Tungstic acid refers to hydrated forms of tungsten trioxide, WO₃. The simplest form, the monohydrate, is WO₃.H₂O, the dihydrate WO₃.2H₂O is also known. The solid state structure of WO₃.H₂O consists of layers of octahedrally coordinated WO₅(H₂O) units where 4 vertices are shared. The dihydrate has the same layer structure with the extra H₂O molecule intercalated between the layers. The monohydrate is a yellow solid and insoluble in water. The classical name for this tungstic acid is 'acid of wolfram'.