Vanishing Water

Developer:

A Plethora of Polymers (1993) by Mike Shaw

Grade Levels:

Discipline:

Physical Science

Objectives:

  1. Students will better appreciate the value of water (not a limitless supply).
  2. Students will better understand the water cycle.
  3. Students will apply the scientific method.
  4. Students will make observations.

Vocabulary:

POLYMER =

ABSORPTION =

Background:

Sodium polyacrylate undergoes a polymerization process in which sodium acrylate and acrylic acid are combined. Water Loc and Water Grabber are commercial products that incorporate the use of a polymer - a poly(sodium acrylate) to retain water over an extended period of time. These polymers are designed throught crosslinking to absorb water until they reach many times their original weight. Manufacturers claim the crystals will absorb 800 times their own weight when placed in distilled water, 300 times its weight in tap water and 60 times its weight in a 0.9 % sodium chloride solution.

Materials:

  • Sodium polyacrylate (available from Flinn Scientific, Inc.) or Water Grabber (available from K-Mart or Wal-Mart stores)
  • 6 polystyrene drinking cups
  • water
  • teaspoon

Before Class:

Place a level teaspoon of sodium polyacrylate crystals into each of four of the drinking cups.

Procedure:

  1. Stack the four prepared cups one atop the other and then place the remaining two empty cups on top of the four with crystals. Do not allow students to investigate the cups before class. As they enter the room, they should only see an apparently normal stack of drinking cups.

  2. After class begins, set up the situation that is appropriate for your needs. An example might be that you are going to see who is the best observer in the room.

  3. Remove the first cup from the top of the stack and ask a student volunteer to fill it approximately three-fourths full of water.

  4. Then remove the second and third cups from the top of the stack (holding cup three in the left hand if the student is standing on the right side in order to avoid detection since the crystals are in cup three).

  5. Hold all three cups out in front of yourselves at relatively the same height and ask the volunteer to pour water from their cup into your cup two. This is easily done and then you pour water back into the volunteer's cup. Ask the volunteer to repeat this step.

  6. The water is now in your cup two. Pour the water into your cup three and then switch the positions of cups two and three several times for effect. Ask all the students to vote for which cup they believe has the water in it. (While you are doing this the polymers undergo a transition phase change when brought in contact with water and are converted into a gel-like substance. This subsequent solidification of the gel prevents further pouring from cup two to cup three.

  7. You will probably have almost unanimous agreement for cup three. Ask the volunteer to invert his / her cup and follow by inverting your cup two and then cup three. Observe, record and discuss results.

  8. If you decide to repeat the performance, reuse cups one and two , but pull the next cup from the stack which contains crystals for cup three. (I think it is a good idea not to share the secret so that all classes will be surprised.)

Explanation:

Questions:


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