• Question: Why does oil go to the top of water and other substances? Are there any other liquids that do this too?

    Asked by #nerdyweirdo to Chris, Josh, Rebecca, Rob, Susan on 22 Jun 2015. This question was also asked by Aoife.
    • Photo: Rebecca Dewey

      Rebecca Dewey answered on 22 Jun 2015:

      Oil is less dense than water, because the molecules in oil won’t get close enough together that enough of them fit into a small enough space that the density goes up. We describe this as the “specific gravity” of a substance: the ratio between the density of oil and the density of water.

      In addition to that, the way the electrons are arranged on the edges of the molecules mean that oil and water do not mix, and in fact repel each other a bit.

      Ethanol (alcohol) also has a lower specific gravity than water and so will float to the top, but it can be persuaded to mix with water (unlike oil).

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