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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.

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Previous Week's Question Published: 21 March, 2002 Next Week's Question
Solids, liquids and something in between
After mixing 1oz of cornstarch and some water together, why does it get hard when pressure is applied? And then when the pressure is released, the mixture becomes drippy?

To understand why the cornstarch water mixture becomes hard when pressure is applied we must consider the nature of the molecules that make up cornstarch and water. Molecules are the smallest components of a material that can be separated from each other. A given type of molecule is made from a unique combination of atoms.

For example, 1 liter of water contains roughly one million, billion, billion molecules, each composed of one oxygen and two hydrogen atoms.

In solids, the molecules composing the material stick strongly to each other and cannot move. In liquids (such as water), on the other hand, the molecules are less strongly attracted to each other and can rearrange. At room temperature, the relatively small water molecules flow past one another to form a liquid.

The molecules that make up cornstarch are very different from the small water molecules. They consist of long chains of repeating units called sugars. Sucrose or table sugar has two such repeating units per molecule, whereas starch has many, many, more. In pure cornstarch, the sugar chains stick strongly and cannot move past one another, thus starch is a solid.

However, if we add water to starch, the water gets between the starch chains, separates them and allows the chains to slide past one another; the mixture behaves as a liquid.

If we apply pressure to the starch mixture, the water is squeezed out from between the chains and they are able to grab one another. Sliding is prevented and the material behaves as a solid. If we release the pressure, the water can enter between the chains to allow sliding once more.

This behavior is not limited to the molecular scale. A similar phenomena occurs when you run on wet sand at the beach. If you run fast and generate pressure quickly the sand feels hard as water is squeezed out and the sand particles cling to each other. If you step slowly to apply the pressure gradually, the sand particles have time to move past one another -- your foot sinks!