copper sulphate 
(also copper sulfate)
noun [mass noun]
A blue crystalline solid used in electroplating and as a fungicide.
- Chemical formula: CuSO4.5H2O.
- When organic farmers do use one of the chemicals they are allowed to use, such as sulphuric acid or copper sulphate, they do so on a pretty substantial scale, to the serious detriment of neighbouring wildlife.
- Apart from the blue crystals of copper sulphate that would appear to spray the potatoes against blight, chemicals belonged to another world.