In chemistry, chromism is a process that induces a change, often reversible, in the colors of compounds. In most cases, chromism is based on a change in the electron states of molecules, especially the π- or d-electron state, so this phenomenon is induced by various external stimuli which can alter the electron density of substances. It is known that there are many natural compounds that have chromism, and many artificial compounds with specific chromism have been synthesized to date. It is usually synonymous with chromotropism, the (reversible) change in color of a substance due to the physical and chemical properties of its ambient surrounding medium, such as temperature and pressure, light, solvent, and presence of ions and electrons. Chromism is classified by what kind of stimuli are used. Examples of the major kinds of chromism are as follows. thermochromism is chromism that is induced by heat, that is, a change of temperature. This is the most common chromism of all. photochromism is induced by light irradiation. This phenomenon is based on the isomerization between two different molecular structures, light-induced formation of color centers in crystals, precipitation of metal particles in a glass, or other mechanisms. electrochromism is induced by the gain and loss of electrons. This phenomenon occurs in compounds with redox active sites, such as metal ions or organic radicals. solvatochromism depends on the polarity of the solvent. Most solvatochromic compounds are metal complexes. There are many more chromisms and these are listed below in . The output from the chromisms described above is observed by a change in the absorption spectra of the chromic material. An increasingly important group of chromisms are those where changes are displayed in their emission spectra. Hence they are called fluorochromisms, exemplified by solvatofluorochromism, electrofluorochromism and mechanofluorochromism. Chromic phenomena Chromic phenomena are those phenomena in which color is produced when light interacts with materials, often called chromic materials in a variety of ways. These can be categorized under the following five headings: Stimulated (reversible) color change The absorption and reflection of light The absorption of energy followed by the emission of light The absorption of light and energy transfer (or conversion) The manipulation of light. Color change phenomena Those phenomena which involve the change in color of a chemical compound under an external stimulus fall under the generic term of chromisms. They take their individual names from the type of the external influence,