Abstract

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 1700's.

Colloidal suspensions respond differently to different forces, as seen in this cool Oobleck video by Blake (Kids 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, those same 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 like 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?

Terms, Concepts, and Questions to Start Background Research

To do this type of experiment you should know what the following terms mean. Have an adult help you search the internet, or take you to your local library to find out more!

• starch
• colloid
• physical properties
• solid
• liquid

• How does starch behave as a colloidal material?
• What are the physical properties of colloidal materials?
• Do different sources of starch change the properties of the colloid?

Bibliography

• Exploratorium, 1998. "Science Explorer: Outrageous Ooze — Is This Stuff a Liquid or a Solid?," San Francisco: Exploratorium. [accessed July 3, 2006]
  http://www.exploratorium.edu/science_explorer/ooze.html
• Watch this video to see Blake explain how to make Oobleck, show you its physical properties, and talk about matter:
  Kids Science, 2006. "Kids Science Episode 1: Cornstarch Suspension," YouTube.com [accessed July 18, 2006]
  http://www.youtube.com/watch?v=fazPiaHvFcg
• Watch this video to see how high frequency waves cause very odd behavior in Oobleck, from this paper by researchers at the Center for Nonlinear Dynamics, University of Texas at Austin:
  Merkt, F.; Deegan, R.D.; Goldman, D.; Rericha, E., and Swinney, H.L., 2004. "Persistent holes in a fluid," Phys. Rev. Letters. 92 184501 [accessed July 3, 2006]
  http://chaos.ph.utexas.edu/research/vibrated_cornstarch.htm
• Seuss, 1949. Bartholomew and the Oobleck, New York: Random House.

Materials and Equipment

• water
• cups
• measuring spoons
• medicine dropper or small syringe
• stir stick
• different types of starches:
  • corn starch
  • potato starch
  • tapioca starch
  • rice starch

Experimental Procedure

1. You will need to determine the amount of water required to form a colloid from each type of starch, one at a time.
2. Starting with the corn starch, add 1 tbsp of the starch to a small cup or bowl.
3. Add water one drop at a time, stirring frequently and counting the number of drops until the mixture forms a colloid. You will recognize the colloid when you stir with your stir stick. At first, the mixture will look solid and separate into clumps, but then the clumps will start to flow together again like a liquid.
4. When you reach the proper consistency for a colloid, stop adding water and record the number of drops in a data table:

   Type of Starch	Drops of Water	Observations & Physical Properties
   corn
   potato
   rice
   etc...

5. Play around with the colloidal material. Poke it with your finger and put some in your hand. What does it look like? How does it move? What does it feel like? Write down some of the physical properties in your data table.
6. Repeat steps 2–5 with each of the other starches and record your data in the data table.
7. Make a bar graph of your data to compare the different starches. Place the number of drops of water on the left side (Y-axis) of the graph. Draw a bar for each type of starch up to the matching number of drops. Compare the bars on the graph representing the different starches. Are they the same or different?

Variations

• Experiment with the different properties of the colloids. Are they solid or liquid? Do they respond differently to different kinds of pressure? Push slowly into the colloid with your finger. How does the colloid respond? Poke the colloid hard with your finger. Does it respond differently? Do all of the colloids respond similarly and have similar properties? Can you find other ways of testing the physical properties of colloidal materials?
• Starches are often used to make gels. What happens if you increase the temperature of your colloidal material? Try heating up the colloidal starch mixtures. How do they change? Are there new physical properties that you can observe? Do starches from different plant sources gel at the same temperature and have the same consistency? For more on gels, try the Science Buddies project Are You Gellin'?
• Clay earth behaves like a colloidal material when it has just the right amount of water in it. Can you find the critical amount of water needed to made a colloidal clay soil? Colloids can appear solid against strong downward forces, but are very weak against lateral forces that push sideways. Try testing your colloid against these two different directional forces. Which direction is your colloid the weakest? If your colloid were a clay soil, how would this contribute to landslides or earthquakes? Can you engineer a way to make colloidal soils resistant to lateral forces during earthquakes and landslides?

• For more science project ideas in this area of science, see Materials Science Project Ideas.

Credits

Sara Agee, Ph.D., Science Buddies

Last edit date: 2006-08-04 12:30:00