Zinc hydroxide, amphoteric properties

Amphoteric Hydroxides. A hydroxide, such as zinc hydroxide, which can combine with acids to form salts and also with bases to form salts is called an amphoteric hydroxide. The amphoteric properties of a metal hydroxide are determined by the stability of the hydroxide complex of the metal. 

The metallic properties increase down any column and towards the left in any row on the periodic table. One important metallic property is that metal oxides are base anhydrides. A base anhydride will produce a base in water. These are not oxidation-reduction reactions. Many metal oxides are too insoluble for them to produce any significant amount of base. However, most metal oxides, even those that are not soluble in water, will behave as bases to acids. A few metal oxides, and their hydroxides, are amphoteric. Amphoteric means they may behave either as a base or as an acid. Amphoterism is important for aluminum, beryllium, and zinc. Complications occur whenever the oxidation number of the metal exceeds +4 as in the oxides that tend to be acidic. 

Students may have learnt about ionic precipitation reactions and the formation and properties of ammonia in the topic on Acids, bases and salts . They also need to draw upon knowledge of amphoteric compounds (those that react with both acids and alkalis) to understand the further reactions of the insoluble hydroxides of zinc, aluminium and lead with excess sodium hydroxide solution to form soluble salts. The reactions of the insoluble hydroxides of copper and zinc with excess ammonia solution are similar in that soluble salts are also formed. 

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