This illustrates an example of the changes of colour which an element can undergo according to its physical condition. Iodine is a greyish-black non-metallic element and is a member of the halogen group in the Periodic Table. On the right, iodine is present as dark highly reflective crystals, which readily sublimes (see the small crystals at the top of the flask). In the centre of the picture the iodine is disolved in potasium iodide solution, while on the left it is disolved in carbon disulphide.
© Science Museum / Science & Society Picture Library