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Density and Buoyancy: Making Eggs Float

Media Type:
Video

Running Time: 3m 02s
Size: 3.2 MB

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Source: ZOOM

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Have you ever noticed that it's easier to float in the ocean than in fresh water? In this video segment adapted from ZOOM, two cast members add salt to tap water to illustrate the effect of salt on water and the buoyancy of an egg.

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Density and Buoyancy: Making Eggs Float (Document)

Background Essay

There's nothing magical about density. Simply put, it is the mass of a substance or object relative to the amount of space it takes up.

If two substances differ enough, their relative densities are easy to determine. For example, a piece of lead, which is a dense metal, feels much heavier than a piece of cork of about the same size. An analysis of the densities of the two objects would show that one cubic centimeter of lead, about the size of a small marble, has a mass of more than 11 grams, while one cubic centimeter of cork weighs about one-quarter of a gram.

When materials have such different densities, it is easy to predict which will sink and which will float. But what about other substances? Is there a way to predict whether or not a substance will sink or float in water or any other fluid?

Indeed there is. An object or substance will sink in a fluid if it weighs more than the fluid it displaces when fully submerged. In other words, if a one-cubic-centimeter object weighs more than one cubic centimeter of a fluid -- the amount it would push out of the way if submerged -- it will sink in that fluid. If it weighs less than the same amount of fluid, it will float.

Unlike the densities of solids, which remain relatively constant, the densities of many fluids, in particular gases, can be changed. Heating a gas and allowing it to expand lowers its density. Cooling it or compressing it causes its density to increase.

Dissolving a substance in a fluid also alters the fluid's density. When salt is dissolved in fresh water, for example, the density of the resulting saltwater solution is greater than that of the fresh water. An object that has a density only slightly higher than that of fresh water, such as an egg, will float in salt water if there is enough salt in the solution to make it denser than the egg.

Discussion Questions

• How did the ZOOM cast members decide how much salt to add?
• Why did they decide to use salt?
• What did they need to measure to compare the densities of salt and fresh water?
• How did weighing the water help them compare densities of salt and fresh water?
• Try this activity with sand instead of salt. Does an egg float in a mixture of sand and water? Why or why not?

• Transcript

Have you ever tried to float in an ocean? How about in a lake? Did you notice a difference? I've tried to float in a lake and in an ocean, and I thought it was a lot easier in the ocean, because the salt helps hold me up. And Katie N. of Preston, Washington, says that salt can do amazing things when put to the test. She says that we can use it to have an egg float in water. Here's what happens when we drop an egg in a glass of water without any salt in it. Bye-bye, egg. It sinks right to the bottom. I know. Now let's see if we can add salt and if that'll make it float. So do you want to add one tablespoon first? Sure, we'll try that. So that's one. Now we'll stir it up. It kind of went up. I know, it, like, jumped, kind of. Let's just stir it a tiny bit. It's cool, it looks kind of like a cloud. Yeah. Oh, it didn't make too much of a difference, so why don't you try another tablespoon? Two, whoa, yeah, it goes way up. It takes a little while for the salt to dissolve into the water, then it starts floating. Okay. Three. Oh, wow, that went way up. Stay up, little eggy. Stay up. It's, like, right in the middle, kind of. Yeah. This is our fourth. Oh! Wow! It floated! Four-- that's right at the top. It's so cool. Jim of Arizona also experimented with this, and he sent us this Sci Scoop. He said that when we add salt into the water, we're smooshing more molecules into the water. It makes the water more dense, so when there was no salt in the water, the egg was more dense than the water and the egg sank. But when we added salt to the water, the water was more dense than the egg, and the egg floated. Here's a cup of fresh water and here's a cup of salt water. They're taking up the same amount of space, but if you weigh them, you can see that the salt water weighs more than the fresh water. Okay. So I'll fill it up on this side. Okay, so that's that. And...Fill it up right over...Fill it to the 200. Yup. See? Look how much the salt water...Look at the difference. So they're the same amount of water, but there's more stuff inside the salt water, so the salt water's more dense, so it's heavier. Try this at home. Think of a question that you want answered, like, would sugar dissolved in water help an egg float? How about soap? Make a prediction. Test it out.

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