If \(\mathrm{NaOH}\) is added to an aqueous solution of zinc ions, a white precipitate appears and on adding excess of \(\mathrm{NaOH}\), the precipitate dissolves. In this solution, zinc exists in the: (a) Anionic part (b) Cationic part (c) Both in cationic and anionic parts (d) There is no zinc in the solution

Chemical precipitation is a process in which a solid material, known as a precipitate, separates from a solution. It occurs when the concentration of certain ions exceeds their solubility, leading to the formation of an insoluble compound. In our exercise, when sodium hydroxide (), a base, is added to a solution containing zinc ions (), a white precipitate of zinc hydroxide () forms. This is due to the reaction between the zinc ions and hydroxide ions () which produce an insoluble salt, zinc hydroxide. This can be illustrated by the reaction equation: .

Precipitation reactions are fundamental in different fields such as water treatment, where they are used to remove impurities, and in analytical chemistry for qualitative analysis of certain ions.

Complex ion formation is a concept where a central metal ion is surrounded by a certain number of ligands, which are ions or molecules with a lone pair of electrons that can form a coordinate bond with the metal ion. In the exercise, when an excess of ) is added, the initially formed zinc hydroxide precipitate dissolves, forming a complex ion called zincate (). The equation for this reaction is shown as: .

The hydroxide ions act as ligands, donating their lone pairs to bond with the zinc ion. Complex ion formation can lead to an increase in the solubility of some precipitates, as is the case with zinc hydroxide in the presence of excess hydroxide ions.

The solubility of zinc hydroxide is a subject of interest in coordination chemistry and chemical equilibria. It is a sparingly soluble compound under neutral conditions. However, its solubility can change under different pH values. In a basic solution, such as after the addition of ), the solubility increases due to the formation of a complex ion.

In the scenario provided by the exercise, the zinc hydroxide formed initially is not very soluble. Yet, the presence of excess ) changes the chemistry significantly, leading to the dissolution of the hydroxide as a zincate ion. This reflects the principle that the solubility of a precipitate can be affected by the pH of the solution and the presence of complexing agents.

Coordination chemistry is the study of compounds formed between metal ions and ligands. These compounds, known as coordination complexes, have a wide range of applications and implications in various fields, including biology, medicine, and materials science. In our exercise, the complex ion is the central zinc ion surrounded by hydroxide ions as ligands, forming a coordination complex.

The number of ligands around the metal ion and their arrangement is described by coordination numbers and geometries, respectively. Complex ions like zincate, where hydroxide ions are coordinated to a metal ion, are key examples of the principles of coordination chemistry at work. They illustrate how the properties of a metal ion in solution, including its solubility and reactivity, are deeply influenced by its coordination environment.