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Spheres of Influence

Starring

You and Your Students!

Script By

Vicki Cobb, Education World Science Editor

Synopsis

Oil and water: a mix for learning.

Genre

Chemistry, Earth Science

Props Required

  • a large clear glass jar with a screw-top lid, all labels removed
  • vegetable oil to fill the jar
  • water
  • blue, red, or green food coloring
  • teaspoon
  • magnifying lenses

Setting the Scene (Background)

It's an accepted fact that oil and water don't mix. You can use this activity -- guaranteed to enthrall and fascinate your students -- for hands-on science fun. The activity is full of science implications.

This is the first of three related activities that will give you a better understanding of homogenized milk, salad dressing, face cream, and butter making. See the other two activities in this series:

Stage Direction


Show-Biz Science is scripted by popular children's book writer Vicki Cobb. Click to learn more about Vicki or to read a brief synopsis of her philosophy of teaching science.

Visit our archive of archive of Show-Biz Science Activities. Watch for a new activity each week. Then chat with Vicki -- share your feedback and ask your questions about teaching science -- on our special Showbiz-Science message board.

Be sure to visit Vicki's Kids' Science Page for more great science fun, a complete list of her books, and information about how you can invite Vicki to come to your school. And don't miss her library of science videos too. Or visit Vicki and other great authors of nonfiction for children at the INK Think Tank.
 

I've set up this activity as a demonstration; if you want to have your students do it themselves, have them work in groups of three or four. Each group will need its own jar, vegetable oil, colored water, teaspoons, and magnifying lenses.

The Plot

Act I
Drop a few drops of food coloring into a glass half filled (or less) with water. Then drop a teaspoon of colored water into the jar of oil. The water instantly becomes a spectacularly beautiful and perfect colored sphere! (You might see several spheres that slowly drift to the bottom of the jar of oil.) Use the teaspoon to drop several more samples of colored water of different amounts into the jar.

Following are some questions to ask about what your students are seeing:

  • What shape are the drops?
  • Why are they spheres? (Let your students share their thoughts before answering. They are spheres for the same reason the earth, moon, planets, and stars are all spheres. A sphere is nature's most perfect shape. Absent all other forces, liquids will assume the shape of a sphere. It is the most efficient shape, too, because it has the smallest surface area for the largest volume.)
  • Which is heavier -- the oil or the water? (the water) How do you know? (because the water sinks to the bottom)

Act II
It's clear that the oil and water are not mixing. What happens if you try to mix them by adding a force? Screw the top on the jar and give it a single very hard shake. The force of that shake is enough to break up the water in to a number of small spheres that swirl through the oil.

  • Have you mixed the oil and water? (no, the force has just broken up the water into many smaller spheres)
  • What happens if you just let the jar rest? (The water spheres settle to the bottom.)
  • Are all the spheres on the bottom? (If you look closely, you might see some very tiny spheres near the top of the oil. Those spheres have not settled out. They are so small that the buoyant force of the oil is strong enough to keep them afloat.)
Does that give your students an idea?

Act III
With the top securely screwed on, shake the jar as hard as you can for several minutes. When you stop shaking the jar, it will appear as if the oil and water have been thoroughly mixed. Look closely with a magnifying glass at the edges of the mixture. You will see that you have succeeded in breaking the water up into gazillions of extremely tiny drops that stay suspended in the oil for a while before settling out. Each water droplet is surrounded by oil.

What happens over time to the water droplets?

Behind the Scenes

You have just created a suspension of water in oil. If the water droplets are small enough, the suspension is fairly stable and is called an emulsion. You can add a few drops of detergent and shake again. Detergent is an emulsifying agent that can stabilize the mixture even further.

The End

Here's an application that will bring this lesson home to your kids:
Milk from a cow contains cream, which is mostly fat. The cream rises to the surface where it can be skimmed off (to make skim milk). Years ago, milk was delivered to families' homes; it was delivered each day in glass bottles. Before drinking that milk, you had to shake it up to mix in the cream. Today we can buy homogenized milk in many places. Homogenized milk is a milk-cream mixture that has been shaken up so much that the cream droplets are so small they are permanently suspended in the milk. Homogenized milk is an emulsion. The protein in the milk acts as an emulsifying agent.

See additional experiments with food in Vicki Cobb's book Science Experiments You Can Eat (HarperCollins, 1994).

Coming Next Week: I'll tell you how all this emulsion stuff applies to salad dressing.


Article By Vicki Cobb
Education World®
Copyright © 2004 Education World
 

09/24/2004

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