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On September 17th, 1998 the Ithaca Journal ran its first "Ask A Scientist!" article in which Professor Neil Ashcroft , who was then the director of CCMR, answered the question "What is Jupiter made of?" Since then, we have received over 1,000 questions from students and adults from all over the world. Select questions are answered weekly and published in the Ithaca Journal and on our web site. "Ask A Scientist!" reaches more than 21,000 Central New York residents through the Ithaca Journal and countless others around the world throught the "Ask a Scientist!" web site.

Across disciplines and across the state, from Nobel Prize winning scientist David Lee to notable science education advocate Bill Nye, researchers and scientists have been called on to respond to these questions. For more than seven years, kids - and a few adults - have been submitting their queries to find out the answer to life's everyday questions.

Previous Week's Question Published: 12 September, 2007 Next Week's Question
Jell-O: Not solid or liquid, but a facinating hydrogel
Question
Is jello an amorphous solid? If not, what is it?

Question
Jell-OŽ is a fascinating material that is neither solid nor liquid. An amorphous solid is a material in which the arrangement of atoms or molecules is not periodic. Examples of amorphous solids that are found around the house include glass windows and candle wax. In comparison, in crystalline solids such as salt and sugar the atoms and molecules are arranged in periodic lattices. Jell-OŽ in its powdered form is an amorphous solid, but when that powder is dissolved in hot water and allowed to set it forms what is classified as a hydrogel. So now the question becomes, what is a hydrogel? The phenomenon of gelation (the formation of a gel) has been known for hundreds, if not thousands, of years, in particular by cooks. If you have ever boiled a chicken carcass in water to make broth, upon cooling, the solution solidifies into a jelly-like mass. Or if you have taken berries or apples in the summer and cooked them down with lots of sugar to make a jam or jelly, you have also formed a hydrogel. In 1926, Dr. Dorothy Jordon Lloyd, a British scientist who studied gelatin and gelation, wrote, "The colloid condition, the 'gel,' is one which it is easier to recognize than to define. . ." Jell-OŽ and other gels are solid-like in that they do not exhibit any flow. Unlike solids, however, small molecules and salts can easily diffuse through a gel, as they would through a liquid.

What is happening at the molecular scale to give rise to the unusual properties of a hydrogel? Gelatin comes from the protein collagen, the main protein in connective tissues such as bones and tendons. When the gelatin is dissolved in hot water, the long polymer chains can move freely in solution. As the solution cools, the motion of the chains also slows down, allowing some chains to encounter each other and become entangled. As the degree of entanglement increases, the liquid is trapped and immobilized. The result is a material that behaves somewhat like a solid and somewhat like a liquid. In the case of the chicken stock, as the carcass is boiled, collagen protein chains are leached out of the skin and bones. Gelation is sometimes reversible, as in the case of Jell-OŽ and chicken soup, which means that as the temperature rises, the chains disentangle and the gel melts.

In addition to gelatin, there are a number of other natural and synthetic polymers that can form gels. For example, there are several vegetarian, edible hydrogels that are made from agar, alginate, and carrageen, which are all polysaccharides (polymers chains made from sugars) that are extracted from seaweeds. Many cutting-edge chefs incorporate gels into their dishes by taking advantage of the science of gels to create unique tastes and textures. For example, by choosing a gel with a melting point near that of body temperature, they make small capsules that melt in the mouth, releasing flavor. For more information on gelatin and other food-related hydrogels, I recommend the book "On Food and Cooking" by Harold McGee.

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