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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.

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    $\begingroup$ Two questions : 1) What was the final temperature of your oven ? 2) Are you sure that there was no ammonia in the vicinity ? Rather small amounts of ammonia may react with copper ions to produce complex ions whose color is a different sort of blue. $\endgroup$
    – Maurice
    Feb 13 at 20:28
  • $\begingroup$ I dried it at around 50 C. There was definitely no ammonia in the vicinity. $\endgroup$
    – Kantura
    Feb 13 at 20:48
  • $\begingroup$ @Maurice Sorry, it turned green not purple, I don't know why I typed purple. Major typo. $\endgroup$
    – Kantura
    Feb 13 at 20:49
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    $\begingroup$ The hydrate is blue , anhydrous is whitish. $\endgroup$ Feb 13 at 21:45
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    $\begingroup$ @Kantura. And maybe tomorrow you will suddenly remember that the temperature was not 50°C and that it was not copper sulfate, but maybe who knows ... $\endgroup$
    – Maurice
    Feb 13 at 21:53
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Municipal water treatment often includes both chlorine and ammonia, creating chloramines for a stronger effect.

The color from the reaction with ammonium hydroxide is here - I don't think this matches what you describe.

However, the production of copper chloride (this video shows a different method) seems to be consistent with a green color. You can search more images online to see if that matches what you saw.

There's a reason why chemistry labs like distilled water. :)

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