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Grades K–2 · Materials & Matter

Why does this oil stay above the water?

Oil and water are both liquids, but these samples do not blend into one even liquid. In the picture, the oil makes a layer above the water. Both liquids are still there.

P22 visual explanationThese oil and water samples do not dissolve into one uniform liquid. Oil droplets can be temporarily dispersed, then gather again. The less-dense oil forms the upper layer when settled; both liquids remain.Oil and water can remain as separate liquidsOilWaterThis oil has less mass in the same-sized space.Settling orders the layers; it does not make either vanish.

In the diagramOil and water can remain as separate liquidsOilWaterThis oil has less mass in the same-sized space.Settling orders the layers; it does not make either vanish.

These settled samples form two layers. The oil is less dense than the water, so it lies above it.

The oil has less mass than water in the same-sized space. We say that this oil is less dense. Once these liquids settle, the less-dense oil sits above the denser water.

Oil can also be spread through water as little drops. Those drops have not turned into water. Without something keeping the drops apart, they can join again and form a separate layer.

About this explanation

Starting knowledge: Recognizes liquids, layers, and comparison of mass in equal-sized spaces.

Sources

  • American Chemical Society — The Density of Liquids
  • American Chemical Society — Density: Sink and Float for Liquids
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