Hg^2+ is soft acid and thus akin to bind to soft base S^2- to form HgS. The relatively harder acids Zn^2+ and Cd^2+ are more inclined to react with hard bases such as CO3^2-, O^2- to form carbonates and oxides. Solution Hg^2+ is soft acid and thus akin to bind to soft base S^2- to form HgS. The relatively harder acids Zn^2+ and Cd^2+ are more inclined to react with hard bases such as CO3^2-, O^2- to form carbonates and oxides..