What is green vitriol?

Definition:
Vitriol is the generic name for a group of chemical compounds made up of sulphates of various metals, originally iron or copper. The colour of certain minerals was used to distinguish them, such as green vitriol for hydrated Iron(II) sulphate.

Structural formula:
1. The structural formula of green vitriol is$FeS{O}_4.7H_{2}O$ .
2. The pure anhydrous form is white. When the salt comes in contact with water, it regains its color as per the metal valency, green in this case.
Final Answer:

Green vitriol is hydrated Iron(II) sulphate.