354 THEORY OF SOLUTION AND CRYSTALLISATION. 



8. When a substance produces with a given solvent a coloured 

 solution, some very remarkable changes are frequently observed, 

 which in many cases afford useful information as to the state in 

 which the dissolved solid exists in the liquid. A few familiar 

 examples will suffice. Anhydrous copper sulphate is white, but 

 wetted with water it becomes blue, and the bine salt yields a blue 

 solution. Anhydrous copper chloride is brown, but it gives a 

 green mass with a little water, which yields a green solution, 

 becoming, however, blue upon dilution. Black cobalt iodide 

 slightly moistened becomes green, and on the addition of more 

 water yields a red solution. 



The coloured substances that are formed in each of these cases 

 are so many definite compounds of the salt with water. Their 

 composition, and that of a few other similar compounds, is 

 represented by the formuloe given below — 



Copper Sulphate. 

 Anhydrous ... ... ... Cu SO4 



Blue (Blue Vitriol) ... ... Cu SO4 5 OHg 



Copper Chloride. 



Brown ... ... ... ... Cu CI2 



Green ... ... ... ... Cu CI2 OH2 (?) 



Blue ... ... ... ... CUCI22OH2 



Cobalt Iodide. 



Black ... ... . ... Co I2 



Green ... .,. .. ... Co I2 2OH2 



Red ... ... ... ... C0I26OH2 



Cobalt Chloride. 



Pale Blue ... ... ... Co CI2 



Indigo Blue ... ... ... Co CI2 2OH2 



Reddish Violet ... ... . . . Co CI2 4OH2 



Red ... ... ... ... C0CI26OH2 



