Copper Sulphate
Copper sulphate, blue stone, blue vitriol are all common names for pentahydrated cupric sulphate, CuSO45H2O, which is the best known and the most widely used of the copper salts. Indeed it is often the starting raw material for the production of many of the other copper salts. Copper sulphate crystals are used in a familiar colour change process: this is the blue copper sulphate to be found in many a school chemistry lab
The Green Lion
"Take the vitriol of Venus... and add thereto the elements water and air. Resolve, and set to putrefy for a month according to the instructions.... Separate and you will soon see two colours, namely white and red. The red is above the white. The red tincture of vitriol is so powerful that it reddens all white bodies, and whitens all red ones, which is wonderful. Work upon this tincture by means of a retort, and you will perceive a blackness issue forth. Treat it again by means of the retort, repeating the operation until it comes out whitish... Rectify until you find the true, clear Green Lion."
Vitriol of Venus
In this description of colour changes by Paracelsus (1493-1541) the processes are half-obscured by the secretive phrasing of alchemy (vitriol of Venus).
A chemistry experiment - look in the flask
Copper sulphate in solution is blue. Add copper sulphate crystals to water. After several hours the copper sulphate will have dissolved, but will be more concentrated at the bottom. After several days diffusion will result in the concentration within the flask being the same everywhere, with uniform reflection of light. Adding a piece of zinc metal to the solution will create a displacement reaction. The solution loses its colour as copper is displaced by zinc and the zinc metal gets coated in red-brown copper metal.
