JON BAUGH / SCIENCE PHOTO LIBRARY JON BAUGH / SCIENCE PHOTO LIBRARY
Animation of a mixture of water and hexane, a hydrocarbon, showing their immiscibility. When the liquids are mixed, they separate out into two layers, with the denser water beneath. Their immiscibility is due to their chemical make-up. Water is a polar molecule, and has a separation of electric charge. Negative charge is concentrated on the oxygen atom (red), and positive charge (blue) is concentrated on the hydrogens. As the molecule is a V-shape rather than completely symmetrical, this means it has positive and negative ends, which attract the opposite ends of adjacent molecules, forming short-lived hydrogen bonds (black lines). Hexane, however, is a symmetrical molecule made up of six identical carbon atoms (black), bonded to hydrogen atoms. There is no separation of charge on the molecule, making it non-polar. Hexane molecules interact with each other only through the much weaker van der Waals forces (green glow), due to very short-lived and fluctuating variations in electron density around the molecule. Each molecule prefers to be surrounded by others that will interact in the same way, so the two liquids do not mix. Both, however, are miscible with some molecules containing polar and non-polar parts. This can be seen in the mixture of water with ethanol (clip K004 5234) and hexane with ethanol (K003 5378).
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