• Question: what is Polymorphism and allotropy?

    Asked by to Daren, Lynne, Phillip, Simon on 19 Jun 2014. This question was also asked by .
    • Photo: Daren Fearon

      Daren Fearon answered on 19 Jun 2014:

      In chemistry and crystallography, polymorphism is the ability of a molecule to pack in different ways to form a crystal. This can effect things like solubility.

      Allotropy is the ability of an element to exist in different forms. A good example of this is carbon, which can exist as soot, or graphite or diamonds!

    • Photo: anon

      anon answered on 19 Jun 2014:

      A good way of understand chemistry words is to break them down into little bits. Poly means many and morph means form – so polymorphism means many forms – or many arrangements of the same thing.

      Daren covered allotrope perfectly – I can’t answer it any better than that!

    • Photo: Lynne Thomas

      Lynne Thomas answered on 19 Jun 2014:

      A crystal is essentially a three dimensional brick wall where atoms or molecules are the bricks. If you think in two dimenions, and about paving a driveway, you can put the bricks together so that all the bricks are parallel like in a brick wall. Or you can tilt them and make a herring-bone pattern. The same size and shape of brick can therfore be put together in different ways and still fill all of the driveway. This can also happen with atoms and molecules when they go into a solid. If its different ways of arranging just one type of atom (or a single element), they are called allotropes. If it is molecules they are polymorphs.

      Polymorphs are all around you and can have very different properties. One good example is the cocoa butter in chocolate. There are six polymorphs of chocolate and the different arrangements of thre molecules affects the melting point. Four polymorphs melt below room temperature so would make a messy chocolate bar! One melts at your body temperature (the one we want so it melts in your mouth!) and one melts a few degrees above body temperature. There is a problem that the highest melting temperature form is the most stable form which means that if you leave the molecules to their own devices, they want to arrange like this. So to make chocolate, they have to force it to be the right solid and this is called tempering and involves heating and cooling and stirring in the right way. And if your chocolate melts and then resolidifies or is left for a long time, you get a white coating on the surface called blooming and it doesn’t taste quite right. All it is is a rearrangement of thr cocoa butter molecules in the solid and this is why chocolate has a shelf life!

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