Sharon Endebak

Solid Water

Science Concept:

A polymer absorbs a large amount of water by the process called osmosis.

Materials:

two - 400 mL beakers

sodium polyacrylate (This same experiment may be performed by using the stuffing found in the seat of baby diapers as a replacement for the pure chemical)

table salt

water

food coloring

stirring rod

Directions:

1. Measure 300 mL of water in one of the 400 mL beakers.

2. Add a few drops of food coloring to the water.

3. Measure out 5 -7 grams of the sodium polyacrylate and pour into the other 400 mL

beaker.

4. Pour the water quickly into the beaker containing the sodium polyacrylate from

a height of about 12 inches with a lot of vigorous splashing. This is to ensure good mixing as stirring after the addition of the water does not work properly.

5. Turn the beaker upside down to demonstrate how it has become solid.

6. Measure out about 10 grams of table salt.

7. Pour the salt onto the solid water gel and stir until the mixture becomes a

liquid again.

8. Pour the liquid back into the first beaker again to demonstrate that it is a liquid

again.

Introduction:

Have you ever wanted to freeze water in just a few seconds? (Do demo) Well, instead of ice, now you can at least make the water solid.

Explanation:

The white powder is a polymer of sodium polyacrylate. The particles have a membrane of the polyacrylate which surrounds the sodium ions. By the process of osmosis, the water is attracted to the sodium polyacrylate because it contains sodium ions (an ion that you would find in table salt). It expands the crystals of the powder and makes it into solid like gel. This is an example of an osmosis process reaction involving a polymer. Sodium polyacrylate contains a high number of sodium ions within each particle. Water is highly attracted to sodium ions. So when the water is poured into the beaker containing the sodium polyacrylate, it moves into the individual powder particles and expands the polymer particles to become a solid like gel.

When the table salt was added to the gel, it caused it to turn back to a liquid because salt has a higher concentration of sodium ions on the outside of the polyacrylate particles. The water was attracted to the salt and this caused the water to leave the polymer particles which then collapsed and become a liquid again. This process is called osmosis.

The same material is found in baby diapers - I wonder why?

Safety:

These chemicals are not harmful, but still practice good cautionary technique.

Disposal:

If the solution has completely turned back into a liquid, it may be disposed of into the sink. However, if it is still partly solid, it should be disposed of in the garbage.

Reference: Flinn Scientific Chem Fax - Sodium Polyacrylate, Pub. No. 755.10