Key concepts
Chemistry
Gels
Molecules
Solutions

Introduction
Have you ever wondered how gels are made? You probably have several kinds of products around your house—some that you eat—that use gels: puddings, diapers, shoe insoles, packaging, ice cream, toothpaste and many more.

A gel is a mixture of solid particles suspended in liquid. The solid particles in the gel can absorb water, causing the gel to swell and increase in volume. If you ever dunked a diaper in a tub of water, you have seen this in action; the diaper will swell as it absorbs the water, and if you cut the diaper open you will see the pieces of gel that are absorbing the water. These properties of gels make them quite useful for a variety of purposes. What are some of the things you can think of that gels can help do?

Background
Gels are very common because they have so many uses—not only around the house but also in science. Around the house you might also find gels in: knee pads, JELL-O, bike seats, Sterno fuel, air fresheners, hair products, cold packs, toys and medicine gel capsules. In the medical field, bio-gels—gels that contain medicines—are being studied that can be used in the body for repair or to deliver medicines. Gels, specifically aerogels, have been used on space probes to collect star dust and return it to Earth for study. Gels have even been designed to clean up radioactive contamination.

A simple gel can be made using cornstarch and water, and varying the ratio will change its properties. Cornstarch, as its name implies, is starch derived from corn. Starch is a commonly consumed carbohydrate that is made of many bonded molecules of glucose (a type of sugar). It is made by most plants for use as an energy source. When water is added and it is heated, some of the bonds between the glucose molecules break and the starch undergoes "gelatinization," which means, you have yourself your very own, homemade gel!

Materials
•    Cornstarch
•    Water
•    Measuring spoons and cups
•    Two stir sticks or forks
•    Two microwave-safe bowls
•    Microwave oven
•    Oven mitts or small towel
•    Waxed paper
•    A flat, hard surface (such as a kitchen countertop)
•    Scratch piece of paper and pen (optional)
•    Graph paper (optional)

Preparation
•    To one bowl, add one third cup and three teaspoons (or one tablespoon) of water. To the same bowl add one teaspoon of cornstarch. This bowl now contains 5 percent cornstarch.
•    To the second bowl, add one third cup of water. To the same bowl add four teaspoons (or one tablespoon and one teaspoon) of cornstarch. This bowl contains 20 percent cornstarch.
•    Each bowl should have the same total volume. Keep track of which bowl is which. You might want to make small labels using a scratch piece of paper and a pen.
•    Use extreme caution when microwaving the gels. As you heat the gel it can become superheated and cause a burn.
 
Procedure
•    Stir each bowl thoroughly (using a different stir stick for each bowl). How do the 5 percent and 20 percent cornstarch solutions look and feel different from, or similar to, each other?
•    Heat each bowl separately in the microwave on high for a total of one minute and 30 seconds. Do this by heating each bowl for 30 seconds, then stopping the microwave and stirring the bowl, followed by heating for another 30 seconds, stirring again, and then heating for a final 30 seconds. Use caution when stirring the bowls and handling them as the gels can become superheated.
•    Carefully remove each hot bowl from the microwave using oven mitts or a small towel. Stir the contents of each bowl after removing it. How did the cornstarch solution in each bowl change? How does each solution look in appearance and feel when you stir it?
•    Allow the solutions to cool to room temperature before touching them as they will be very hot. It may take about five to 10 minutes for them to cool. As they cool, do you notice any more changes?
•    When the solutions have cooled to room temperature, touch them with your fingers. How does the 5 percent solution feel compared with the 20 percent solution?
•    Put a piece of waxed paper on a flat, hard surface and carefully place the gel solutions, one at a time, onto the wax paper and allow each gel to spread out. If you want to quantify how much each gel spreads out, put a piece of graph paper over the waxed paper before placing the gel solutions on the waxed paper.
•    Leave the gels undisturbed for about five minutes. How much did the 5 percent solution spread out compared with the 20 percent solution? How else are the two solutions similar or different?
•    If you used graph paper, you can trace around the gels when they are done spreading and count the number of squares that are inside the traced area. Which solution covered more squares? How many more squares did it cover?
•    Extra: Starches can come from many different sources, such as corn, potatoes, rice and tapioca. In this activity you used cornstarch, but you can try this activity with other kinds of starches. Do you get similar gels using different starches?
•    Extra: Gels can be made out of many different materials, such as gelatin, agar, diaper filling, tapioca, seaweed, or fruit pectin. You can try making gels out of these different materials. How are gels made from different materials different? How are they similar?

Observations and results
Before microwaving, was the 20 percent cornstarch solution less opaque, whiter and harder to stir than the 5 percent solution? After microwaving, did both solutions turn clear? Did the 20 percent solution become much thicker than the 5 percent solution? After it cooled, did the 20 percent solution spread out a lot less than the 5 percent one?

Cornstarch is made up of many molecules of glucose, specifically amylopectin and amylose. When starch is heated with water, the starch granules swell and burst, causing them to break down and release the glucose molecules into the water. Consequently, the starch molecules interact with more water, increasing the randomness of the solution. This process is known as gelatinization. When the heated solution of cornstarch and water cools down, the amylose molecules can bind each other again to create a molecular mesh. The more amylose molecules there are, the firmer, or more viscous, the mesh will be. After heating, a solution with more starch in it, such as the 20 percent cornstarch solution compared with the 5 percent one, will be firmer and stickier. Because it is more viscous, the 20 percent solution will spread out much less than the 5 percent one once cooled. Different gels can be made using different starches because starches' consistencies vary with the proportions of amylose and amylopectin that comprise them.

What could you use your gels for?

Cornstarch and water also gain interesting properties when they're mixed at room temperature. Try the activity "It's a Solid… It's a Liquid… It's Oobleck!" to have more science fun with these simple ingredients.

Cleanup
Solutions with cornstarch can be very sticky and messy. The 20 percent cornstarch gel should be composted or thrown in the trash. The 5 percent solution can be thrown away or diluted with water and rinsed down the drain.

More to explore
"The Page That Dripped Slime" from Bizarre Stuff You Can Make in Your Kitchen
"Water Structure and Science," from Martin Chaplin at London South Bank University
"How Play-Doh Modeling Compound Works: Starch Chemistry," from HowStuffWorks, Inc.
"Are You Gellin'?" from Science Buddies
"It's a Solid… It's a Liquid… It's Oobleck!" from Scientific American


This activity brought to you in partnership with Science Buddies

Editor’s Note (10/24/22): This article has been edited after posting to correct the descriptions of amylose.