You and Your Students!
Vicki Cobb, Education World Science Editor
Oil and water: a mix for learning.
Chemistry, Earth Science
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:
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.
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:
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.
Does that give your students an idea?
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.
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
Copyright © 2004 Education World