When you mix copper chloride (CuCl₂) and ammonium hydroxide (NH₄OH), you'll observe a fascinating chemical reaction that produces a beautiful blue precipitate. Here's a breakdown of what happens:

Reaction:

CuCl₂ (aq) + 4NH₄OH (aq) → [Cu(NH₃)₄]²⁺ (aq) + 2Cl⁻ (aq) + 4H₂O (l)

Explanation:

1. Initial State: Copper chloride (CuCl₂) dissolves in water to form copper(II) ions (Cu²⁺) and chloride ions (Cl⁻). Ammonium hydroxide (NH₄OH) is a weak base that exists in equilibrium with ammonia (NH₃) and water.

2. Formation of Tetraamminecopper(II) Ion: When ammonium hydroxide is added, ammonia molecules (NH₃) from the equilibrium react with copper(II) ions to form a complex ion called tetraamminecopper(II) ion ([Cu(NH₃)₄]²⁺). This complex ion is a deep blue color.

3. Precipitation: As more ammonium hydroxide is added, the concentration of ammonia increases, favoring the formation of the tetraamminecopper(II) complex. However, the concentration of chloride ions also increases, and in excess ammonia, the solubility of the complex is reduced. This leads to the precipitation of a light blue solid, which is likely a mixture of [Cu(NH₃)₄]Cl₂ and [Cu(NH₃)₄](OH)₂.

Observations:

* Initially, the solution turns a light blue as the tetraamminecopper(II) complex forms.

* As more ammonium hydroxide is added, the solution becomes a deeper blue.

* A light blue precipitate will eventually form, indicating the formation of the insoluble copper complex.

Note: The reaction is reversible. If you add a strong acid, such as hydrochloric acid, to the solution, the equilibrium will shift back towards the formation of copper(II) ions and the blue color will disappear.