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In terms of density:

Diagram of four boxes showing density of molecules packed in boxes

Diagram of density formula: mass divided by volume

The dots above (representing molecules) show one reason why one material has more or less density than another. Can you see how the molecules of the densest material, water, are more closely spaced than the others? Notice that the molecules of the gasses (air and helium) are much farther apart than those of the liquids (water and oil). Another reason why one material could have a higher density is that the molecules themselves have different masses. This is what causes most of the difference between air and helium.

Note: This illustration is meant to represent that the spacing between atoms and molecules of the materials are different. It is not meant to depict the exact spacing or shape of the molecules of each substance.

Now let's pour two of the materials, oil and water, into one container. But first, predict what might happen. OK, I'm ready to pour.

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