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Soak up this free disappearing water activity

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Page 2 Expected Results: The diaper powder turns to a damp white gel, which remains stuck to the bottom of the cup upon addition of water. Diaper powder absorbs ~900 times its own mass of water. It absorbs so well because it is a polymer which means its molecules are made of small, repeating units that form long chains. In its dry crystal form, sodium polyacrylate is coiled. When water is added, the long chains of sodium polyacrylate stretch out because each repeating unit in sodium polyacrylate contains an area that can support a negative charge. By adding water to the polymer, these areas form negatively charged ions that repel each other and cause the polymer to stretch out. As the polymer stretches, more water molecules can associate with its areas of negative charge. The unique shape of water also means it can associate with two polymer chains; two polymer chains surround each layer of water molecules. Each row of water molecules forms a bridge between the two chains. Water's ability to link between chains is called cross-linking, which allows the polymer to absorb a lot of water. Follow up/Extension: 1. Students can try to release the trapped water from the polymer chain network by adding salt to the gel. Ions present in the salt displace the water molecules within the gel because they're more strongly attracted to the polymer than water. 2. Have students design a lab to determine the absorbing capacity of the powder and what conditions affect absorbance. 3. Fill a clear container with plain water and put some of the largest hydrated crystals into it. The crystals will appear to vanish! Students can learn how the concept of refraction helps explain their observation. Questions for students: 1. Describe the process of diffusion when water and salt are introduced to the polymer? 2. How could the characteristics of superabsorbent polymers be useful in agriculture and retaining moisture in the soil? Disposal/Clean-Up: The cup and contents can be disposed of in the garbage. Disappearing Water (continued) + ward ' s science 5100 West Henrietta Road • PO Box 92912 • Rochester, New York 14692-9012 • p: 800 962-2660 • wardsci.com Find materials for this activity at wardsci.com. Discover more free activities at wardsworld.com

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