Zinc hydroxide (\(\text{Zn}(\text{OH})_2\)) is an inorganic compound often encountered as a white, powdery solid. The question of its solubility in water has a nuanced answer. While basic chemistry classifies zinc hydroxide as insoluble, this term requires careful definition when discussing its true behavior in pure water. The amount that actually dissolves is exceedingly small under normal conditions.
The General Answer and Solubility Rules
Standard chemical rules classify zinc hydroxide as insoluble in water, placing it in the category of sparingly soluble compounds. This classification is rooted in the general behavior of metal hydroxides in aqueous solutions. Most metal hydroxides, which contain a metal ion bonded to hydroxide (\(\text{OH}^-\)) ions, do not dissolve significantly when placed in water.
Only the hydroxides of Group 1 metals (such as sodium or potassium) and the heavier Group 2 metals (like barium) readily dissolve. Zinc is a transition metal, and its hydroxide does not meet the criteria for high solubility.
The term “insoluble” is a qualitative description indicating that dissolution is below an observable threshold. For practical purposes, when a powder fails to disappear in water, it is labeled insoluble. This distinction separates compounds that dissolve freely from those that remain largely solid.
The Chemistry of Limited Solubility
Moving beyond the label of “insoluble” reveals a quantitative reality: a minute amount of zinc hydroxide does dissolve in pure water. This limited dissolution is described by an equilibrium established between the solid compound and its constituent ions. Solid \(\text{Zn}(\text{OH})_2\) dissociates into zinc ions (\(\text{Zn}^{2+}\)) and hydroxide ions (\(\text{OH}^-\)), but this process is highly restricted.
The extent of this limited solubility is measured by the Solubility Product Constant (\(K_{sp}\)). This value is the equilibrium constant for the dissolution reaction, and for zinc hydroxide, the \(K_{sp}\) is approximately \(3.0 \times 10^{-17}\) at standard temperature. The smallness of this number defines its limited solubility.
A \(K_{sp}\) of \(3.0 \times 10^{-17}\) demonstrates that the concentration of dissolved zinc ions is incredibly low. In pure water, the molar solubility confirms this to be only a few millionths of a mole per liter. This tiny concentration is measurable with sensitive instruments but is chemically insignificant for most casual observations, which is why the precipitate remains visibly solid.
The Amphoteric Nature of Zinc Hydroxide
The limited solubility of zinc hydroxide changes dramatically when the acidity or basicity of the solution is altered. Zinc hydroxide is an amphoteric compound, meaning it reacts chemically with both acids and bases. This property allows \(\text{Zn}(\text{OH})_2\) to dissolve readily in solutions with a very low pH (acidic) or a very high pH (basic), even though it is insoluble in neutral water.
When placed in a strong acid, \(\text{Zn}(\text{OH})_2\) acts as a base. The hydroxide ions react with the acid’s hydrogen ions (\(\text{H}^+\)) to form water. This neutralization pulls the \(\text{OH}^-\) ions out of the dissolution equilibrium, causing the solid zinc hydroxide to dissolve completely to form a soluble zinc salt and water. The resulting clear solution contains free, highly soluble \(\text{Zn}^{2+}\) ions.
The dissolution in a strong base involves a coordination reaction. When excess hydroxide ions are added, the solid zinc hydroxide acts as a weak acid, accepting additional hydroxide ions. This reaction creates a new, soluble complex ion known as the tetrahydroxozincate ion (\(\text{[Zn(OH)}_4]^{2-}\)).
The formation of the tetrahydroxozincate ion causes the white precipitate to redissolve in excess base, resulting in a clear, colorless solution. This complexation reaction is the major exception to the general rule of insolubility.