.

The first material the Explorers examined started off as a fine white powder. When the club assistant poured water into the bowl with it, however, it soon expanded enormously and spilled completely out of the bowl. After a minute its growth was over, and the Explorers were delighted to find that it now felt almost exactly like snow—fairly warm snow, but snow nonetheless. The other fascinating thing about it was that it did not feel wet at all—the water had been absorbed so efficiently that no trace of dampness was present. Mr. Ramsey told the group that one of the names and uses for this material is “fake snow.”

Next up was a set of small hard clear crystals. When put in water, these also expand greatly into “jelly marbles,” round balls about a centimeter in diameter. They feel slightly squishy and slippery, as they are made almost entirely of water.

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These jelly marbles had one more surprise in store for the club. Mr. Ramsey had a bunch of them in a bowl, and another bowl that was seemingly empty other than water. When the members put their hands into the water, though, they discovered that it was full of more of these marbles—virtually invisible marbles, but still there.

This ability to hide within water is due to something called their refraction index. Refraction is the bending of light waves, and it occurs because the speed of the light waves changes depending on the material it is passing through. For example, light travels at one speed through a vacuum, another speed through glass, another speed through air, and yet another speed through water. If you have ever seen an object in water that was not exactly where you thought it would be when you tried to touch it—either in a pool, or a bowl of water—or seen a spoon in water that looked bent where the water and air touch, then you have observed the effects of refraction. A material’s refraction index is a measure of how much light bends when it passes through it.

Imagine dropping a clear glass marble into clear water. Both are clear, but you will still be able to see the marble in the glass, because it will refract the light differently. The jelly marbles, though, are almost completely made of water, and so their refraction index is nearly exactly the same as water’s. Drop one of them—or a whole pile of them—into the water, and they essentially disappear. You can see through the water to the other side of the container just the same as if they weren’t there at all.

Finally, the Explorers turned their attention to another set of small, hard crystals. These are the same as the jelly marbles except for two things: first, they grow into small chunks of jelly rather than spheres; and second, these are colored. Each pair of Explorers dumped three sets of these crystals—red, blue, and yellow—into bowls of water. Over 30-45 minutes, the crystals swelled as they absorbed the water. After playing with the gooshy pile of jelly crystals a bit, each of the students took a clear plastic water bottle and scooped all of the yellow ones into their bottle, followed by one of the other colors and then the final color, until each bottle had three layers of jelly crystals. Mr. Ramsey told them to take their bottles home and observe them for a couple of weeks, as over time the colors from each layer would slowly mix, forming secondary colors of orange or green or purple depending on the order of the layers.

Each student also took a few of the jelly marbles home in a baggie, and there was time left in the meeting for just one final reminder. This was all a demonstration of physical changes, and therefore each of the activities could be reversed to get back the original materials. If the jelly marbles or crystals are laid out on a sheet of wax paper, they will slowly (over a period of days or even weeks, depending on the humidity) shrink back to their original form and size as the water evaporates. The same thing will happen to the fake snow. Each of the materials can then be used again.

To see some photos from this activity, check out the Gallery.

The link below leads to the science supply web site where we obtained our jelly marbles, jelly crystals, and fake snow.

Science Supplies