Abstract

Objective

The goal of this science fair project is to determine what salt concentration will float an egg.

Introduction

Did you know that if you put an egg in a cup of tap water, it will sink to the bottom? But, if you add enough salt, the egg will float back up to the surface! Why? Because the density of the egg is higher than the density of tap water, so it sinks. Density (ρ), as shown below in Equation 1, is the mass (m) of a material per unit volume (v). For example, the density of freshwater under standard conditions is approximately 1 gram (g) per cubic centimeter (cm³). In other words, if you filled a 1-cm x 1-cm x 1-cm box with freshwater, the water inside the box would have a mass of 1 g. Adding salt to the water increases the density of the water, because the salt increases the mass without changing the volume very much. With enough added salt, the saltwater solution density is higher than the egg's, and the egg will then float.

Equation 1:

But just how much salt is needed to make an egg float? In this science fair project, you will figure that out by using the technique of making serial dilutions. 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, as shown in Equation 2.

Equation 2:

If the volume of stock solution and the volume of water are equal, you'll be diluting the solution by half. This is called a two-fold dilution. If you want bigger steps, you should use relatively more water; if you want smaller steps, you should use relatively less water. By repeating the process, you can make a whole series of dilutions, which is how the method got its name. In this science fair project, you will start out using two-fold dilutions to find out how much salt will cause an egg to float.

Terms, Concepts, and Questions to Start Background Research

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

• Density
• Mass
• Volume
• Serial dilution
• Stock
• Concentration
• Relative concentration
• Absolute concentration

Questions

• What happens to salt (NaCl) molecules when dissolved in water?
• Why does adding salt to water increase its density?

Bibliography

• Ophardt, C. E. (2003). Density – a Physical Property. Virtual Chembook, Elmhurst College. Retrieved June 19, 2009, from http://www.elmhurst.edu/~chm/vchembook/120Adensity.html
• Kenyon College. (n.d.). Serial Dilutions. Retrieved June 19, 2009, from http://biology.kenyon.edu/courses/biol09/tetrahymena/serialdilution1.htm
• Swenson, H. (n.d.). Why Is the Ocean Salty? U.S. Geological Survey Publication. Retrieved May 4, 2006, from http://www.palomar.edu/oceanography/salty_ocean.htm

Materials and Equipment

• Bag of clear 16-oz. plastic cups
• Table salt
• Water
• Measuring cup, liquid
• Container, 1-quart (qt.)
• Spoon for stirring
• Spoon for egg transfer
• Eggs (5)
• Lab notebook

Experimental Procedure

1. Take one egg out of the refrigerator and allow it to warm to room temperature.
2. Make a stock solution of 1 cup of salt dissolved in 1 qt. of water, as follows:
   1. Pour 3 cups of water into your 1-quart container.
   2. Add 1 cup of salt.
   3. Stir to dissolve.
   4. Add more water to make 1 qt.
   5. Stir to mix completely.
3. Make a two-fold serial dilution of the stock solution, as follows:
   1. Label four of the plastic cups 1–4. Label the fifth 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 2–5.
   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? Example: Cup 2 is made up of half stock solution and half tap water, which is a 50 percent relative salt concentration.
   8. What are the absolute salt concentrations of cups 1–4? (If you want to convert to metric units, 1 cup of salt is about 292 g, and 1 qt. of water is 0.946 liters [L].) 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. Use a soup spoon to lift the egg 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 percent with each step. That means with each step, the new concentration should be 80 percent 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.
10. Repeat the entire procedure with the four other eggs. Plot the densities for all five eggs on a chart. How much variation in density is there from egg to egg?

Variations

• Does a hard-boiled egg float at the same salt concentration as an uncooked one? Hint: You’ll need to measure the same egg before and after hard boiling and be very precise about your serial dilutions.
• 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 test your prediction!)
• Figure out another method of determining the density of an egg. Compare the density measurements for the same eggs using your method and this salt water float 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, PhD, Science Buddies

Sandra Slutz, PhD, Science Buddies

Sources

This science fair project idea came from the following source:

• ONR. (n.d.). Resources: Experiments—Saltwater vs. Freshwater, which is Denser? Retrieved May 4, 2006, from http://www.onr.navy.mil/Focus/ocean/resources/water2.htm

Last edit date: 2010-12-02 10:30:47