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Topic: Does anyone know why cornstarch does not make Oobleck when mixed with oil or alc  (Read 8502 times)

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Offline Caligirl

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Hello everybody!

Water and cornstarch combined will make a non-Newtonian liquid called Oobleck! My son is doing a science project and he wants to know if you can make oobleck with a liquid besides water. We tested rubbing alcohol, hydrogen peroxide and oil.  Non of these produced Oobleck, but we can't find any research explaining why.

Does anyone know why cornstarch does not make Oobleck when mixed with oil or alcohol, or hydrogen peroxide?

Offline Arkcon

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Well, I would start with why the non-Newtonian fluid forms with water, and what it is about starch that forms the non-Newtonian fluid.  And maybe you can find the interference that oil gives to prevent the non-Newtonian fluid.

Sorry to answer a question with more questions, but that's what we do here. We want to help people learn.  That's detailed in the Forum Rules{click}
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline Caligirl

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Thank you for your reply!  We have been researching for days now with very little solid evidence of the what is happening.  He is 9 years old and I am not a chemist :)  Can you tell me is it lack of surface tension of the oil?  And if that explains the oil, what explains the alcohol or peroxide?

Any resources you can direct me to would be so helpful!

Offline thetada

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My money's on the role of intermolecular forces, and specifically hydrogen bonding (or lack of). The below link is informative in that topic:

http://www.chemguide.co.uk/atoms/bonding/hbond.html

Offline billnotgatez

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@Caligirl
I am curious
What brands of starch, oil, rubbing alcohol, and hydrogen peroxide did you use?  I assume you used tap water.

Offline Caligirl

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tap water- yes.  Mazola corn oil. Kroger cornstarch and hydrogen peroxide. 70% isopropyl alcohol (CVS brand).  Why?

Offline P

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I'm with thetada on this - I reckon it's the hydrogen bonding that occurs with water...  could be wrong, but it is a good bet I reckon.
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Offline thetada

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I've really enjoyed thinking about this question, thanks for posting it  Caligirl.

I set it as a class assignment for my A-level chemistry students, and this was the answer I judged to best address each separate point:

[Just a quick note: my students speak English as an additional language. When he says "squeeze the molecule out of the cornstarch molecule", I think he means, squeeze them from between the molecules.]

"When pressure is applied to Oobleck, the water molecules squeezes out of the cornstarch molecules and this gives ability to cornstarch molecules to join together and form a solid.
This cannot be happend whit oil because oil is a large molecule so it can't easily squeeze out of the corn starch . And about the alcohol, because of low power of hydrogen bonding in alcohol, they can't squeeze out and make bond with each other , so they go somewhere else and act as a lubricant again , so the mixture cannot become solid.
And about H2O2 because it probably reacts with cornstarch and makes another mixture!"

Offline thetada

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And this was the explanation which best addressed each separate point from my other class....

"It is possible the water molecule's small size, polarity, and ability to form H bonds allow oobleck to form. The water molecules surround the cornstarch. When it is pressed down on, they are "squeezed out" from between cornstarch molecules, leaving only the solid cornstarch. The same principle applies when simething is drawn quickly across the surface.
•Oil is a large, non-polar molecule that cannot form H bonds. This means it cannot surround the cornstarch molecules as closely, or form h bonds, which would form, break and reform as molecules slide past each other.
•Alcohol is also a polar, like water. However, the proportion of the molecule that can form H bonds is smaller than water's. Besides, alcohol is much larger than water, and again, cannot surround the CS as closely.
•Hydrogen peroxide is polar and small. In concentrated hydrogen peroxide, an oobleck-like product is formed, supporting the first theory. However, hydrogen peroxide is very reactive, which may mean it reacts with the CS it comes into contact with, explaining the not-quite-oobleck qualities.
—Side note on ammonia: the reactions may be with the CS hydroxyl groups and produce ketones, some of which smell like nail polish/polish remover. The mixture did have a faint smell that would not be expected from either hydrogen peroxide or CS on their own, but this is inconclusive.
•Ammonia is small, polar, and unreactive. When an ammonia solution (in water) was mixed w CS, oobleck was produced. This supports the hypothesis that the properties of oobleck are due to how closely the molecules in the liquid component of the suspension can surround the CS, and whether or not they can form H bonds.
—To test this: mix a liquid made of small, non-polar, non-h bond-forming molecules with CS, see if it makes oobleck or not. This might show whether the molecule's size or polarity has a greater effect on its oobleck-forming abilities. "

Offline thetada

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Something else interesting, while I was discussing this with my class, we tested hypotheses by trying to make oobleck with other solvents. We reasoned that a greater proportion of each molecule of ethanol can form hydrogen bonds compared to isopropyl alcohol molecules, but ethanol still did not produce any oobleck. Next, we reasoned that a compound comparable in size to water, which could also form hydrogen bonds, might form oobleck. Consequently, we tried making it in concentrated ammonia and... SUCCESS! Also, when we tried making oobleck with 100% hydrogen peroxide, we did get some oobleck-like properties, but it definitely wasn't the real thing.

It's interesting because hydrogen peroxide is small like water and also forms hydrogen bonds. I wonder if its greater reactivity is responsible for its comparative poor performance as an oobleck-solvent. Maybe it reacts with the cornstarch molecules?
« Last Edit: March 23, 2016, 12:02:23 PM by thetada »

Offline P

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It's interesting because hydrogen peroxide is small like water and also forms hydrogen bonds. I wonder if its greater reactivity is responsible for its comparative poor performance as an oobleck-solvent. Maybe it reacts with the cornstarch molecules?

I would expect the dipole moment in the H2O molecule to be stronger..  again - just a hunch - I might look it up if I get time later. Stronger dipole - stronger H-bonds?
Tonight I’m going to party like it’s on sale for $19.99!

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Offline thetada

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overall H-bonding should be stronger between water molecules, too, because the ratio of oxygen and hydrogen atoms maximises number of H bonds.

btw I've written a blog post on the activity as a teaching tool bit.ly/1RzirsZ

Offline Caligirl

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Thank you all for your interest in our topic and your thoughtful replies!  My 9 year old chose this topic and I didn't hesitate to OK it, because I imagined that there would be plenty of previous research we could cite.  I am shocked that there isnt any, especially since so many science teachers do tackle the topic of oobleck.

thetada - special thanks you to you and your students!  I'm going to try to explain it to my 9 year old so he can explain it at the science fair!  :)

Offline thetada

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you're welcome, it made a great pair of lessons! I hope your son is successful with his science fair.

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