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Density and Buoyancy: Testing Liquids

Resource for Grades K-8

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Running Time: 4m 21s
Size: 13.0 MB

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Density and Buoyancy: Testing Liquids (Document)

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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 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. 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.

If they don't mix, fluids will also either sink in or float on top of other fluids. Just as with solids, the densities of the two fluids determines the buoyancy of one in the other. If a given volume of one fluid is heavier than the same volume of another fluid, the first fluid will sink in the second. If it is lighter, it will float.

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Discussion Questions

What did you see when the plastic building block was put into the glass? Exactly how was it floating? How much of it was in the water? Was the water really helping to hold it up?

How does weighing the liquids tell you about their densities?

If one cup (or 250 ml) of one liquid is heavier than one cup (or 250 ml) of another liquid, how do their densities compare?

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Transcript

(electricity crackling)

RACHEL: Do you think a grape would sink or float in corn syrup? How about water or oil? Well, Paul H. of Eagan, Minnesota wants us to find out.

See, this is corn syrup and it's clear. This is water, and it's blue because we put food coloring in it.

This is oil, and it's yellow.

KALEIGH: Here we go— Okay— so, this is corn syrup. And here comes the water. Oh, that's cool. Almost there?

ZOOMer: The color is really neat.

RACHEL: Awesome. And now for the oil. And... It's so cool. Mm-hmm.

KALEIGH: Density is one of the things that makes things float. These three liquids can float on top of each other because they have different densities.

The most dense is the corn syrup the least dense is the oil, and the density of the water is in between the corn syrup and the oil. Since the oil is the least dense it's able to float on top of everything.

We're going to drop these grapes and see in which liquid it floats and in which liquid it sinks. Drop one in the syrup. (gasps) It floats.

RACHEL: Oh, my goodness.

KALEIGH: Okay, so, let's see if it floats in the water. Here we go. Oh, it sinks. Ready? Oh. It sinks.

If we were right, the grape should fall through the oil and the water and float on top of the corn syrup. So, here. Let's drop it in. Awesome! That's so cool. Look at that.

RACHEL: That's cool, you see, like, bubbles inside, too.

KALEIGH: Yeah— now, we have some plastic building blocks some metal nuts and some styrofoam.

So, which liquids do you think they'll float in?

RACHEL: Um, well, first let me check the weight for... for, like... I think... whoa, this is much heavier than the grape. Oh, yeah. So I would think this one would just sink straight down right through. but I think the plastic building block is less dense than the grape. I think it might float between the water and the oil.

KALEIGH: Here, let's do the metal nut. Ready?

Oh, right to the bottom. Yep, you were right— nice job. Let's drop it in. Let go. Uh-oh, hold on. Let me get those air bubbles out. So, I'll push it down a little. There we go.

RACHEL: Oh, we were right.

KALEIGH: Cool.

RACHEL: It's between the water and the oil.

KALEIGH: Let's see what happens when we push the grape down. Here, I'll reach all the way down there.

RACHEL: (chuckling)

KALEIGH: Oh, it just floats back up.

RACHEL: Oh, that's awesome.

KALEIGH: Yeah, that's pretty neat. I need to wipe my hands. Thank you. Okay. So, want to try the styrofoam?

RACHEL: Yeah.

KALEIGH: Okay, do you want to drop it in? Yep, go ahead. Yeah, push that one down, too. I don't think that one wants to go down. Yeah, I think that one definitely floats on top of the oil.

RACHEL: Oh, that's gross.

BOTH: (both laughing)

KALEIGH: That's really neat.

RACHEL: That's awesome. It's so cool to look at.

KALEIGH: There's one on each level. Yeah. So, the Styrofoam's on top. Then the building block... and the grape... and the nut.

BOTH: Cool.

KALEIGH: The corn syrup is the most dense, the oil is the least dense, and the water is in between the corn syrup and the oil. So, if you're still wondering what density is here you go. See that these three liquids take up the same amount of space in their containers.

But the denser one, the corn syrup, is heavier. So, here's the water... and the corn syrup. Whoa. That's why the water is able to float on top of the corn syrup. And here's the oil. The oil is lighter than the water and that's why the oil can float on top of the water.