Next time you go to the supermarket take a close look at the aisle of baby supplies. You will notice that there are many different kinds of diapers. Some varieties have elastic to hug babies’ legs; others have super-adhesive bands that won’t come undone. Some brands even offer fresh scents or bright colors. One feature that all disposable diapers have in common is a high absorbency. Thanks to a special chemical inside the lining, diapers can hold a lot of liquid without leaking.
If you have a younger brother or sister who is still in diapers or who has been recently potty-trained, here is a fun chemistry experiment that you can do to learn more about the chemistry of diapers.
A clean disposable diaper, a gallon-size zipper-lock bag, a small clear cup, water, teaspoon, salt.
1) Open up the diaper and lay it flat on the table. (The next step can be a bit messy, so you may want to protect the table with a sheet of newspaper.)
2) Carefully cut through the cotton lining of the diaper and remove all of the stuffing material.
3) Place this cotton-like material into the zipper-lock bag. (Before sealing the bag, carefully shake the remaining diaper over bag so that any loose powder from the diaper falls into the bag.)
4) Seal the bag and shake it vigorously for at least one minute. You should begin to see a white powder collecting in the bottom of the bag.
5) Without opening the bag, carefully move the cotton material toward the top of the bag. Hold this material in your hand while shaking the bag again. This will allow additional powder to settle to the bottom of the bag without sticking to the cotton.
6) Once you have accumulated at least a teaspoon of white powder in the bottom of the bag, open the bag and remove the cotton.
7) Carefully pour the white powder from the bag into a clear cup.
8)Add a teaspoon of water to the white powder. What do you notice?
9)Add another teaspoon of water to the powder. What happens to the powder? How does it change?
10) Keep adding teaspoons of water one at a time until the powder can’t absorb any more.
Now Try This:
Try this activity with different brands of diapers. Do some brands hold more water than others? What happens if you add salt to the cup full of gel? Try it and find out.
What’s Going On?
The white powder in the diaper is a type of polymer called sodium polyacrylate. Polymers are long chains of molecules that are linked together. Unlike most other types of polymers, sodium polyacrylate is hydrophilic, or water-loving. When water is added to the powder, the tightly-coiled polymer unwinds and creates a hydrogel by forming chains around the water molecules and holding onto them like a net. This allows the polymer to absorb hundreds of times its weight in water, making it the perfect ingredient for baby diapers! To turn the gel back into a powder form, simply add table salt. The presence of sodium chloride greatly decreases the ability of sodium polyacrylate to absorb and retain water. Baking soda and vinegar have a similar effect.
Debbie De Roma is the education manager at the Reuben H. Fleet Science Center.