Copper hydroxide is an inorganic compound recognizable by its pale blue to bluish-green solid appearance. The direct answer to whether copper hydroxide is soluble in water is that it is not; it is considered virtually insoluble in standard, neutral water. This lack of solubility is a defining physical property of the compound and dictates many of its applications and chemical behaviors. Solubility describes the maximum amount of a substance that can dissolve in a given amount of solvent.
The Simple Answer: Why Copper Hydroxide is Insoluble
When discussing the solubility of ionic compounds like copper hydroxide, chemists note that while many compounds dissolve easily, certain groups are exceptions. Copper hydroxide falls into the category of metal hydroxides, which are generally insoluble or only slightly soluble in water.
Only hydroxides of the alkali metals (Group 1 elements) and the heavier alkaline earth metals, such as barium, are readily soluble. Copper, as a transition metal, forms a hydroxide that does not dissociate significantly into its copper ions and hydroxide ions when placed in water. While a minuscule amount does dissolve, the concentration is so vanishingly small that for all practical purposes, the substance is classified as insoluble.
The Underlying Chemistry of Insolubility
The behavior of copper hydroxide in water is determined by a competition between two energetic forces: lattice energy and hydration energy. Lattice energy is the energy required to break apart the strong electrostatic forces holding the positive copper ions and negative hydroxide ions together in the solid crystal structure. This energy must be supplied to the system to separate the ions.
Hydration energy is the energy released when the individual ions are surrounded and stabilized by polar water molecules, a process called solvation. For an ionic compound to dissolve, the energy released during hydration must be greater than or at least equal to the lattice energy required for separation. When hydration energy exceeds lattice energy, the dissolution process is energetically favorable, and the compound dissolves.
In the case of copper hydroxide, the lattice energy is significantly larger than the hydration energy of the resulting ions. The small size and relatively high charge density of the copper ion allow it to form a very strong, tightly packed crystal lattice with the small hydroxide ions. This strong attraction makes the crystal structure difficult to break apart. Because the energy gained from surrounding the ions with water molecules is insufficient to overcome this strong lattice force, the compound remains largely undissolved in neutral water.
Ways to Make Copper Hydroxide Dissolve
Although copper hydroxide is insoluble in neutral water, its chemical nature allows it to dissolve readily under specific chemical conditions. One common method involves adding an acid, such as sulfuric acid or nitric acid, to the solid compound. Copper hydroxide acts as a base and reacts with the acid in a simple neutralization reaction, forming a soluble copper salt and water. For example, the reaction with sulfuric acid yields the highly soluble copper(II) sulfate. This process effectively removes the hydroxide ions from the equilibrium, driving the solid to dissolve as it is consumed in the reaction.
A second, more distinctive method involves adding a concentrated solution of a powerful base, such as aqueous ammonia. While this may seem counterintuitive, the copper ion has a strong affinity for ammonia molecules, which act as ligands. The insoluble copper hydroxide reacts with excess ammonia to form a completely different chemical species, a soluble coordination compound. This resulting species is the deep blue tetraamminecopper(II) complex, which is often used as a striking visual confirmation of copper’s presence in a solution.
Common Applications of Solid Copper Hydroxide
The insolubility of copper hydroxide is a desirable property that makes it valuable in several practical applications. Its stability as a solid means it can be applied to surfaces without being immediately washed away by rain or irrigation. This characteristic has led to its extensive use in agriculture as an effective fungicide and bactericide.
Copper hydroxide is used to protect crops from various fungal and bacterial diseases. When applied to plants, the solid compound releases small amounts of copper ions onto the leaf surface, which are toxic to fungal spores and bacteria. Beyond agriculture, its vivid blue color makes it a suitable pigment and colorant in ceramics and paints. Furthermore, it is employed as a wood preservative, protecting the material from decay and rot.