Decomposition of solids involves a series of reactions, which transform the original substance and produce quantitatively new phases. The decomposition usually results in the formation of solutions or melts; sometimes new solid phases are formed. The reactions are generally accompanied by the formation of gaseous phases. The conversion of the original component is either complete (usually in fusion decompositions) or an undecomposed fraction with the original or a changed phase composition remains (in acid decompositions).