Key concepts
Density
Mass
Volume
Concentration
Buoyancy
Water

Introduction
Have you ever wondered why some objects float in water and others sink? It has to do with the density of the objects compared with the density of the water surrounding them. If an object is less dense than the water around it, it will float. Because salt water is denser than freshwater, some things float more easily in the ocean—or extremely salty bodies of the water, such as the Dead Sea. You can make your own dense water by adding salt to tap 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.

Background
If you put an egg in a cup of tap water, it will sink to the bottom. Why is this? Because the density of the egg is higher than the density of tap water, so it 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.

But, if you add enough salt to the water, the egg will actually float back up to the surface! Adding salt to the water increases the density of the solution 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 becomes higher than the egg's, so 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 solutions with varying concentrations of salt in them.

Materials

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

Preparation

• 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.
• Pour one and one half cups of water into your large container.
• Add one half cup of salt to the large container and stir to dissolve some of the salt (it will not all dissolve yet).
• Add one more cup of water to the large container (making two and one half cups total) and stir to dissolve the remaining salt. The salt should be completely dissolved before you go on to the next step. It may take several (five to 10) 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?
• Arrange the five cups on a surface, going in a line from left to right. Label the cups 1 to 5. If you are using plastic cups, you can use a permanent marker to label them. If you are using nondisposable cups, you can use masking tape and a pen to label them.

Procedure

• Add three quarters cup of the salty solution you prepared to cup 1.
• Add three quarters cup of plain tap water to cups 2 through 5. (Cup 5 will be plain tap water.)
• Add three quarters cup of the salty solution you prepared to cup 2 and mix it. What is the salt concentration in cup two compared with cup one?
• Add three quarters cup of the salt solution from cup 2 to cup 3 and mix it. What is the salt concentration in cup 3 compared with cups 1 and 2?
• Add three quarters cup of the salt solution from cup 3 to cup 4 and mix it. What is the salt concentration in cup 4 compared with the other cups?
• Use a soup spoon to place an egg in cup 5. Does the egg float?
• Use the spoon to take the egg out and place it in cup 4. Does the egg float?
• Repeat this process with cups 3, 2 and then 1. 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 saltwater solutions to try and determine the egg’s density. To do this, start your solution 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 hard-boiled and uncooked eggs. Do you get the same results with other eggs or is there some variation between different eggs? For testing hard-boiled versus raw eggs, you should test the same egg, first raw and then after hard-boiling it to investigate any differences.
• Extra: Find out how much salt there is in seawater. From the results of your activity, do you think an egg would float or sink in seawater?

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Observations and results
Did the egg float in cup 1 and 2, but not in cups 3, 4 or 5?

You likely saw that the egg floated best in cup 1, floated a little less in cup 2 (but part of it was above the surface) and did not float in the other cups. Cup 1 had the undiluted salty solution that you originally prepared, which was one half cup of salt in two and one half cups water total. The concentrations of the salt solutions in cups 2 to 4 were halved as you increased in cup number; for example, the concentration of the salt in cup 2 was half that of cup 1, and the concentration of the salt in cup 3 was half again of cup 2. (Cup 5 had plain tap water.) The egg should have sunk in cups 3, 4 and 5 because the density of the egg was higher than the density of the solutions (or plain tap water) in those cups. Cups 1 and 2 had more salt in them than the other cups (with cup 1 having the most salt), which means these solutions were denser. The egg should have floated (with part of it above the water surface) 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 3 and that in cup 2.

More to explore
What Is Density?, from Charles E. Ophardt, Elmhurst College
Why Is the Ocean Salty?, from Herbert Swenson, U.S. Geological Survey Publication
Fun, Science Activities for You and Your Family, from Science Buddies
How Salty Does the Sea Have to Be for an Egg to Float?, from Science Buddies

This activity brought to you in partnership with Science Buddies