Violet tungsten oxide solubility refers to solubility in water, but it is insoluble in water, so solubility is 0. It is also insoluble in ethanol. However, tungsten oxides belong to acidic oxides, so it is soluble in alkali solution. How to judge if metal oxides are soluble in water: apart from alkali metal element such as sodium, potassium, cesium, other metal oxides are insoluble in water.
Solubility means under certain temperature, mass of solid material soluble in 100g solvent when reaches saturation condition. Solubleness refers to the soluble extent of one material.