Hands-on Science Carnival 2010 Activity Stations: Chemistry

Polymer Chemistry II: Super Absorbent Polymers

[Shopping List: Pampers or generic diapers; cloth baby diapers or cotton washcloths; sponges; paper plates; scissors; standard polyacrylic acid powder; instant snow powder; clear spheres and crystals; fishbowl; Solo cups and lids; buckets of water; cups for pouring water]

Baby Diaper Dissection (do this only a few times, or as mini-workshop)) and/or Polyacrylic Acid Water Absorption

1. Open a Pampers diaper and inspect. Compare to a cloth diaper and a sponge.
2. Add some water to each. How much can each absorb?
3. To understand why the Pampers can hold so much more water, carefully cut one open along the edges and separate the layers.
4. Carefully collect as much of the powder as you can and place it into a Solo cup.
5. Add water, a little at a time and observe what happens. Keep adding water until no more can be absorbed.
6. You can also use the commercial polyacrylic acid powder instead.

Instant Snow

1. Put a small amount of instant snow powder in a Solo cup.
2. Add a small amount of water and observe what happens.
3. Add more water, feel the consistency of the "snow", and finally add as much water as it will absorb, a little at a time.

When finished, the kids can take their samples home by putting a lid on their cup. They may want to pour some out. Once at home they can leave it to dry out for a couple of days and once again turn into just a small amount of powder. Then they can add water and repeat the experiment.

Clear Spheres (Alien Eyeballs) and Clear Crystals

1. Prepare a fishbowl full of clear spheres in water in advance. It doesn't take many, so don't go crazy. They start out as 3 mm diameter beads, but will grow to 1" or more in diameter in about 24 hours.
2. If desired, prepare a second container with clear crystals.
3. Look in the bowl. Can you see anything?
4. Put your hand into the bowl. What do you feel?
5. Have a smaller bowl with spheres at various stages of growth, as the shapes they assume are very interesting to watch.

Let kids take a couple full-sized spheres or crystals home in a Solo cup with a lid. They can also take 2 or 3 dry sphere beads (put them in the same cup). By the time they get home they should be starting to grow, so they can transfer them to a larger glass or bowl, add more water, and let them grow to full size, check every couple of hours to observe the shapes.

What's Happening: All of these samples are made from the same basics material, polyacrylic acid, which is a long chain polymer molecule which is extremely hydrophilic, i.e. it attracts water. Even more amazing however, is that it can bond with water molecules and lock them into huge lattice-like structures, absorbing several hundred times its own weight in water. There's 4-5 grams of dry powder in Pampers diapers (stuck in the cottony fluff in the middle layer), and that's why even a newborn sized diaper can hold as much as 16 ounces of water. The instant snow is exactly the same material, just ground up into a much finer powder which causes it to form smaller molecular structures when it absorbs water, giving it the look and feel of snow. It's actually used in movies when they want snow in a scene without the need for cold temperatures.

The clear spheres and crystal spheres are the same basic material, but prepared a little differently to form the shapes you see when they absorb water. The spheres start as 3 or 4 mm beads, but grow to more that 1" in diameter over 24 hours or so. The crystals show similar growth. Because they are composed almost entirely of water when fully grown, they are essentially invisible in a bowl of water (as long as both the spheres and the liquid water are clean). Water is water. The index of refraction of the liquid water is the same as the water in the sphere, so light rays are not refracted and the spheres are indistinguishable from the rest of the water. The chunky crystals are used in moisture-control potting soil that you can but at any garden supply store.

Variations: If the clear spheres are allowed to dry out (dehydrate), they will shrink back to the tiny beads, and can be reused. Same with the baby diaper powder and instant snow. If the spheres or crystals are grown in colored water, they will be colored. if they are then placed into a bowl with fresh clear water, they will slowly fade as the food-colored water molecules diffuse out and clear water molecules diffuse in. Of course the clear water in the bowl will become colored, so you will need to change it frequently. You could also start with fully grown clear spheres in fresh colored water to watch the sphere slowly become colored. Also try putting several clear spheres into a bowl with fresh water and add a little milk or non-dairy creamer powder. Shine a laser pointer beam into the water for an interesting effect. You can also shine a laser beam into one sphere to demonstrate the internal reflections and refraction that occur in a raindrop to produce a rainbow.