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Slush Powder

Students will learn how to make water "disappear".

This is a "magic trick" and is meant to engage students. It works best with lots of enthusiasm!

Slush powder is a super absorbent chemical, and can absorb 100-1000 times its weight in water. The powder, made of sodium polyacrylate, is a polymer, Polymers are large molecules made of repeating units. Slush powder gets its name from its ability to store water as a slushy matrix, and is used in diapers and "Instant Snow". Sodium polyacrylate is a non-toxic chemical, also known commercially as SapGel or Waterlock.

Slush powder polymer's long chains of acrylic acid subunits cage sodium atoms within them. On the "inside" of the molecule there is a high concentration of sodium compared to the low concentration of sodium on the "outside" of the molecule. As a result, when you add water, surrounding the polymers, with positively charged hydrogen ions in the water, the sodium ions are free to go on the move and the tightly organised polymer chains begin to unravel. Water moves into the spaces between polymers and a stiff swollen gel forms.

As water moves in and sodium moves out, in order to reduce the concentration imbalance: a process called osmosis. Osmosis is the spontaneous movement of water into an area of high solute concentration from low concentration.

Due to osmosis, more and more water is pulled into the molecule until the ratio of sodium atoms to water molecules is so small that the amount of sodium on the "inside" of the molecule is negligible. Water movement via osmosis stops when the ratio of water and sodium inside and outside the polymer is equal. It takes a lot of water to do this — that is why sodium polyacrylate is advertised to absorb up to 1000 times its own weight of water. This influx of water makes the polymer swell and take up more space.

Objectives

  • Name some practical uses for absorbent polymers.

  • Investigate the properties of common polymers.

  • Describe in general terms what occurs during a polymerization reaction.

Materials

  • Per Demo or Group:
    3 identical, opaque cups
    slush powder
    water
    * Slush powder, sometimes sold as “diaper polymer” or “water gel” is available at Teacher Source, craft or magic shops

Key Questions

  • Is this a physical or chemical change?
  • Why would a molecule with many parts be particularly good at being absorbent?
  • Can students think of other uses for slush powder?

What To Do

Preparation:

  1. Before students arrive, set out 3 cups in a spot where students can see the cups without seeing inside.
  2. Put a teaspoon of slush powder in one of the 3 cups.

Instructions:

  1. Show the students there is nothing in the cups by tilting each cup toward the students (don’t let them see the bottom where one cup has powder).
  2. Make a big show when pouring 3/4 of a cup of water into one of the cups that doesn’t have the powder in it. Say you are going to make this water disappear.
  3. Aloud, chant a magic word over the liquid (e.g. Abracadabra).
  4. Wonder if it worked — it didn’t. To show the failure, transfer the water into the empty cup that has no powder in.
  5. Explain that you will try again. Can the students help you remember the magic word? Chant different magic words until you announce that you remember the correct magic word: “underpants” (or another word of your choice).
  6. Theatrically pour the water from the second cup into the cup containing the powder and then use the correct magic word.
  7. Once the water mixes with the power it will form a gel that will stay in the cup when the cup is turned upside down.
  8. Turn the cup upside down. Announce that the water has disappeared.
  9. Ask students where the water has gone.
  10. If possible use students’ suggestions to explain that the water hasn’t disappeared, but has been absorbed by the slush powder.
  11. Dispose of slush powder gel in the garbage, not in the sink.

Extensions

  • Add salt to the gel to make it liquefy. The water that was absorbed into the polymer moves out by osmosis to re-balance the salt concentration.
  • Cut apart a disposable diaper to find the slush powder inside. Test cloth vs. disposable diapers. How much water can each diaper hold? Which is better?

Other Resources

The University of Southern Mississippi | Polymer Science Learning Center |Make a Virtual Polymer