«The Copper and the Arsenic Groups.»—Ions whose sulphides are insoluble in dilute acids. The precipitating agent for the two groups is H_{2}S, ‹in acid solution›. The ‹sulphides of the arsenic group are soluble in a mixture of› (NH_{4})_{2}S ‹and› (NH_{4})_{2}S_{x} and are separated thereby from the sulphides of the ‹copper group›. «The Copper Group.»—«Hg»^{2+}, («Pb»^{2+}), «Bi»^{3+}, «Cu»^{2+}, «Cd»^{2+}. «The Arsenic Group.»—«As»^{3+}, «As»^{5+}, «Sb»^{3+}, «Sb»^{5+}, «Sn»^{2+}, «Sn»^{4+}, «Pt»^{2+}, «Pt»^{4+}, «Au»^{+}, «Au»^{3+}. «The Aluminium and the Zinc Groups.»—Ions whose sulphides or hydroxides are insoluble in neutral or slightly alkaline (ammonium hydroxide) solutions. The two groups are precipitated together by a mixture of NH_{4}Cl, NH_{4}OH, (NH_{4})_{2}S. The ‹aluminium group› may be ‹separated› from the ‹zinc group› by treatment of solutions of the chlorides or nitrates with ‹barium carbonate›. «The Aluminium Group.»—«Fe»^{3+}, «Al»^{3+}, «Cr»^{3+}. «The Zinc Group.»—«Fe»^{2+}, «Ni»^{2+}, «Co»^{2+}, «Mn»^{2+}, «Zn»^{2+}. «The Alkaline Earth Group.»—Ions whose carbonates and phosphates are insoluble in neutral or alkaline solutions. The precipitating agent is NH_{4}Cl, NH_{4}OH, (NH_{4})_{2}CO_{3} for «Ba»^{2+}, «Sr»^{2+}, «Ca»^{2+}, and (NH_{4})_{2}HPO_{4} for «Mg»^{2+}.[327] «The Alkalies.»—Ions whose chlorides, sulphides, hydroxides, carbonates and phosphates are soluble. «K»^{+}, «Na»^{+}, «NH_{4}»^{+}. In considering the analytical reactions and the analysis of these groups of metal ions, we shall take up the groups in the order reversed to that given in the table. We shall begin with the group of alkali metals, follow this group with the alkaline earths, then take the aluminium and zinc groups, the copper and silver groups, and finish with the arsenic group. This order is chosen because the chemistry of the reactions involved is simplest in the groups to be studied first and grows more complicated as we advance to those to be studied later. It is not intended to discuss in detail all the reactions and methods; our attention will be limited rather to the study of ‹typical general relations›, and the student is expected to acquire the power to apply the general conclusions reached, to any specific case demanding it. [p159] «The Alkali Group.»—The group includes the ions of «sodium» and «potassium», the most common and most important of the alkali metals, and the «ammonium»-ion. Characteristic of the ions of the group is the fact that all of their common salts are easily soluble in water. They remain in solution in systematic analysis, while other metal ions are removed in the form of various insoluble precipitates.