The reason why anhydrous copper sulfate changes from white to blue when dissolved in water is mainly due to a chemical reaction between copper ions and water molecules to form hydrated copper ions. Specifically, this process can be summarized as follows:

Dissolution of anhydrous copper sulfate:

Anhydrous copper sulfate (CuSO₄) is a white or off-white powder that is easily soluble in water.

When anhydrous copper sulfate is dissolved in water, it releases copper ions (Cu²⁺) and sulfate ions (SO₄²⁻).

Formation of hydrated copper ions:

Copper ions (Cu²⁺) are not stable in aqueous solution, and they will combine with water molecules to form hydrated copper ions.

In this process, copper ions act as central ions and water molecules act as ligands, which are combined together through coordination bonds to form hydrated copper ions.

For tetrahydrated copper ions ([Cu(H₂O)₄]²⁺), the coordination number of copper ions is 4, that is, each copper ion is combined with 4 water molecules.

Color change:

Hydrated copper ions appear blue in solution, which is due to the electronic structure and energy state of hydrated copper ions.

Therefore, when anhydrous copper sulfate is dissolved in water and forms hydrated copper ions, the color of the solution changes from white to blue.

Formation of crystalline hydrates:

As the reaction proceeds, hydrated copper ions and sulfate ions may further combine to form crystalline hydrates, such as copper sulfate pentahydrate (CuSO₄·5H₂O).

Copper sulfate pentahydrate is also a blue crystal, and its formation further confirms the phenomenon that anhydrous copper sulfate changes color when it encounters water.

In summary, the reason why anhydrous copper sulfate changes from white to blue when dissolved in water is that copper ions combine with water molecules to form hydrated copper ions, and hydrated copper ions appear blue in solution. This phenomenon is not only of theoretical significance, but also plays an important role in practical applications, such as detecting the presence of moisture.