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Question:
IN SCIENCE In 7th grade science class at St John's Lutheran in La Grange, we conducted an activity that consisted of two ingredients corn starch and water. It made a substance that is a liquid and a solid we . We observed that the mixture under pressure would be a solid have a solid state. If you rolled it into a ball, it would be puttylike. y-like. When you stop rolling it , it oozes out of your hands and drips into the pan. It is like when the pressure was decreased in the mixture , it would return to a solid liquid state. We wonder4ed why it has these properties. What we like I would like to know its , why it does this and what makes these changes occur. Do you know?



Replies:
I think this kind of behavior happens most often when you have very long molecules (the corn starch) in a solution (the water). The long starch molecules get tangled up in one another when you bother the system (with pressure or stirring) but relax and allow the whole solution to flow when left alone for a while. This whole area of study is known as "rheology" - the study of "condensed matter" that "flows". Most real materials can be classified on one side or the other of the liquid-solid boundary under normal circumstances, but actually any solid will "flow" given enough applied stress and enough time, and most fluids actually can behave rigidly when hit hard enough and fast enough.

Arthur Smith



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