Why might mixtures of cornstarch and water react like solids sometimes and liquids other times?

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ncchemist eNotes educator| Certified Educator

Cornstarch and water mix together to form a rather interesting mixture.  It looks like a white homogeneous mixture like a thick paint.  When you handle it quickly and roughly with lots of motion (like rapidly stirring) it will form a near solid that can be picked up and temporarily handled like a solid.  When you allow it to "relax" and sit still the apparent solid will slowly ooze back into liquid form and transfer and pour like a liquid.  This behaviour is due to the fact that cornstarch is a polymer.  In other words it is made up of molecules that are long linear chains of atoms.  When agitated these polymer chains interact against each other to give the mixture solid-like properties.  When at rest these polymer chains relax against one another and give the mixture liquid-like flow properties.  This is a common demonstration activity in chemistry classes.