NOVA Online | Balloon Race Around The World

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Here's what happens when you mix two of the materials, oil and water, together: (you'll see the action four times)

Gif animation of liquids of different densities separating

Gif animation of liquids of different densities separating

If you poured your materials into a container, they would arrange themselves like this:

GIF animation of the four boxes stacked vertically, boxes filling, water, oil, air, helium from bottom to top

GIF animation of the four boxes stacked vertically, boxes filling, water, oil, air, helium from bottom to top

Look what floats and what sinks! Helium floats on top of everything because it has the lowest density. And the water sinks to the bottom because it has the highest density.

Think about these everyday demonstrations of density . . .

A birthday balloon full of helium: Helium has a lower density than air. That's why a helium balloon rises in air.

A scuba diver under water: Air has a lower density than water. That's why the diver's air bubbles rise to the surface of the water.

A puddle with an oil slick on top: Did you ever notice how streaks of oil sit on top of a puddle? That's because the oil has a lower density than water, so it rises to the top of the puddle.

"But what about hot air?" you're probably saying. Well, you already know that a hot air balloon floats. What, then, would you conclude?

A floating hot air balloon has a lower density than the air which surrounds it.
A floating hot air balloon has a higher density than the air which surrounds it.