Acid Black 2, also known as Nigrosine, is a synthetic dye. It is indeed water-soluble, which means it can dissolve in water to form a colored solution. Acid dyes like Acid Black 2 are commonly used in various applications, including dyeing textiles, leather, and paper, as well as in some ink formulations. They are called "acid dyes" because they require an acidic environment, typically in the form of an acid, to fix the color onto the substrate (e.g., fabric fibers). The acidic conditions are necessary for the dye molecules to interact with the substrate and form a stable bond.