Three colorless solutions are combined into a large beaker and the mixture becomes amber, then blue-black, and then colorless again. This sequence of color changes will repeat with a period of ...

However, once all the acid has reacted, the copper(II) oxide remains behind as seen by black powder at the bottom of the beaker. The blue colour is due to the formation of a soluble salt.

Compounds with a phenol group will form a blue, violet, purple, green, or red-brown color upon addition of aqueous ferric chloride. This reaction can be used as a test for phenol groups. Mix several ...