Have you ever thought about why a disposable diaper is so great at holding onto the liquid that a baby can put out? Why is it so much better than a sponge?
Materials:
• Disposable Diaper
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Caution: The experimenter will need help getting the powder out of the diaper. The powder should not be inhaled or ingested. If the powder gets on the hands, wipe it off and then wash the hands. The powder is not toxic but it will absorb a lot of water.
Experiment:
Using the baby diaper, begin to pull it apart. The edge of the diaper is where different layers can be disengaged. You may need a scissors to get through some of the layers. The top layer is a fabric that covers a cotton layer that sits on top of a powder. Once you have exposed the cotton layer put it in the baggie and then pull the cotton away from the rest of the diaper. Seal the bag and shake the baggie to try and free the powder from the cotton. You should be able to recover at least a teaspoon of the powder. Once you see the powder in the bottom of the bag, the diaper and the cotton remnants can be removed. Pour the powder into the cup. Add a small amount of water (tablespoon or so) and see what happens. Swirl the cup if nothing appears to happen. Add more water a little at a time. Once the powder doesn’t seem capable of holding more water, add a tablespoon of salt and stir with a spoon.
What’s happening?
You should have seen that the water is significantly absorbed by the small amount of powder. This powder is a polymer called sodium polyacrylate. A polymer is a very long chain molecule and this polymer has many negative charges associated with it and the same number of positive sodium ions. The water being a highly polar molecule is strongly attracted to the positive and negative (represented by dashes in the picture) charges. Because these negative charges cannot move from the polymer, the water molecules are stuck to the polymer and cannot slide past each other like they do in ordinary water and so the polymer becomes more of a gel. Once salt is added, the highly condensed negative chloride ions can get more water molecules surrounding them and because they aren’t attached to something, now the water molecules can slide past each other again – albeit holding onto the chloride ions. A sponge is made out of polymers as well but there are no positive or negative charges and so they will not hold onto the water as tightly.
Extension:
Think about how else we might use this material in the real world such as adding a water-logged sodium polyacrylate gel to the rooftops of houses that are in danger of being in a forest fire. Are there any other good ideas?
Reference:
https://www.acs.org/education/whatischemistry/adventures-in-chemistry/experiments/diapers.html
To view past “ChemShorts for Kids” activities, go to:
https://chicagoacs.org/ChemShorts.
