Question

Write the chemical formula of hydrated copper sulphate and anhydrous copper sulphate. Giving an activity to illustrate how these are interconvertible.

Answer 1

Hint: Remember that the hydrate form of any compound A is given by $A.{{\operatorname{xH}}_{2}}O$ whereas the anhydrous forms of compound do not involve any water molecules in the chemical formula. The number of water molecules involved in the hydrated form depends from compound to compound.

Complete step by step solution:
Let us first observe the physical and chemical properties of Copper Sulphate before moving onto the specifics of this question.
Copper (II) sulphate, also known as copper sulphate, is an inorganic compound with the chemical formula \[CuS{{O}_{4}}.\operatorname{x}({{H}_{2}}O)\], where x can range from 0 to 5. The pentahydrate ($x=5$) is the most common form. Older names for this compound include blue vitriol, bluestone, vitriol of copper, and Roman vitriol.

The pentahydrate (\[CuS{{O}_{4}}\cdot 5{{H}_{2}}O\]), is the most commonly encountered salt and is bright blue in colour. It exothermically dissolves in water to give the aqua complex \[{{[Cu{{({{H}_{2}}O)}_{6}}]}^{2+}}\], which has an octahedral molecular geometry. The structure of the solid pentahydrate reveals a polymeric structure wherein copper is again octahedral but bound to four water ligands.

Anhydrous copper sulphate on the other hand is a grey powder. It is commonly used as a test for water. If it turns blue (which is chemically its hydrated form) when subjected to any liquid, then that liquid is water.

The interconversion of the hydrated and unhydrated form is fairly simple. When water is added to the unhydrated form (greyish white) it changes into its hydrated form (blue). To change back you just have to heat it again. The reaction is as follows:
\[CuS{{O}_{4}}\cdot 5{{H}_{2}}OCuS{{O}_{4}}+5{{H}_{2}}O\]


Note:
The hydrated form of copper sulphate should not be interpreted as its chemical structure. The water is added in the crystal lattice in between the molecules of copper sulphate. The form\[CuS{{O}_{4}}\cdot 5{{H}_{2}}O\] is actually a double salt. It can be proved by dissolving this in water. You can experimentally determine that once it is dissolved, the water molecules present in the crystal lattice are gone into the solution.