Try This Eggsperiment to Measure the Density of Salt Water

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• Science Fair Project Guide

Project Summary

Difficulty	5
Time required	Very Short (a day or less)
Prerequisites	None
Material Availability	Readily available
Cost	Very Low (under $20)
Safety	No issues

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Objective

The goal of this project is to measure the density of salt water by determining what salt concentration will float an egg.

Introduction

If you put an egg in a cup of water, it will sink to the bottom. The density of the egg is higher than the density of tap water, so it sinks. Adding salt to the water increases the density of the water. With enough added salt, the salt water solution density is higher than the egg's, and the egg will float. In this project, you will use the technique of making serial dilutions to figure out how much added salt is needed to float an egg.

A serial dilution is a method for accurately diluting a solution in regular steps. You add a known amount of your starting (or stock) solution to a known amount of water and mix them. The new concentration will be determined by the ratio of the volume of stock solution to the total volume:

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• density,
• flotation,
• ions.

Questions

• What are the atomic weights of the elements used for the solutions in this experiment (H, O, N, Cl)?
• What happens to salt (NaCl) molecules when dissolved in water?
• Why does adding salt to water increase its density?

Bibliography

• Swenson, H., date unknown. "Why Is the Ocean Salty?" U.S. Geological Survey Publication [accessed May 4, 2006] http://www.palomar.edu/oceanography/salty_ocean.htm.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• a bag of clear 16 oz plastic cups,
• table salt,
• water,
• measuring cup,
• quart container,
• spoon for stirring,
• spoon for egg transfer,
• 1 egg.

Experimental Procedure

1. Take an egg from the refrigerator and allow to warm to room temperature.
2. Make a stock solution of 1 cup of salt dissolved in 1 quart of water.
   1. Pour 3 cups of water into your quart container.
   2. Add 1 cup of salt.
   3. Stir to dissolve.
   4. Add enough water to make one quart.
   5. Stir to mix completely.
3. Make a two-fold serial dilution of the stock solution.
   1. Label 4 of the plastic cups, 1–4. Label the 5th cup, "tap water." Cup 1 will be for the stock solution, cups 2–4 will be for the dilutions.
   2. Add 3/4 cup of your stock salt solution to cup 1.
   3. Add 3/4 cup plain tap water to cups 2–4.
   4. Measure out 3/4 cup stock solution, and add it to cup #2. Mix.
   5. Measure out 3/4 cup of the solution from cup 2 and add it to cup 3. Mix.
   6. Measure out 3/4 cup of the solution from cup 3 and add it to cup 4. Mix.
   7. What are the relative salt concentrations of cups 1–4?
   8. What are the absolute salt concentrations of cups 1–4? (If you want to convert to metric units, a cup of salt is about 292 grams, and 1 quart of water is 0.946 liter.) Write these concentrations down in your lab notebook.
4. Now, starting with cup 5 and working your way up, test the egg in each solution to see if it will float. You can use a soup spoon to lift it in and out of the cups.
5. In which cup did the egg first float? (Save this solution for step 7.) If the egg floated in more than one cup, did you notice any difference in how it floated?
6. Now you know within a factor of 2 how much salt it takes to float an egg. How can you narrow down the range further to get a more precise estimate? By doing another serial dilution, of course.
7. This time you will start your dilution with same the salt concentration in which the egg first floated.
   1. Figure out a new serial dilution with smaller steps. For example, you could try diluting the solution by 20% with each step. That means with each step the new concentration should be 80% of the original concentration.
   2. What amounts of stock solution and water do you need to use? (Remember that you will need enough solution to more than cover the egg.)
   3. Write up your new dilution procedure in your lab notebook, including the calculated salt concentrations for each cup.
   4. Make the new dilution series. Remember to start with salt concentration where the egg first floated. (If you don't have enough solution from the original serial dilution, make some more by starting from the stock solution.)
8. As before, test the egg in each cup, starting with the lowest salt concentration. In which cup did the egg float first?
9. If you want, make another dilution series with even smaller steps to improve the precision of your estimate.

Variations

• Does a cold egg (fresh from the refigerator) float at the same salt concentration as a warm one?
• Find out how much salt there is in sea water. From the results of your experiment, predict whether an egg would float or sink in sea water. (If you live close enough to the ocean, you can get collect some sea water and try it.)
• Here is another method you can try. You'll need a hydrometer (available from aquarium stores that sell marine fish), a graduated cylinder, and a scale for weighing the egg. A hydrometer is a device used to measure the density of a liquid. It's a sealed glass cylinder, weighted at the bottom so that it floats upright. The density of the liquid is read from a printed scale at the level the hydrometer floats in the liquid. Measure the density of the egg (see the Science Buddies project, How Dense Is the Incredible Edible Egg?, for an easy way to do this). Make a concentrated salt solution by adding 1/2 cup of table salt to 1 pint of water. Use the hydrometer to measure the density of this solution. Rinse and dry the hydrometer, then measure the density of a plain tap water solution. Use a ratio to calculate how much of each solution do you need to mix in order to float the egg. Measure, mix and test.
• For another way of looking at salt water density, see the Science Buddies project Can Water Float on Water?.

• For more science project ideas in this area of science, see Ocean Sciences Project Ideas

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

• Project idea from: ONR, date unknown. "Resources: Experiments - Saltwater vs. Freshwater, which is Denser?," Office of Naval Research [accessed May 4, 2006] http://www.onr.navy.mil/Focus/ocean/resources/water2.htm.

Last edit date: 2006-08-15 17:11:47

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