Question

What change in color is observed when copper sulfate is heated? Is it a chemical change or a physical one?

Answer 1

Hint: A chemical change is a change that includes formation or breakage of chemical bond, while a physical change is just change in physical state of a compound. Copper sulfate has water of crystallization in its crystal structure.

Complete answer:
> Copper sulfate that we normally see in a lab is blue in color and it is called hydrated copper sulfate. It contains water molecules in its crystal structure. Its molecular formula can be given as \[CuS{O_4} \cdot 5{H_2}O\].
> So, when it is heated, hydrated copper sulfate loses water of crystallization, means water molecules and the new product is anhydrous copper sulfate means it does not have any water molecules included in its crystal structure. Reaction can be given as

\[CuS{O_4} \cdot 5{H_2}O \to CuS{O_4}\]

> So, anhydrous copper sulfate is white in color.
> That is the reason why we see color change from blue to white when we heat copper sulfate.
> Now, there is hydrogen bonding present between oxygen atoms of sulfate anion and hydrogen atoms of water. That hydrogen bonding is broken while we heat the compound. So, as bond breaking is involved in the reaction, we can say that it is a chemical change.

Additional Information There are many such salts available that have water of crystallization.
Water of crystallization is nothing but water molecules that are involved in the formation of crystal structure of the salt. In some of the compounds like copper sulfate, they are actually responsible for the color of the compound.

Note: Remember that copper sulfate that we use normally or we use in a lab is hydrate copper sulfate and its chemical formula is \[CuS{O_4} \cdot 5{H_2}O\]. Whenever we take or calculate the molecular weight of this compound, the weight of water molecules are also considered.