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Determine the Effect of Emulsifiers on a Heterogeneous Mixture

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Author: Janice VanCleave

So You Want to Do a Project about Emulsifiers!

Let's Explore

Purpose

To determine the effect of detergent on an oil-and-water mixture.

Materials

  • masking tape
  • pen
  • 2 empty 20-ounce (600-mL) water bottles with caps
  • tap water
  • 1 teaspoon (5 mL) liquid cooking oil
  • 1/2-teaspoon (2.5-mL) measuring spoon
  • 1 teaspoon (5 mL) dishwashing liquid

Procedure

  1. Use the tape and pen to label the bottles "A" and "B."
  2. Fill bottle A about three-fourths full with water and add 1;2 teaspoon (2.5 mL) of oil to the bottle of water.
  3. Repeat step 2 with bottle B.
  4. Add the dishwashing liquid to bottle B, and then seal each bottle with its cap.
  5. Shake each bottle 20 times, and then allow both bottles to stand for 3 minutes.
  6. Observe the contents of each bottle.

Results

At first the oil and water in each bottle mixed. After standing, the contents of bottle A separated into two layers, while the contents of bottle B remained mixed.

Why?

It is commonly said that oil and water do not mix. When oil and water are put in the same container and stirred or shaken, the oil breaks up into globules. This combination of two or more substances that is not the same throughout is called a heterogeneous mixture. After standing, the oil and water separate into two layers. Dishwashing liquid is a detergent (a chemical that cleans, especially if it removes oily dirt). Detergent molecules are emulsifiers, which means they cause oil and water not to separate. In this investigation, the detergent keeps the oil suspended in small globules in the water. This occurs because the detergent molecules have two ends, one end that repels (pushes away) oil but attracts water and an opposite end that repels water but attracts oil.

Cleanup

Shaking the bottles causes some of the oil to break into microscopic globules. In bottle B, the detergent molecules cover the oil globules by embedding their oil-attracting ends in the oil globules. Their water-attracting ends stick out and have a negative charge. Since like charges repel each other, the negatively charged ends of the detergent molecules that cover the oil globules repel each other. The oil globules stay suspended in the water. In bottle A, the oil globules coalesce Goin together) and float to the top of the bottle.

Cleanup

For Further Investigation

There are different kinds of dishwashing liquids. Which cleans oily dishes better—thick or thin dishwashing liquid? A project question might be, What effect, if any, does the viscosity (measure of thickness) of detergent have on how it emulsifies?

Clues for your Investigation

  1. Repeat the investigation with detergents of different viscosities.
  2. To measure the viscosity of a detergent, put a drop of dishwashing liquid on one end of a metal baking sheet. Raise this end of the pan about 450 and watch the drop move down the pan. Rate the viscosity of each detergent by how fast the drops flow down the pan, with the most viscous detergent being the one with the slowest-moving drops.
  3. To rate the emulsifying ability of each detergent, add 1/2 teaspoon (2.5 mL) of oil to the bottle at a time. Continue adding oil until no more oil mixes with the water. When the additional oil no longer mixes with the water, do not include that amount of oil in your total.

Cleanup

References and Project Books

Churchill, E. Richard. 365 Simple Science Experiments with Everyday Materials. New York: Black Dog & Leventhal, 1997.

Potter, Jean. Nature in a Nutshell for Kids. New York: Wiley, 1995.

Reader's Digest. Why in the World? Pleasantville, N.Y.: Reader's Digest, 1994.

Soucie, Gary. Whats the Difference between Lenses and Prisms and Other Scientific Things? New York: Wiley, 1995.

VanCleave, Janice.]anice VanCleaves Food and Nutrition for Every Kid. New York: Wiley, 1999.

Janice VanCleave's Molecules. New York: Wiley, 1993.

Wellnitz, William R. Homemade Slime and Rubber Bones! Blue Ridge Summit, Pa.: Tab Books, 1993.

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