"Blue Vitriol" is the common commercial name for which of the following chemical substances?

The correct option is

Copper Sulphate

Explanation

In historical chemistry, the term "Vitriol" refers to certain hydrated sulfate salts of metals. These salts are characterized by their glassy, crystalline appearance and distinct colors.

Analysis of Options:

• Copper Sulphate (CuSO₄·5H₂O): This is chemically known as Copper(II) sulphate pentahydrate. It is commonly called Blue Vitriol due to its bright blue crystals. It is frequently used in agriculture as a fungicide (e.g., Bordeaux mixture).
• Iron Sulphate (FeSO₄·7H₂O): This is known as Green Vitriol (Ferrous sulphate).
• Magnesium Oxide (MgO): This is commonly referred to as Magnesia.
• Calcium Carbonate (CaCO₃): This is the primary component of limestone, chalk, and marble.

Key Takeaway:
The common names for hydrated sulphate salts are based on their color: Blue Vitriol is Copper Sulphate, Green Vitriol is Iron Sulphate, and White Vitriol is Zinc Sulphate.