Zinc hydroxide (Zn(OH)₂) is not soluble in excess sodium hydroxide solution. It is actually soluble in excess of strong bases, such as sodium hydroxide (NaOH), due to the formation of a complex ion. Here's why:

1. Formation of a Complex Ion:

- Zinc hydroxide is amphoteric, meaning it can act as both an acid and a base.

- When reacted with excess NaOH, it acts as an acid.

- The hydroxide ions (OH-) from NaOH react with zinc hydroxide to form the soluble tetrahydroxozincate(II) ion [Zn(OH)₄]²⁻.

2. Equilibrium Shift:

- The reaction between zinc hydroxide and hydroxide ions is an equilibrium reaction.

- The addition of excess NaOH shifts the equilibrium to the right, favoring the formation of the complex ion and increasing the solubility of zinc hydroxide.

The overall reaction:

Zn(OH)₂(s) + 4OH⁻(aq) ⇌ [Zn(OH)₄]²⁻(aq)

In summary:

Zinc hydroxide is soluble in excess sodium hydroxide solution because it forms a soluble complex ion, [Zn(OH)₄]²⁻, due to the reaction with hydroxide ions. This reaction is driven by the equilibrium shift caused by the excess of NaOH.