Many chemicals form hydrates, where water molecules are bound to the chemical molecules. These hydrates often have different colors and different properties. For instance, anhydrous stannic chloride is a corrosive, fuming liquid, while the pentahydrate is an acidic white solid.
Copper(II) chloride has two forms: a brown anhydrous form and a blue-green dihydrate form. Because of excess HCl, my copper(II) chloride hydrate is more green than blue. When it is heated gently, it turns brown and releases both water vapor and white fumes, showing that the excess HCl and the water have been released from the crystal structure.
Here is a picture of the resulting product, with the original dihydrate for comparison.
Since the dehydration required heat, the reaction is likely endothermic. Therefore, rehydration should be exothermic. It is. However, because the anhydrous copper(II) chloride had been heated too vigorously, there was some decomposition to HCl and CuO and the solution formed when it was placed in water was cloudy. Here is the video of the reaction when two drops of water are added to the bulk of the anhydrous substance.
I then tried burning a crystal of copper(II) chloride with a magnifying glass. The copper(II) chloride refracted the green coloration for a moment before they turned brown as they were dehydrated. Then they turned black and melted. The resulting liquid was quite mobile. When cooled, it forms a black amorphous solid.
This dissolves in water, forming a mixture of copper(II) chloride solution (green), copper(II) oxychloride precipitate (green), and copper(I) chloride precipitate (white). The copper(I) turns to the oxychloride upon exposure to air. The black lump completely dissolves.
If I do the same with copper sulfate, I will post the results in this article.