Home

Home
Common Compounds
Exam Guide
FAQ
Features
Glossary
Construction Kits
Companion Notes
Just Ask Antoine!
Simulations
Slide Index
Toolbox
Tutorial Index

FAQ
Introduction
Measurement
Matter
Atoms & ions
Compounds
Chemical change
The mole
Gases
Energy & change
The quantum theory
Electrons in atoms
The periodic table
Chemical bonds
Solids
Liquids
Solutions
Acids & bases
Redox reactions
Reaction rates
Organic chemistry
Everyday chemistry
Inorganic chemistry
Environmental chemistry
Laboratory
History of chemistry
Miscellaneous


Home :FAQ :LiquidsPrint | Comment
Previous Question Next Question

Why does starch paste thicken when stirred?

I am studying bonding properties in various compounds in my high school A.P. chem course. Our instructor conducted an experiment using corn starch and a small amount of water. The resulting substance had a fairly liquidy consistency however when pressure was applied to the material, it hardened momentarily before returning to it's original state. While handling the material quickly it remained a solid, but liquified once at rest. What causes this phase change phenomenon in corn starch?
Mike

Cornstarch/water paste is an example of a non-Newtonian fluid, that is, a liquid whose viscosity changes, depending on how much force you use to get the liquid to flow. Viscosity is a measure of the liquid's resistance to flow (molasses has high viscosity; water's viscosity is much lower).

Pressing on the cornstarch suspension squeezes the water from between the particles. It resists flow because now the particles are grinding against each other. Releasing the pressure allows the water to seep between the particles again and the suspension flows more easily.

You can see the same effect with wet sand. The sand hardens under your feet. But if you lean over and draw a line in the sand with your finger, you'll see that it's rather soft.

Author: Fred Senese senese@antoine.frostburg.edu



General Chemistry Online! Why does starch paste thicken when stirred?

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/liquids/faq/starch-thicken.shtml