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.
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