Question

a blue compound that turns black when heated​

Answer 1

Copper sulfate is normally found in the form of blue crystals, copper (II) sulfate pentahydrate. When you heat copper sulfate pentahydrate it turns white as the water which is driven off by the heat. The white solid remaining is anhydrous copper sulfate. On further heating, it changes to black color.

Answer 2

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Cu ²+ ion on strongly heating leaves a black residue.

Copper sulfate is normally found in the form of blue crystals, copper (II) sulfate pentahydrate. When you heat copper sulfate pentahydrate it turns white as the water which is driven off by the heat. The white solid remaining is anhydrous copper sulfate. On further heating, it changes to black color. If you add water to the anhydrous copper sulfate an exothermic reaction occurs, you can feel the test-tube getting hot, as the blue copper sulfate pentahydrate is re-formed.