CuSO4 (s) vs CuSO4 (aq)
Copper sulfate is also known as cupric sulfate. Copper sulfate is a salt of copper +2 ion and sulfate anion. When a copper +2 solution and a sulfate solution (potassium sulfate) are mixed together, a copper sulfate solution will result. Copper sulfate has several types of compounds, which differ by the number of water molecules associated with it. When copper sulfate isn’t associated with any water molecules, it is known as the anhydrous form. This is in a powder form and has a gray-white color. Anhydrous copper sulfate has a molar mass of 159.62 g/mol. Depending on the number of water molecules, the physical properties of the salt may vary.
Copper sulfate solid form can be found in various molecular formulas as mentioned in the introduction. However, among them, most commonly occurring form is the pentahydrate (CuSO4·5H2O). This has a beautiful bright blue color and an attractive crystal structure. The molar mass of this solid is 249.70 g/mol. Naturally this pentahydrate form is present as chalcanthite. Further, there are two other hydrated forms of copper sulfate solids which are very rare. Among them, bonattite is a trihydrated salt and boothite is a heptahydrated salt. The pentahydrate copper sulfite has a melting point of 150 °C, but it tends to decompose before this temperature by removing four water molecules. The blue color of the crystal comes from the water molecules. When heated to about 200 oC, all the water molecules evaporate, and the gray-white color anhydrous form is obtained. Solid copper sulfate readily dissolves in water, to produce an aqueous solution. This salt has many agricultural uses. For example, Copper sulfate pentahydrate is a good fungicide.
When the solid form of copper sulfate dissolves in water, it gives the aqueous solution of copper sulfate, which is blue in color. In this solution, the copper +2 ions will exist as aqua complexes. The complex that is formed can be written as [Cu(H2O)6]2+. This is an octahedral complex, where six water ligands are arranged around the copper +2 ion octahedrally. Since the aqua ligands have no charge, the overall complex gets the charge of copper, which is +2. When solid copper sulfate dissolves in water, it releases heat to outside; therefore, the solvation is exothermic. Aqueous solutions of copper sulfate are important in chemical reagents. For example, Fehling’s reagent and Benedicts’ reagent contain copper sulfate. These are used to test reducing sugar. So in the presence of a reducing sugar, Cu2+ will be reduced to Cu +. Further, this is also used in Biuret reagent for testing proteins.
What is the difference between CuSO4 (s) and CuSO4 (aq)?
• Often CuSO4 (s) is a blue color crystal. But CuSO4 (aq) is a blue color solution.
• Often in CuSO4 (s), there are five water molecules. But there can be varying number of water molecules or sometimes no water molecules in the compound. In aqueous copper sulfate, there are six water molecules form complexes with copper ions.