Temperature at which a solid changes to a liquid. A completely pure crystalline substance has an exact temperature at which it melts. Impure substances and Material without the regular, ordered structure of crystalline solids. Amorphous substances, like glass, lack a definite repeating pattern in their atomic structures (crystallinity). There may be small regions of order, but, overall there is disorder. substances will begin melting at one temperature and finish at another. For these substances the term “melting range” is more appropriate. Under some conditions a material will sublimate rather than melt, moving directly from a solid to gas state.