I dried out some blue copper sulphate in an oven to see if it would turn white which it did. I can't remember exactly but I left it overnight in the oven at around 50 C. It was white the following day.
When I put some water drop by drop on it it did not return to the original bright blue. Instead it took on a more green colour.
Why did this happen ?
The copper sulphate was bought from an chemistry chemicals supplier and the water was normal tap water which was treated for domestic use in a city. Water is drinkable.
There was definitely no ammonia in the vicinity.