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Float an Egg
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Developer:
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Great Ideas from DuPont Teaching Fellows
Compiled by the Kinston, NC
DuPont Communication Team
March, 1994
| Charles Scaife
Department of Chemistry
Union College
Schenectady, NY 12308
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Grade Levels:
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2 through 4
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Discipline:
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Physical Science
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Goals:
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Try to float an egg in water and in salt water.
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Background:
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Pure water has very specific properties. Pure water is colorless, odorless, and tasteless. Pure water freezes at a certain temperature, 0°C. Pure water boils at a certain temperature, 100°C. Pure water has a certain density (mass divided by volume) equal to 1 gram per milliliter; thus, each milliliter of pure water weighs 1 gram near room temperature.
The density of an object compared to the density of water determines whether the object will float in the water or sink into the water. An object which is less dense than water floats in the water. An object which is more dense than water sinks into the water. An object which is about the same density as water barely floats but almost completely under the surface of the water.
Many properties of water change as substances are dissolved in the water. You will observe a change of one of the properties mentioned above in this experiment. This experiment and related ones are discussed by various authors (1, 2, 3).
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Materials:
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1-pint Glass jar or wide-mouth plastic container
Raw chicken egg in the shell
Measuring spoon (1 teaspoon)
Table salt
Stirring stick (wooden skewer, wooden splint, or even a straw)
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Explanation:
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The raw egg has a higher density than pure water as illustrated by the observation that the raw egg sinks in pure water. The mass of water displaced by the egg is less than the egg's mass, and the resultant upward buoyant force on the egg is too small to allow it to float. However, the raw egg must eventually have a lower density than the salt water as illustrated by the observation that the raw egg floats in salt water containing sufficient salt. In this case the mass of salt water displaced is sufficiently large as to equal the egg's mass, and the resultant upward buoyant force is large enough to cause the egg to float.
Since objects more dense than water sink and objects less dense than water float, adding table salt to the pure water increases the density of the water.
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Questions:
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How does the density of the raw egg compare to the density of pure water?
What is the difference between the pure water in which the egg sinks and the salt water in which the egg floats?
Do you expect the egg to float in rubbing alcohol? Why, or why not?
Would a person float better in pure water or in salt water? Why? If you ever swam in Great Salt Lake in Utah, describe your experiences to the class. Do your experiences agree with your conclusion to the previous part of Question 4?
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References:
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Tik L. Liem, Invitations to Science Inquiry,
2nd ed., Science Inquiry Enterprises, Chino
Hills, CA, 1987, (4.38) p. 128.
Janice Pratt VanCleave, Chemistry for Every Kid: 101 Easy Experiments Tjat Really Work, Wiley, New York, NY, 1989, pp. 30-31.
Mickey Sarquis and Jerry Sarquis, Fun with Chemistry: A Guidebook of K-12 Activities, vol. 1, Institute for Chemical Education, University of Wisconsin, Madison, WI, 1991, pp. 43-45.
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Procedure:
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Fill the glass jar or plastic container about two-thirds full with pure water. Put a raw chicken egg beside the container, and ask students to predict whether the egg will sink or float. Then give the students a chance to check their prediction. Tell them to place the egg carefully into the water and observe what happens.
Ask students to predict whether or not the egg will float when table salt is added to the water. Then give the students some table salt to test their prediction. Tell them to add one level teaspoon of table salt to the water, stir carefully and thoroughly, and observe what happens. Repeat this process of adding one teaspoon of salt at a time, stirring, and observing what happens until the egg floats. How many teaspoons of table salt are required to float the egg?
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