Key Concepts
Density, mass, volume, concentration, buoyancy, water

Introduction

Have you ever wondered why some objects float on top of the ocean, and other objects sink to the bottom? It has to do with the density of the objects compared to the density of the salt water surrounding them in the ocean. If you add salt to plain water, it increases the density of the water. In fact, if you add enough salt, you can make the water so dense that an egg will actually float in it! Explore how this works in this science activity. 

This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

Background

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 is this? Because the density of the egg is higher than the density of tap water, the egg sinks. Density is the mass of a material per unit volume. For example, the density of freshwater under standard conditions is approximately one gram per cubic centimeter. Adding salt to the water increases the density of the water because the salt increases the mass without changing the volume very much. 

When enough salt is added to the water, the saltwater solution’s density is higher than the egg’s, and the egg will then float! The ability of something, like the egg, to float in water or some other liquid is known as buoyancy. But just how much salt is needed to make an egg float? In this science activity, you’ll figure that out by making salt solutions with varying concentrations of salt in them.  

Materials

  • An egg
  • Water
  • Measuring cup
  • Large container, such as a large bowl or cooking pot. It must be able to hold at least three cups.
  • ½ cup table salt
  • Five cups that hold at least 16-oz. each
  • Permanent marker (if you are using plastic cups) or masking tape and a pen to label the cups
  • Three spoons for mixing salty solutions
  • Soup spoon for egg transfers

Preparation

  1. Take the egg out of the refrigerator and allow it to warm to room temperature. Be sure to always wash your hands after handling uncooked eggs because they may carry Salmonella. 
  2. Pour 1 ½ cups of water into your large container. 
  3. Add ½ cup of salt to the large container and stir to dissolve some of the salt (it will not all dissolve yet). 
  4. Add one more cup of water to the large container (making 2 ½ cups total) and stir to dissolve the rest of the salt. The salt should be completely dissolved before you go on to the next step. It may take several (five to ten) minutes of stirring, so you may need to be patient. Why do you think it’s important to start out with a solution that has such a high concentration of salt?
  5. Arrange the five cups on a surface, going in a line from left to right. Label the cups one to five. If you are using plastic cups, you can use a permanent marker to label them. If you are using non-disposable cups, you can use masking tape and a pen to label them.

Procedure

  1. Add ¾ cup of the salty solution you prepared to cup one.
  2. Add ¾ cup of plain tap water to cups two to five. (Cup five will be plain tap water.)
  3. Add ¾ cup of the salty solution you prepared to cup two and mix it. What is the salt concentration in cup two compared to cup one?
  4. Add ¾ cup of the salt solution from cup two to cup three and mix it. What is the salt concentration in cup three compared to cups one and two?
  5. Add ¾ cup of the salt solution from cup three to cup four and mix it. What is the salt concentration in cup four compared to the other cups?
  6. Use a soup spoon to place an egg in cup five. Does the egg float? 
  7. Use the spoon to take the egg out and place it in cup four. Does the egg float?
  8. Repeat this process with cups three, two and then one. In which cup does the egg first float? If the egg floated in more than one cup, did you notice any difference in how it floated? What does this tell you about the density of the egg?

Extra: In this science activity you figured out, within a factor of two, how much salt it takes to float an egg. You could narrow down the range further by testing additional salt water dilutions to try and determine the density of the egg. To do this, start your dilution with the salt concentration in which the egg first floated and make a new dilution series, as you did before. Now in which cup does the egg first float? What does this tell you about the density of the egg?

Extra: Repeat this activity using several more eggs, possibly both hardboiled eggs and uncooked eggs. Do you get the same results with other eggs, or is there some variation between different eggs? For testing hardboiled vs. raw eggs, you should test the same egg raw and then after hard-boiling it to investigate any differences.

Extra: Find out how much salt there is in sea water. From the results of your activity, do you think an egg would float or sink in sea water?

Observations and Results

Did the egg float in cup one and two, but not in cup three, four or five? 

You likely saw that the egg floated best in cup one, floated a little less in cup two (but still had part of the egg above the water level), and did not float in the other cups. Cup one had the undiluted salty solution that you originally prepared, which was ½ cup salt in 2 ½ cups water total. The concentrations of the salt solutions in cups two to four were halved as you increased in cup number; for example, the concentration of the salt in cup two was half that of cup one, and the concentration of the salt in cup three was half that of cup two. (Cup five had plain tap water.) The egg should have sank in cups three, four and five because the density of the egg was higher than the density of the solutions (or plain tap water) in those cups. Cups one and two had more salt in them than the other cups (with cup one having the most salt), which means these solutions were denser. The egg should have floated (with part of it above the water level) in these two cups because the solutions were denser than the egg. The actual density of the egg is in between the density of the solution in cup three and the solution in cup two.

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Credits

Teisha Rowland, PhD, Science Buddies

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Key Concepts
Density, mass, volume, concentration, buoyancy, water
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