How is iodine described?

Iodine is accurately described as a dark grey crystalline solid or a purple vapour. At room temperature, iodine typically exists as a solid, forming shiny, metallic-looking crystals. When heated, it sublimates, transitioning directly from a solid to a gas, resulting in a distinctive purple vapour. This property of sublimation is characteristic of iodine, distinguishing it from substances that are found in other states, such as gases or liquids, at room temperature.

Understanding iodine's physical state and behaviour helps to emphasize its dual nature; as a solid, it can demonstrate crystalline structures, while the vapour displays the unique colour that is often associated with iodine's gaseous form. The other descriptions do not accurately capture these properties, making the response that describes iodine as both a dark grey crystalline solid and a purple vapour the appropriate choice.