Polyacrylate+demo

 (adapted from an OMSI pocket demo and a perennial Tyler Lab polymer demo):

Background:[[image:163px-Sodium-polyacrylate.svg.png align="right"]]

 * Polymers** are long-chain molecules that consist of many ("poly") units ("mer") which repeat over and over to form long chains. **Sodium polyacrylate** is a polymer where the repeating unit is [-CH2-CH(COO-)-]. The negatively charged //carboxylate// group attached to the polymer backbone is balanced by the presence of positively-charged sodium (Na+) ions. Because water is attracted to both the sodium and carboxylate ions, this polymer is able to absorb large amounts of water. In fact, it can absorb hundreds of times more water than its own weight. This polymer is used in disposable diapers and is added to soils to help them retain water. Once the solid polymer absorbs enough water, it forms a //gel//.

Materials Needed:

 * 3 opaque disposable cups. Small plastic picnic cups work well.
 * About 10g sodium polyacrylate polymer. This can be purchased from some garden stores, or you can get it from a disposable diaper:
 * Cut open a (unused) disposable diaper. Remove the insides and place them in a gallon ziplock bag. Shake really hard for a few minutes. The polyacrylate particles will shake free and fall to the bottom of the bag. Dispose of the rest of the diaper.

Procedure:

 * Get 3 opaque cups. Place about 3g polyacrylate in one of the cups, and leave the other two empty.
 * Place the cups in a row in front of the class. Pour water into the cup containing polyacrylate.
 * Rearrange the cups for a while. Swirl them as you rearrange them. Be sure the polyacrylate absorbs all the water.

Extras:

 * Once the polymer has absorbed enough water to make a goopy gel, add about 1g of table salt (NaCl) and stir.
 * The extra sodium (Na+) ions compete for the water, and cause the water to desorb from the polymer. This returns the mixture to more of a liquid/solid mixture.
 * Because of this, the presence of salt lowers the amount of water the polymer can absorb. Since urine contains a lot of salt, it cannot be absorbed in diapers as easily as pure water would.