 In this lesson, we will learn how to test for the presence of transition metal ions and the ammonium ion. A flame test may be used to test for the presence of group 1 and group 2 metal ions, as well as for the copper 2 plus ion. To test for transition metal ions, we can add aqueous sodium hydroxide to the unknown solutions. The color of the metal hydroxide precipitate formed can help us determine the identity of the transition metal ion. General equation for the reaction is metal ion plus hydroxide ion gives a metal hydroxide precipitate. Addition of sodium hydroxide to a solution containing copper 2 plus ions creates copper 2 hydroxide, which is a blue precipitate. Iron 2 hydroxide is a green precipitate, which slowly becomes reddish brown upon standing. Iron 3 hydroxide is a reddish brown precipitate. Zinc hydroxide is white. Here is a challenge. Why does iron 2 hydroxide go from green to reddish brown upon standing? Please pause the lesson to think about this and resume when you are done. Iron 2 hydroxide goes from green to reddish brown due to the oxidation of iron 2 plus to iron 3 plus. Hence, on standing iron 2 hydroxide oxidizes to iron 3 hydroxide. Some non-transition metal ions that form a precipitate upon the addition of aqueous sodium hydroxide are the calcium ions, the aluminum ion, and the magnesium ion. These resulting metal hydroxide precipitates are white. Further addition of sodium hydroxide will dissolve aluminum hydroxide. Calcium hydroxide and magnesium hydroxide can be compared using a flame test. Calcium ions will give a brick-red flame whereas magnesium ions do not give a flame color. To test for the ammonium ion add aqueous sodium hydroxide and warm in a water bath. Ammonia is formed and its presence can be detected using red litmus paper which turns blue. Alternatively, a glass rod may be dipped in concentrated hydrochloric acid and held near the mouth of the reaction vessel. Ammonium chloride in the form of a thick white smoke should form. The color of the metal hydroxide precipitate produced can help us determine the identity of the metal ion. Most of these metal ions are transition metal ions. The ammonium ion can be tested using the addition of aqueous sodium hydroxide which forms ammonia.