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We made "Oobleck" in class (corn starch, H2O, food coloring). We were arguing whether it was a liquid or a solid. Can you determine whether it is a liquid or solid based on the molecules movement?

Asked by: Luka Willem Rizzuto
School: MacArthur Elementary School
Grade: 3
Teacher: Mrs. Coleman
Hobbies/Interests: Luka enjoys writing, playing with Hot Wheels cars, and riding his bike. His family includes his mom, dad, and brother Maxwell who is six.
Career Interest: Luka would like to become a scientist

Answer from Stanley N. Salthe

Visiting Scientist, Binghamton University

Mr. Salthe's area of research is natural philosophy. He is also interested in ecology, evolutionary biology, semiotics, systems science, and thermodynamics. He received his Ph.D from Columbia Univeristy and has a wife, Barbara, and two children, Becky and Eric. Mr. Salthe enjoys woodland gardening, nature walks, and all of the arts.

If you could see the molecules, you would find that they are indeed in motion.  This motion increases the warmer the temperature is, and slows down as the temperature drops.  Your “Oobleck” would become fully liquid if you heated it up, and it would get more rigid if you cooled it down.  In between, it would be a gel.  In the heated up state, the molecules in the liquid would be seen jiggling about furiously, and even moving between each other, so that one molecule might trace a path across your field of vision. In the cooled down state, you would see that the molecules don’t trace a path, but just stay in place, moving slightly as if shivering.  It is supposed that if you could cool the molecules down to absolute zero degrees, they would have no motion at all. These are fundamental facts of physics, studied in the subfield of thermodynamics, which roughly means, action related to temperature.  In the hottest situations we would have explosions, with molecules flying off in every direction and dancing wildly. Our world is relatively cool, with only occasional explosions.  We ourselves are pretty warm, even compared to most other living things.  This means the molecules that make us up are moving quite a lot.  But, since they are very big molecules (proteins and nucleic acids) compared to the molecules found in cornstarch, we don’t get shaken apart.  Their motions are important in creating our metabolism, which is the basic chemical activity in all-living things.  Some kinds of molecules make solid objects at room temperature, some make liquids, and some, like those that make up us, are in between. Regardless, all would get more rigid as temperature cools, and more active as it warms up.  

Last Updated: 3/1/17