May 10, 2006

Salad Dressing: A Liquid Suspended in a Liquid

If you shake oil and water together and then let them stand, they will separate into two layers. Liquids that do not form solutions are said to be immiscible

Classic French salad dressing (or vinaigrette) is a mixture of oil and vinegar and seasoning. Vinegar is a water-based substance (an acid) and is immiscible with oil. In order for all the flavors of the dressing to be evenly spread through a salad, it must be thoroughly mixed. A vinaigrette is usually given a number of hard shakes and immediately poured on a salad before the two liquids have a chance to separate.

This experiment is designed to answer the question: Does the size of the droplets of two immiscible liquids affect the rate of separation into layers?

Materials & Equipment

• 1/3 cup vinegar
• 1/2 teaspoon salt
• 1/4 teaspoon pepper
• 1/4 teaspoon garlic powder
• 1/4 teaspoon paprika
• 1 cup salad oil
• measuring cups and spoons
• a jar with a tight cover
• watch or clock with a second hand
• a small bowl
• an eggbeater or electric mixer
• a magnifying glass

Procedure

1. Put the vinegar in the jar and add the salt, pepper, garlic powder, and paprika. Screw the lid on tight and shake.
2. Pour in the salad oil and let the mixture stand for a few minutes.

Observations

Where does the oil go? Which do you think would be heavier, a cup of water or a cup of oil? Can you think of a way to check your guess?

Procedure

1. Cover the jar and shake about 10 times. Use the watch to see how long it takes for the mixture to separate. Can you see which is the continuous phase and which is the discontinuous?
2. Shake the jar hard about 20 times. Does the dressing take more or less time to separate? Look for droplets of vinegar suspended in the oil.
3. Shake the jar different numbers of times and examine the size of the droplets immediately after shaking. When does the dressing have the smallest droplets?
4. Put the mixture in a small bowl and beat hard for about 4 minutes with an eggbeater or an electric mixer. Quickly pour the dressing back into the jar. Examine the droplets with a magnifying glass. How long does the dressing take to separate into two layers? What did shaking and beating do to the size of the droplets?

You can use the French dressing over a tossed green salad. Mix well and use just enough to coat the leaves lightly.

Reprinted with permission from Science Experiments You Can Eat by Vicki Cobb. Text © 1972, 1994 by Vicki Cobb (www.vickicobb.com). Published by HarperCollins.

Talk Back: Do you have any comments about this activity? Send them to us using the form below.

I have my parent's permission to submit this.
First name:		Age:
City:		State:	AK AL AR AZ CA CO CT DC DE FL GA HI IA ID IL IN KS KY LA MA MD ME MI MN MO MS MT NC ND NE NH NJ NM NV NY OH OK OR PA RI SC SD TN TX UT VA VT WA WI WV WY
E-mail:
Comment: