Air is a mixture of gases, and when it is polluted, it also contains substances that are harmful to us. Most of the solid substances in nature turn into a liquid and then a gaseous state when heated. However, there are exceptions. When iodine is heated, it does not change from a solid to a liquid state, but to a gaseous state, and this physical change is called sublimation. Heating iodine releases purple vapors, which then turn into a solid state in contact with a cold balloon.