Yellow tungsten oxide is an acidic oxide. It can’t be soluble in water, slightly soluble in acid. Yellow tungsten oxide can react with alkali to form the corresponding soluble tungstate, so yellow tungsten oxide can be soluble in alkali.
Solubility is the degree to which a substance can be dissolved. If a solute is dissolved in a solvent well, we say that the substance is soluble. If it is not much dissolved, it is said that the substance is slightly soluble. If it is difficult to dissolve, then it is said that this material is insoluble or difficult to dissolve. In a certain temperature, the solubility of a solid substance in the 100g solvent to reach saturation state is the quality of the dissolved.
In the picture above, number 1 presents to yellow tungsten oxide, number 2 means the YTO dissolving process, number 3 shows the final solution that YTO dissolved in alkali (such as sodium hydroxide).