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This week's LabZone activity
Feb. 28, 2007
Getting Critical Over Colloids
What is a colloid? If you have made Oobleck out of corn starch and water, then you know that a colloid is a mixture that acts like a solid and a liquid at the same time! What is the critical factor in making a colloidal material? Will different sources of starch change the recipe?
Objective
The purpose of this project is to find the critical point for colloidal mixtures composed of different types of starches.
Introduction
What do ketchup, Oobleck, and quicksand have in common? They are all made up of tiny, solid particles suspended in water. Chemists call this type of mixture a colloidal suspension, and the amount of solid and water to use is called the critical concentration. The critical concentration for each colloidal material is unique and depends on many different factors.
Colloids have very interesting physical properties. One of the more interesting physical properties of colloidal materials is that sometimes they seem to be solid and other times they seem to be a liquid. Because of this odd behavior, colloids are called non-Newtonian fluids, because they break the rules of ideal fluids described by Isaac Newton in the 1700s.
Colloidal suspensions respond differently to different forces, as seen in this cool Oobleck video by Blake (Kid Science, 2006). A fast, hard force will cause the colloid to appear solid, but a slow, even force will cause the colloidal material to flow like a liquid. This can be dangerous if you live in an area with clay soil, because sideways forces during a flood or earthquake can cause the earth to suddenly become very unstable!
As it turns out, colloidal materials are very common. Even though they have such strange physical properties, these properties make them very useful products and materials. Foam, gel, glue, and clay are all examples of colloidal materials. There are many colloidal materials found in food products, like: marshmallows, mayonnaise, pudding, milk, butter, and jelly. Building materials such as cement, stucco, plaster, and paint are colloidal materials. Even our bodies and other living organisms are made of colloidal materials! They are everywhere!
In this experiment, you will learn about a very simple colloidal material: starch suspended in water. You will test starch from different plant sources (corn, potato, rice, tapioca) to see if the colloids share similar physical properties. You will measure the amount of water needed to make a colloid out of each type of starch. Will these colloidal suspensions be the same or different?
Experimental Procedure and Additional Information
Used with permission. Copyright © 2002-2007 Kenneth Lafferty Hess Family Charitable Foundation. All rights reserved.
Sohn, Emily. 2006. Putting the squeeze on toothpaste. Science News for Kids (Nov. 29). Available at http://www.sciencenewsforkids.org/articles/20061129/Feature1.asp .
For more science project ideas, go to http://www.sciencebuddies.org/mentoring/areas_of_science.shtml .
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